Airport and Air Traffic Control System

Airport and Air Traffic Control System

January 1982

NTIS order #PB82-207606

Library of Congress Catalog Card Number 82-600545

For sale by the Superintendent of Documents,U.S. Government Printing Office, Washington, D.C. 20402

Foreword

Air transportation is expected to continue growing during the next two decades. Indealing with this growth it will be important to ensure safety and minimize the costs ofthe system to the Government and airspace users. Large investments are now antici-pated in both airports and air traffic control systems, investments that require unusu-ally long leadtimes. For these reasons the House Committee on Appropriations has re-quested that OTA conduct an assessment of airport capacity and related air traffic con-trol issues.

This subject is, more than most, a moving target. There have been rapid changesin Federal Aviation Administration (FAA) plans in recent years, and these plans havebeen further complicated by airline deregulation and the aftermath of the ProfessionalAir Traffic Controllers Organization strike. These events affect future plans becausethey influence the rate of growth and where that growth will occur. There also con-tinue to be rapid and significant changes in the aviation, telecommunications, anddata-processing technologies on which the system relies. In addition, these plans arecoming before Congress during a period of increasing budgetary constraints.

This assessment is intended to provide a perspective on both airport developmentaid and FAA’s proposed air traffic control system modernization. In both areas thereare questions of how much improvement will be needed, how soon it will be needed,and how the funding of improvements will be allocated among airspace users.

Director

. . .Ill

Airport and Air Traffic Control Advisory Panel Members

Raymond L. Bisplinghoff, ChairmanVice President and Director of R&D, Tyco Laboratories

Jesse BorthwickExecutive DirectorNational Association of Noise Control Officials

Secor D. BrowneSecor D. Browne Associates, Inc.

Jack EndersPresidentThe Mitre Corp.

Matthew FinucaneAviation Consumer Action Project

William T. HardakerAssistant Vice President, Air Navigation/Traffic

ControlAir Transport Association

William Horn, Jr.National Business Aircraft Association, Inc.

Jack D. HowellAir Line Pilots Association, International

Alton G. Keel, Jr.Assistant Secretary of the Air ForceResearch, Development and Logistics

Clifton A. MooreGeneral ManagerDepartment of AirportsCity of Los Angeles

Thomas L. OnetoPlanning OfficerAircraft Owners and Pilots Association

Robert E. PoliPresidentProfessional Air Traffic Controllers Association

Gilbert F. QuinbyConsultant

Janet St. MarkPresidentSMS Associates

David S. StemplerAirline Passengers Association

Richard TaylorVice PresidentBoeing Commercial Airplane Co.

David ThomasGeneral Aviation Manufacturers Association

iv

Airport and Air Traffic Control System Project Staff

John Andelin, Assistant Director, OTAScience, Information, and Natural Resources Division

William Mills, Project Director

Marsha Fenn M. Karen Gamble Larry L. Jenney Paul B. Phelps Zalman Shaven

Contractors

Adib Kanafani, Institute of Transportation Studies, University of California at BerkeleyVincent Volpicelli, Port Authority of New York and New Jersey

Jerry D. WardRobert Simpson, Flight Transportation Laboratory, Massachusetts Institute of Technology

John Heritage, EditorR. Bryan Harrison

OTA Publishing Staff

John C. Holmes, Publishing Officer

John Bergling Kathie Boss Debra M. Datcher Joe Henson

Contents

Chapter

1. Executive Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Introduction and Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. The National Airspace System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Aviation Growth Scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

S. Technology and the Future Evolution of the ATC System . . . . . . . . . . . . . . . . . . .

6. Airport Capacity Alternatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7. Policy Implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page

3

9

25

45

67

101

125

ACRONYMS

AATF

ACARS

ACAS

ADAP

AERA

ANCLUC

ARINCARTCCARTS

ASRATAATARS

ATCATCRBS

BCAS

CDTI

CFCDABS

DARCDMEDODDOTDPIF&EFAA

FARFSSGAGPSICAO

IFR

Airport and Airways TrustFund, trust fund

ARINC /Communications Ad-dressing Reporting System

Airborne Collision AvoidanceSystem

Airport Development Aid Pro-gram

automated en route air trafficcontrol

airport noise comparability andland use

Aeronautical Radio, Inc.air route traffic control centerAutomated Radar Terminal

System, a computer-drivendisplay system used in ter-minal areas

airport surveillance radarAir Transport AssociationAutomatic Traffic Advisory

and Resolution Serviceair traffic controlAir Traffic Control Radar

Beacon SystemBeacon Collision Avoidance

Systemcockpit display of traffic infor-

mationcentral flow controlDiscrete Address Beacon Sys-

tem (Mode S)Direct Access Radar Channeldistance measuring equipmentDepartment of DefenseDepartment of Transportationdisposable personal incomefacilities and equipmentFederal Aviation Administra-

tionFederal Air Regulationflight service stationsgeneral aviationGlobal Positioning SystemInternational Civil Aviation

OrganizationInstrument Flight Rules

ILSINSITU

MLSMode S

NASCOM

NASNASPNOTAMsO&MOMB

PANCAP

PATCO

PIREPPMSPSRRCAG

RE&D

ROIRNAVSACDRS

SMSA

SSRTACAN

TCATCAS

TRACONTRBTri-Modal BCAS

VFRVOR

VORTAC

Instrument Landing Systeminertial navigation systemInternational Telecommunica-

tion UnionMicrowave Landing Systema digital data link system

(formerly DABS)National Airspace Communica-

tions SystemNational Airspace SystemNational Airport System PlanNotices to Airmenoperation and maintenanceOffice of Management and

Budgetpractical annual capacity of an

airportProfessional Air Traffic Con-

trollers Organization

Performance Measuring Systemprimary surveillance radarremote communication air-

groundresearch, engineering, and

developmentreturn on investmentarea navigationStandard Air Carrier Delay

Reporting SystemStandard Metropolitan Station

Areasecondary surveillance radarTactical Control and Naviga-

tion Systemterminal control areaTraffic Alert and Collision

Avoidance Systemterminal radar approach controlTransportation Research Boarda variation of the Beacon Col-

lision Avoidance SystemVisual Flight Rulesvery high frequency omnirange

transmittersA TACAN colocated with a

VOR station

. . .Vlll

Chapter 1

EXECUTIVE SUMMARY

Contents

Page

Aviation Growth Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Airport Capacity Alternatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Air Traffic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Funding Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Response to Future Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Chapter 1

EXECUTIVE SUMMARY.

The National Airspace System includes about6,500 public-use airports connected by a net-work of air routes defined by navigational aids.Aircraft operating along these routes and in ter-minal areas near airports are monitored and con-trolled by a system of ground-based surveillanceand communications equipment—the air trafficcontrol (ATC) system—operated by the FederalAviation Administration (FAA).

In 1980, the 435 airports with FAA towershandled some 180,000 takeoffs and landings perday, or roughly 66 million per year, of which 74percent are general aviation flights and 4 percentare military. The remaining 22 percent of opera-tions are commercial flights (air carrier, com-muter, and air taxi) and are heavily concen-trated in a few large airports. The 66 top airportshandle 77 percent of commercial operations and88 percent of passenger enplanements; the 10largest handle 33 percent of operations and 47percent of passengers.

This concentration of air traffic at a few largehubs creates congestion and delay, which in turnincreases airline operating costs and, ultimately,the cost of air travel for the public. As air trafficand fuel prices increase, the cost of these delays

will be magnified. General aviation users of ma-jor hubs also feel the effects of delay in the formof access restrictions imposed during peak hoursto deal with airport congestion.

Concern about these problems, and about thefeasibility and cost of the proposed solutions,prompted the House Committee on Appropria-tions (Subcommittee on Transportation) to re-quest that OTA undertake an assessment of air-port and terminal area capacity and related ATCissues. The Senate Committee on Commerce,Science, and Transportation endorsed the re-quest of the House Committee on Appropria-tions, which directed OTA to concentrate onfour major topics:

● scenarios of future growth in air transporta-tion;

• alternative ways to increase airport and ter-minal area capacity;

● technological and economic alternatives tothe ATC system modifications proposed byFAA; and

• alternatives to the present ATC process.

OTA’s major findings are presented below.

AVIATION GROWTH SCENARIOS

FAA expects air traffic to increase consider-ably over the next 10 to 20 years, and with it thedemand for ATC services. Its plans for modern-izing and expanding the National Airspace Sys-tem are predicated on accommodating contin-ued rapid growth. A key assumption in FAA’sAviation Forecasts has been that there will be noconstraints on future growth and that new facil-ities and equipment will be deployed where andwhen needed to meet demand. FAA forecastshave consistently exceeded actual demand inthe past, however, with lo-year projections ofgrowth as much as 50 percent higher than ac-tually occurred. This raises questions about theusefulness of FAA forecasts as a basis for long-

term planning and about how quickly FAAneeds to proceed with capacity-related improve-ments in its 1982 National Airspace System Plan(NASP).

Most other aviation forecasts generally sup-port FAA’s projections, but some do not. This isnot surprising in light of the uncertainty aboutthe factors that may affect future traffic growth.The Air Transport Association and a major aer-ospace firm have suggested that the U.S. airlineindustry may already be approaching its maturesize, which would mean that air carrier opera-tions may level off or even decline by the end ofthe century. Airline deregulation has destabil-

3

4 ● Airport and Air Traffic Control System

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ized market structure and airline profitability,leading to questions about the ability of the in-dustry to finance badly needed new equipment.There are questions about the future price andavailability of aviation fuel and about the long-term impacts of the Professional Air TrafficControllers Organization walkout.

There is also uncertainty about the future dis-tribution of operations among user groups andamong airports. FAA expects general aviationusers to account for 75 percent of the increase indemand, but there are large uncertainties aboutthe continued growth of the general aviation

fleet. One such uncertainty is the future priceand availability of the aviation gasoline used bysmall personal aircraft. As for air carriers, mar-ket forces and the restrictions imposed followingthe strike have already resulted in a redistribu-tion of operations away from congested hubs tosecond-tier airports that have excess capacity.This new trend, in combination with improvedfacilities for general aviation traffic at relieverairports, could make it possible to accommodatesome increases in aggregated operations withinexisting system capacity.

AIRPORT CAPACITY ALTERNATIVES

At any given airport, delay occurs when de-mand for terminal airspace or runways ap-proaches the capacity to handle aircraft safely.Some delay is normal and inevitable, especiallyduring peak traffic hours or when capacity isreduced because of adverse weather. At somemajor airports, however, the level of demand isnow such that delay is chronic and severe. Thesedelays inconvenience passengers, increase airlineoperating costs, and waste over a hundred mil-lion gallons of fuel each year.

One way to deal with delay is to increase thecapacity of hub areas, either by adding runwaysto an existing airport or by building a new air-port to relieve other, overcrowded airports.Large amounts of land are required, however,and there are strong community objections toairport noise. These factors have made majorairport construction and expansion rare in thepast decade. In addition, building new runwaysor airports requires years of planning (and, insome cases, litigation) before it can be imple-mented. At some airports, however, indepen-dent “stub” runways for propeller aircraft couldincrease effective capacity and minimize land-use and noise problems.

A more immediate way to alleviate delay is tomanage traffic so that demand fits within ex-isting capacity. This could be done througheconomic measures, such as differential pricingschemes to help divert traffic from peak to off-peak hours, or perhaps from congested to under-utilized airports. Administrative measures, suchas hourly quotas or user restrictions, could in-duce a similar reallocation of demand.

Improved ATC technology could also helpease airport congestion. Automated terminal-area metering and spacing, to smooth and ex-pedite the flow of traffic, and the MicrowaveLanding System, to permit more flexible use ofcrowded airspace close to the airport, might per-mit existing capacity to accommodate more op-erations. The magnitude of the potential benefitsvaries widely with local conditions, runwayconfiguration, and traffic mix.

There is no single “best” way to increase capa-city or reduce delay. A variety of measures—economic, administrative, and technological—will be needed and the optimum solution for anygiven airport will be determined largely by localconditions.

Ch. 1—Executive Summary ● 5

AIR TRAFFIC

FAA is planning a program of technologicalimprovements intended to enable the NationalAirspace System to handle a higher volume oftraffic with increased efficiency and safety. Thisnew technology will replace present equipment—some of which has been in use for over 40years—with a modern integrated system thatwill be more reliable and productive. Thisshould allow new or improved forms of serviceto be offered to airspace users. Operating costsshould be lower than with the current generationof ATC equipment, but there would also be ma-jor capital cost requirements. Many of these im-provements can be implemented during the next10 years, but the full modernization programwill not be completed until the late 1990’s.

Two technologies are at the heart of the newgeneration of ATC: 1) advanced computers; and2) a two-way digital data link between aircraftand the ground. Advanced high-speed comput-ers and new software will permit the ATC sys-tem to improve the overall management of traf-fic flow, as well as to formulate tactical measuresthat will ensure conflict-free, expeditious, andfuel-efficient flight paths for individual aircraft.Replacement computers will be installed first inen route ATC centers, then in terminal areas,and finally in a central flow control facility thatwill manage air traffic on a national basis. In ad-dition to safety and capacity benefits, these com-puters will permit a level of automation in ATCthat will greatly reduce the workforce needed tohandle future traffic loads.

The improved data link between aircraft andground facilities will permit a rapid and exten-sive exchange of information and instructionswithout relying exclusively on voice radio forcommunication—for example, transmittal ofclearances and weather information. FAA alsoproposes to use this data link as the basis for theTraffic Alert and Collision Avoidance System(TCAS) which will provide aircraft with anindependent, airborne supplement to ground-based separation assurance.

In terminal areas, the use of the MicrowaveLanding System (MLS) will provide more precise

CONTROL

and reliable guidance for landing in adverseweather conditions. In combination with pro-cedural changes, MLS could also lead to moreefficient use of airport capacity because it allowsaircraft to follow any of several curving or seg-mented approach paths to the runway, therebyeasing some of the constraint imposed by thepresent Instrument Landing System (ILS), whichprovides only straight-line guidance along asingle path.

In general, OTA finds that the ATC systemimprovements proposed by FAA are technolog-ically feasible and desirable with respect to safe-ty, capacity, and productivity, although thereare alternatives that might be equally effective.In most of the programs reviewed, detailed costand benefit information is not yet available,making it difficult to judge the cost effectivenessof the FAA proposals in relation to the possiblealternatives. For the same reason, it is not yetfully clear whether the overall benefits will ex-ceed the capital expenditures needed to effect theimprovements, how the benefits will be distrib-uted among user groups, and how system costwill be allocated. Further information will beneeded on implementation plans and specificcosts and benefits throughout the Congress’ con-sideration of the FAA’s 1982 National AirspaceSystem Plan.

Funding Issues

Based on information available at the end of1981, OTA estimates that the costs of airportdevelopment grants-in-aid, modernization ofATC facilities and equipment, and related re-search and development could average roughly$1.5 billion per year over the next 10 years,about 50 percent higher than the level of recentyears. Congress has several options to providefunding for these programs. One would be tocover these expenditures by general fund ap-propriations. This option, while it would affordthe Congress continuing close control of FAAprograms through the annual appropriationsprocess, might not provide the assured continu-ity of funding needed for undertaking a 10-yearprogram of the scope envisioned by FAA.


Alternative options involve reestablishing, inone form or another, the Airport and AirwaysTrust Fund which expired in October 1980. Pos-sible approaches to reinstituting the trust fundinclude: 1) a user tax structure and tax rates simi-lar to those that existed before; 2) higher user taxrates—raised either uniformly or selectively bytype of user; or 3) a different scheme of taxationthat would levy fees in proportion to benefitsreceived or costs imposed by each type of air-space user.

All of these options are controversial, and thesearch for a solution is complicated by manylong-standing issues about the equity of user

charges and the appropriate distribution of trustfund revenues. Other issues that could emerge inthe debate are how to use the present uncom-mitted balance in the trust fund (amounting toabout $3 billion) and whether to use trust fundmoneys to help meet operating and maintenancecosts. In the past, trust fund allocations derivedfrom user fees have covered only about 15 per-cent of these costs, and many feel that usersshould pay a larger share of them. Others arguethat trust fund moneys should be reserved ex-clusively for capital improvements and R&D ex-penses,

RESPONSE TO FUTURE GROWTH

Basically, there are three forms of action thatcan be taken to affect growth: regulatory, eco-nomic, and technological. Regulatory actions in-clude measures imposed by the Government thatwould restrict the use of airspace or the availa-bility of ATC services according to user class ortypes of activity. Economic measures are thosethat would affect the cost of using the airspaceor that would allow the market forces of com-petitive pricing to determine access to facilitiesand services that are in high demand. Techno-logical responses include not only improvedforms of ground-based and avionic equipment

to increase the efficiency of airspace use, butalso increases in airport capacity through theconstruction of new or improved landing facili-ties. All three approaches are likely to be used;the issue is not which to adopt, but what combi-nation and with what relative emphasis. Ulti-mately, the measures adopted to deal withgrowth will reflect a more fundamental policydecision: is growth to be accommodated wher-ever and whenever it occurs; or is it to be man-aged and directed so as to make the most effec-tive use of existing resources, with the costs fair-ly borne by the beneficiaries.

Chapter 2

INTRODUCTION ANDOVERVIEW

Contents

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Trends and Forecasts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .The Airport Capacity Problem. ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .The ATC Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .The Committee Request . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .OTA’s Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Technological Improvements. ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Freedom of Airspace Use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Automation and Controller Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Funding and Cost Allocation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

LIST OF FIGURES

‘99

1112141414151616181920

Figure No. Page1. Profile of U.S. Airports, 1980. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102. FAA Budget and Funding Sources, 1971-80. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Chapter 2

INTRODUCTION AND OVERVIEW

BACKGROUND

The National Airspace System (NAS) includesabout 6,500 public-use airports serving nearly allcities and small communities in the UnitedStates. Connecting these airports is a network ofair routes, defined by navigational aids, thatchanneI the flow of traffic. Flight along theseroutes, as well as operations in the terminalareas surrounding airports, is monitored andcontrolled by a system of ground-based surveil-lance equipment and communication links—theair traffic control (ATC) system.

With two exceptions (Washington NationalAirport and Dunes International Airport), * U.S.airports used by commercial flights are ownedand operated by local, regional, or State author-ities. Many general aviation (GA) aircraft alsouse these commercial air carrier airports, butmost are served by smaller public airports andby roughly 10,000 privately owned fields. Theair route system and the ATC system are oper-ated by the Federal Aviation Administration

*Washington National and Dunes International are owned bythe Federal Government and operated by the FAA.

(FAA), which has responsibility for assuring thesafe and expeditious movement of aircraft inU.S. airspace and contiguous areas. FAA is alsoresponsible for coordinating the use of airspaceshared by military and civil aviation.

In all, the NAS accommodates about 180,000operations (takeoffs and landings) per day at air-ports with FAA control towers, or roughly 66million per year. Of these, 22 percent are com-mercial flights (scheduled air carrier, commuter,and air taxi), 74 percent are general aviation,and 4 percent are military. Most of the commer-cial operations are concentrated at the top 66airports, which account for over 77 percent ofcommercial operations and 88 percent of passen-ger enplanements. Within this group, airlinetraffic is even more highly concentrated at a fewmajor hubs. As shown in figure 1, the 10 largesthubs handle 33 percent of all operations and 47percent of all passengers.l

‘FAA Statistical Handbook of Aviation, Calendar Year 1980(Washington, D. C.: Federal Aviation Administration, 1981),passim.

TRENDS AND FORECASTS

The use of NAS, as measured by aircraft oper-ations at airports with FAA towers, has grownat an annual rate of about 4 percent in recentyears, due almost entirely to the rapid growth ofthe GA sector.2 FAA expects the rate of growthto slow to about 3 percent per year in the nextdecade, but this would still mean that the con-gestion now experienced at the 5 or 10 largestairports may spread to 10 or 15 additional air-ports by the year 2000. This growth would alsolead to substantial increases in the workload ofthe ATC system. FAA workload forecasts in-dicate that there may be both capacity* and

—‘FAA Aviation Forecasts, Fiscal Years 1981-1992 (Washington,

D. C.: Federal Aviation Administration, 1980), passim.*In a general sense, capacity refers to the number of aircraft that

can be safely accommodated in a given period of time. Airport ca-

safety problems arising from the growth in de-mand for ATC services, problems that will notbe confined to major airports or commercialoperations. Projections show the demand forATC services by GA users could increase by asmuch as 70 percent over the next 10 years.

The accuracy of these forecasts depends onfactors that are difficult to predict reliably, Forexample, the growth in aviation is extremely

pacity is defined as the maximum number of aircraft operations(takeoffs and landings) that can be accommodated in a given peri-od of time on a given runway (or set of runways) under prevailingconditions of wind and weather and in conformance with estab-lished procedures for maintaining safe separation of aircraft. Simi-larly, airspace capacity is defined as the maximum number offlights that can be allowed to pass through a volume of airspaceduring a given period of time without violating minimum separa-tion standards.

9


Figure 1.— Profile of U.S. Airports, 1980a

alncludes heliports, STOL ports, seaplane bases, and mllltary-cwll joint.use fields, excludes facllltles tn Puerlo RICO, Vlrgln Islands, and PaclflcTerritories.

SOURCE FAA Stat/s r/ca/ Handbook, 7980

sensitive to the state of the national economy.The price and availability of fuel could be a seri-ous constraint on all classes of aviation. Thelong-term effects of airline deregulation are un-certain but they could have an important influ-ence on the profitability and competitive struc-ture of the industry. Thus, while there is a con-sensus that air activity as a whole will continueto grow, it is not certain how much growth toexpect, where it will occur, or what strategiesshould be adopted to accommodate it. It doesseem clear, however, that growth of aviation,even at a rather slow rate, gives rise to concernabout future airport capacity, terminal area con-gestion, and the safety and efficiency of the ATCsystem.

Photo credit: Bill Osmun, Air Transport Association

A crowded terminal

Ch. 2—Introduction and Overview • 11

THE AIRPORT CAPACITY PROBLEM

Concentration of air traffic at a few largehubs, brought about by the economics of airtransportation and by the general increase in airtravel, creates congestion and delay. * The cut-back in scheduled flights following the air trafficcontrollers’ strike has caused the problem toabate temporarily, but congestion can be ex-pected to recur when operations return to nor-mal levels, and with it the associated problem ofsafely handling a growing volume of air traffic.Congestion results in delays that increase airlineoperating costs and, ultimately, the cost of airtravel for the public. If fuel prices increase, thecost of these delays will become magnified.Commuter airlines and air taxi services are evenmore vulnerable to delay costs than trunk air-lines, since they have a much smaller base ofpassengers across which to spread these costs.

*Delay occurs whenever aircraft must wait beyond the time theyare scheduled to use an airport or a sector of airspace. In practicalterms, delay is usually defined as occurring whenever some per-centage of aircraft must wait longer than a specified period of time,e.g., 80 percent of the aircraft must wait 4 minutes or longer. Con-gestion occurs as demand (the desired number of operational ap-proaches capacity. An increasing number of aircraft seeking to usean airport or an airspace sector at the same time causes queues tobuild up among aircraft awaiting clearance to proceed.

GA users of major hubs also feeldelay in the form of restrictionsbusy airports imposed during peakwith congestion.

the effects ofon access tohours to deal

Expanding airport capacity, either throughconstruction of new airports or enlargement ofexisting ones, is an obvious but far from easy so-lution. The availability of land for airport ex-pansion is severely limited in major metropoli-tan areas, and the cost of available land is oftenprohibitive. There is also rising communityresistance to airport expansion and constructionon the grounds of noise, surface congestion, andthe diversion of land from other desired pur-poses. Even where these obstacles could be over-come, increasing capacity by building a new air-port is at best a long-range solution—the lead-time from conception to beneficial use of a newairport is often a decade or more.

To deal with the problem of congestion in thenear term, and in a less capital-intensive way,two management approaches may be used. Oneis to shift some of the demand for use of the air-port from peak to off peak hours by administra-tively imposing quotas or by applying differen-

Photo credit: Neal Callahan

Congestion and delay


tial pricing for airport access according to thetime of day. This solution tends to work to theadvantage of major air carriers and against thecommuter and air taxi operators, and even moreheavily against GA users, who complain thatquotas or peak-hour pricing might effectivelypreclude them from using major airports at all.An alternative strategy is to divert some trafficto another airport—for example, from a largemetropolitan hub to GA reliever airports in thevicinity. In several cities the problem is not ageneral shortage of capacity but a dispropor-tionate demand at one airport, while excesscapacity exists at nearby airports that couldserve as satellites or relievers. The difficultyarises in determining who is to be diverted, sincefew potential users of reliever airports would

willingly accept diversion, especially if it im-poses inconvenience or extra cost. One way tomake diversion more attractive would be to im-prove the ground transportation links betweenhubs and reliever airports.

The intractability of the congestion problemand the difficulties of increasing airport capacityor making more efficient use of capacity throughmanagerial techniques have prompted somepeople to look to the ATC system for an alter-nate solution. Through procedural changes ortechnological improvements, the ATC systemmight be able to make more efficient use of theairspace in crowded terminal areas, thereby ex-pediting the flow of traffic to and from runways.

THE ATC PROBLEM

The task of controlling air traffic in congestedterminal areas is greatly complicated when traf-fic consists of a mixture of large and small,piston and jet aircraft. Arriving and departingtraffic, which is descending and climbing alongvarious paths and at different speeds to andfrom en route altitudes, may consist of a com-bination of IFR and VFR traffic. * This trafficmixture is inherently difficult to manage. Effi-ciency dictates that aircraft be moved to andfrom - the runway as expeditiously as possibleand that gaps in traffic be kept to a minimum.Safety, on the other hand, requires a regulartraffic pattern to prevent conflicts, and aminimum safe separation distance to preventfast aircraft from overtaking slower ones. Airturbulence in the form of wake vortices,**which are more severe behind heavier aircraft,requires even greater separation between aircraftthan would be needed if all were a uniform size.The overall result is that ATC procedures neces-sary to assure safety and to manage the work-load also contribute to delays in terminal areas.

—.—“Aircraft operating under Instrument Flight Rules (IFR) and Vis-

ual Flight Rules (VFR).**Eddies and turbulence, generated in the flow of air over wings

and fuselage, can upset the stability of following aircraft. Wakevortices, which are invisible, cannot now be accurately detected,and their movement and duration cannot be reliably predicted.

Technological improvements to the ATC sys-tem could help make fuller use of the physicalcapacity of the airport and reduce controllerworkload. Among these improvements are newsurveillance, communication, navigation, anddata processing equipment that could enhancethe controllers’ ability to separate and directtraffic. The Discrete Address Beacon System(previously know as DABS and now designatedas Mode S) is a new generation of radar equip-ment that permits aircraft to be interrogated in-dividually for information about identity, posi-tion, and altitude. Mode S also provides a two-way data link that could reduce dependence onthe present voice radio channels and provide amuch more rapid and extensive exchange of in-formation between air and ground. Variousforms of proposed airborne systems to detectand avoid potential collisions would provide asupplement to present separation assurancetechniques and reduce some of the controller’sburden in handling a high volume of traffic. Itmay also be possible to provide computer analy-sis of flight plans in advance that would helpresolve conflicts in terminal areas, expedite traf-fic flow, and permit more direct and fuel-savingrouting from origin to destination. Another pro-posed improvement is the addition of specialcockpit displays that would provide a picture of

Ch. 2—Introduction and Overview • 13

traffic in terminal areas and thereby permitpilots to cooperate more effectively with thecontroller or to assume some of the controller’spresent responsibility for separation assuranceand determining flight path in terminal areas.Finally, the Microwave Landing System (MLS)would not only improve the ability to land inconditions of severely reduced visibility, butalso permit multiple or curving approach pathsto the runway instead of the single-file, straight-en approach required with the present Instru-ment Landing System (ILS). In the longer term,proposed new ATC technology might replacethe present system of ground-based radar andradio navigation and surveillance capabilities.

These proposed improvements, if adopted,would require very large investments over thenext two decades. These investments would be

made by the Federal Government, but some ofthe funds could be provided by taxes on airspaceusers, who might also have to purchase newavionics equipment to supplement or replacewhat they already have. Managing the transi-tion to a new generation of ATC would also re-quire careful attention, both to assure continuityof service and to avoid the penalties of excessivecost or unexpected delay. It therefore seemsespecially important to select an evolutionarypath that does not foreclose options prematurelyand does allow flexibility in the choice betweencompeting technologies.

These prospective ATC improvements raiseimportant issues for airspace users. If the re-quired new avionics systems become mandatoryfor access to terminal areas or for general use ofcontrolled airspace, some GA, small commuter,

Photo credit: Federal Aviation Administration

Air controller and screen

14 • Airport and Air Traffic Control System

and air taxi operators may find the cost pro- of the present system as possible. Some possiblehibitive. New civil aviation requirements may improvements might ultimately have to be re-not be entirely compatible with the missions or jected, despite of their potential for increasingcapabilities of military aircraft that share the capacity or enhancing safety, because of the costairspace. There will probably be pressure to pro- to users or infringement of the right of access tolong the transition period and to retain as much the airspace.

THE COMMITTEE REQUEST

Concerns about these problems and about Specifically, the Committee on Appropria-te feasibility and cost of proposed solutions tions requested that OTA make an independentprompted the House Committee on Appropria- assessment in four major areas:tions, - Subcommittee on Transportation, to re- ●

quest that OTA undertake an assessment of air-port and terminal area capacity and related ATC ●

issues. Subsequently, the Senate Committee onCommerce, Science, and Transportation also ex- ●

pressed interest in these issues and endorsed therequest of the House Committee on Appropria-tions. ●

scenarios of future growth in air transporta-tion;alternative ways to increase airport and ter-minal area capacity;technological and economic alternatives tothe ATC system modifications proposed byFAA; andalternatives to the present ATC process.

OTA’s APPROACH

This assessment considers the growth of airtransportation over the remainder of this cen-tury. Particular attention is given to large hubairports, where most of the congestion and delayis expected to occur. For the ATC system, the as-sessment focuses on improvements that wouldaffect the safety and capacity of terminal air-space, but developments in other parts of theATC system (en route and flight informationservices) are also considered, Effects of thesechanges on airspace users (commercial opera-tors, passengers, general aviation, and the mili-tary services) are also examined. Policy optionsand alternative development plans are identifiedand analyzed.

The results of this assessment are presented inthe following five chapters:

Chapter 3. Description of the functions, or-ganization, and operation of NAS with em-phasis on ATC.

Chapter 4. Analysis of possible long-rangetrends in air activity and the effect theymight have on technical, investment, andmanagement decisions.

Chapter 5. Examination of prospective newtechnologies and organizational alterna-tives for the ATC system.

Chapter 6. Analysis of various ways to in-crease airport capacity and their advantagesand disadvantages.

Chapter 7. Discussion of the policy implica-tions that arise from alternative approachesto increasing airport capacity and improv-ing the ATC system.

ISSUES

Expanding, improving, and maintaining the of the Federal Government from the earliestnational system of airways, airports, and air days of aviation. There have been undeniabletraffic control has been an important objective benefits to airspace users and the general public

Ch. 2—Introducflon and Overview • 15——.———

from the greater speed and regularity of airtransportation and from the remarkable recordof safety that has been achieved over the years.The rationale for Federal involvement in the de-velopment and operation of NAS has tradition-ally rested on two grounds: 1) promotion andregulation of interstate and foreign commerce;and 2) enhancement of the capability for na-tional defense. It has been argued on bothgrounds that the Federal Government must takean active role to coordinate the developmentand to manage the operation of the system. Thesystem that has evolved under Federal sponsor-ship and direction is not without its flaws,however, and some observers believe that futuredevelopment should be directed along linesother than those of the past. Many of their con-cerns are embodied in the summary of majorissues which follows; these issues will be treatedin greater detail in subsequent chapters of thereport.

Growth

There is basic agreement among aviation ex-perts that civil aviation in the United States willcontinue to grow, thereby increasing the overalldemand for airport use and ATC services. Thereis considerably less agreement about the rate ofgrowth, the distribution among airspace users,the demands on various types of facilities andthe kinds of services that will be required. As aresult, there are sharp disputes about how to ac-commodate this growth or to influence the formand direction it may take.

FAA’s projections have led it to conclude thatsevere capacity restrictions will manifest them-selves in terminal areas and some parts of the enroute system and that perhaps as many as 20 air-ports may be saturated by 2000. To accommo-date this expected growth, the FAA proposes theaddition of new airport capacity and ATC facil-ities designed to handle higher traffic volumes.However, past FAA forecasts have consistentlyprojected higher rates of growth than have ac-tually materialized, casting doubt on the currentFAA forecasts and the expected demand forATC services through the remainder of this cen-tury. Some observers see trends already devel-oping in a different way. They argue that recent

changes such as airline deregulation, the growthof commuter service, sharp rises in fuel cost, andslower economic growth will either dampengrowth or cause it to develop in a patternsignificantly different from that of the past. Forexample, one suggestion is that in an unregu-lated environment, market forces will cause aredistribution of traffic as users find that delaycosts outweigh the benefits of operating at con-gested hub airports.

GA is the sector of aviation where growth hasbeen the most rapid and where there is most seri-ous concern about accommodating future de-mand. Twenty years ago, GA accounted foronly a small fraction of instrument operations;today it represents slightly over half of all instru-ment operations at FAA facilities, and mostforecast; show GA demand for ATC services in-creasing at rates far higher than those of com-mercial air carriers. Measures to restrict GAactivity at major hubs or to divert it to relieverairports or offpeak hours are certain to be con-troversial. GA users feel that reservations, quo-tas, or differential pricing schemes, would un-fairly deny them access to and use of the air-space system. On the other hand, some believethat GA flights into congested terminal areasshould be limited because they typically carryvery few passengers and so provide less publicbenefit than commercial aviation per operationor per unit of airspace use.

At a more general level, the prospects of traf-fic growth and capacity limitations raise theissue of strategic response to accommodatingfuture demand. In the past, the approach hasbeen essentially to accommodate demand wher-ever and whenever it occurred, i.e., the aim hasbeen to foster growth in civil aviation. Somequestion whether this approach is still desirable,arguing that demand and the growth of air activ-ity should be managed and directed in ways tomake the most productive use of airspace andthe most efficient use of existing facilities.

Basically, there are three forms of action thatcan be taken to influence growth: regulatory,

economic, and technological. Regulatory ac-tions include measures imposed by the Govern-ment that would control the use of the airspaceor the availability of ATC services according to


user class or types of activity. Economic meas-ures are those that would affect the cost or priceof using the airspace or that would allow marketcompetition to determine access to facilities andservices that are in high demand. Technologicalresponses include not only improved forms ofground-based and avionic equipment to increasethe efficiency of airspace use, but also increasesin airport capacity through construction of newor improved landing facilities. All three ap-proaches are likely to be used, and the issue isnot which to adopt but what combination andwith what relative emphasis. Ultimately, thechoice of measures will reflect a more fundamen-tal strategic decision about how to meet increas-ing demand. Chapter 4 presents a further discus-sion of future growth, and chapters 5 and 6 ex-amine the various responses to growth.

Technological Improvements

The many technological improvements of theATC system being contemplated by FAA fallinto four classes:

● navigation and guidance systems;● surveillance;● communication; and● process improvements.

These potential improvements have three majorcharacteristics: 1) most are technologicallysophisticated and require further developmentand testing before they can be operationallydeployed; 2) they will entail very large expendi-tures by the Federal Government to put them inplace and— in most cases—additional costs toairspace users who will have to equip their air-craft with special avionics; and 3) many yearswill be required for full deployment.

There are several controversial aspects ofthese technologies. First, there are purelytechnical and engineering questions that need tobe answered: will these new systems work as in-tended, what are their advantages and disadvan-tages compared to existing technology, and howcan their development be managed so that op-tions are not foreclosed prematurely? As deci-sions are made and implementation proceeds, itwill be necessary to coordinate the programcarefully in order to provide an orderly transi-

tion and to avoid the costs that could result fromdelay or unexpected technical setbacks.

Beyond these technical and managerial mat-ters, there are more fundamental questionsabout the role of FAA in planning and carryingout technological programs of this nature. Con-gress, for example, has questioned FAA’s pro-posed handling of the program for moderniza-tion of its en route computer system, as haveother members of the aviation community. Theyare concerned that FAA is not consulting ade-quately with specific user groups and not takingadvantage of relevant expertise available outsidethe aviation community. Some of them foresee atime when air traffic may have to be curtailedsimply because the technology to handle in-creased traffic with an acceptable level of safetyhas not been properly planned, developed, anddeployed.

On the other side, there are those who defendFAA’s general strategy for ATC modernizationand approve the way in which particular techno-logical programs are being handled. They arguethat deployment must proceed at a cautious paceboth because of the enormous uncertainties thatmust be overcome and because there must becontinuity of operations throughout the transi-tion. In their view, the potential consequences ofabrupt changes or premature decisions are moreserious and, in the long run, more harmful toaviation than temporary curtailments that mayhave to be imposed while technological dif-ficulties are being resolved.

Chapters examines some of the technologicalissues surrounding proposed system improve-ments, and chapter 7 addresses strategy andpolicy options for managing the transition.

Control Philosophy

Perhaps the most fundamental issue underly-ing the proposed improvements in the ATC sys-tem is that of control philosophy—the principlesthat should govern the future operation of thesystem. The philosophy of the present systemfor controlling IFR traffic is embodied in threeoperational characteristics: the system is primar-ily ground-based, highly centralized, and placesgreat emphasis on standardized (i.e., predict-

Ch. 2—lntroduction and Overview • 17

able) behavior by airspace users. In contrast,VFR traffic has little contact with the ATC sys-tem, except with flight service stations and con-trol towers at airports, and operates much as itdid in the early days of aviation, even though itshares airspace with IFR traffic in some in-stances.

As ATC technology evolved the locus of deci-sionmaking under IFR began to shift from thecockpit to the ground. Routes were determinedby the placement of ground-based navigationaids; surveillance was accomplished by reportsto ground centers and later by search radar; andobservers in airport towers began to direct air-craft in landing and takeoff patterns. As the den-sity of air traffic increased, ground-based ATCpersonnel began to take more and more controlover the altitude, route, and speed to be flown.To some extent this transfer of responsibilitywas the inevitable consequence of the technol-ogy employed, but organizational reasons alsodictated ground-based control. Decisions con-cerning not the movement of individual aircraftbut the pattern of traffic as a whole can best bemade by a single person who is in a position toobserve all flights operating throughout avolume of airspace over a span of time. Coor-dination and direction of several aircraft re-quired that a single individual have authorityover others—a role that the pilot of a single air-craft could not be expected to assume or thatother pilots would accept.

Ground basing implies concentration of con-trol at relatively few locations, and the trend hasbeen for centralization to increase over time.Again, the reasons are both technological andorganizational: centralization is organizationallyadvantageous because it consolidates functional-ly similar activities and allows technical speciali-zation, both of which lead to greater efficiencyand reliability of operation. For example, enroute traffic in continental U.S. airspace is nowcontrolled from 20 regional centers (ARTCCs,and proposed ATC system improvements wouldlead to even further consolidation, with en routeand terminal control eventually merging into asingle type of facility. A similar trend towardcentralization can be observed in FAA’s plans toconsolidate flight service station activities at

about 60 sites, compared to the present disper-sion at over 300 locations.

Perhaps the best example of the trend towardcentralization is the growing importance of theCentral Flow Control (CFC) facility at FAAheadquarters in Washington, D. C., which actsas a nerve center for the entire airspace system.With the aid of computers, CFC reviews the na-tional weather picture and anticipated aircraftoperations for the coming day and determinesthe incidence and cost (extra fuel consumed) ofdelays that could occur because of weather andair traffic demand. This results in a daily opera-tional master plan that smooths demand amongairports and allows delays to be taken on theground at the point of departure rather than inholding patterns at the destination. The value ofthis capability was demonstrated when capacityquotas were imposed as a consequence of theAugust 1981 air traffic controllers’ strike. CFCallowed a national airspace utilization plan to bedeveloped, with detailed instructions to airportsand en route centers on how to manage trafficand minimize the adverse effects of the capacityrestrictions,

A system characteristic that accompaniesground-based centralization of control authorityis standardization of performance. FAA operat-ing procedures specify the behavior of pilots andcontrollers in every circumstance, which in-creases the reliability of system operation byreducing uncertainty and by routinizing nearlyevery form of air-ground transaction. Safety isthe prime motivating factor, but capacity and ef-ficiency are also highly important considera-tions. Controller workload is reduced when therange of possibilities they have to deal with islimited, and this in turn permits a given volumeof traffic to be handled with less stress or, alter-nately, an increase in the number of aircraft eachcontroller can safely handle. Either way, the effi-ciency of the ATC system (measured in terms ofhourly throughput or controller productivity) isincreased, with a corresponding reduction insystem operating cost.

Despite the advantages of ground-basing, cen-tralization, and standardization, there are com-plaints about the control philosophy of the pre-


sent system. Pilots complain that a ground-based system detracts from their control overthe conduct of the flight. Centralization mayalso be a problem if, by concentrating controlfacilities or flight services, the personnel on theground are less able to provide particularized in-structions or to take action based on localizedknowledge of flight conditions. Standardization,by definition, limits the flexibility of responseand the freedom to pursue individual or specialcourses of action.

The prospective changes in ATC technologyare viewed with mixed feelings by airspace usersand air traffic controllers. Technology thatwould increase the level of automation could, onone hand, promote greater centralization andstandardization of control functions and couldlead to increases in safety, capacity, or efficien-cy, On the other, automation could serve to in-crease ground authority still further and toreduce the flexibility of the system in dealingwith nonroutine events. Technology like colli-sion avoidance systems or cockpit displays oftraffic information could give back to the pilotcritical information (and hence control respon-sibility) and might enhance the pilot’s ability tocooperate more effectively with the ground-based controller. At the moment, these devicesare thought of as backups in the event of con-troller or system error, but their prospective usealso raises the possibility of independent pilotactions that might contravene controller instruc-tions or disrupt the overall pattern of traffic.

Chapter S, which deals with these and otherforms of advanced aviation technology forground-based and airborne application, treatsthe issues that arise from prospective changes indistribution of control between the air and theground or from further centralization of ATCfunctions and services.

Freedom of Airspace Use

The rising demand for ATC services and theprospect of congestion at more and more majorairports are the basic stimuli for many of thetechnological improvements and proceduralchanges now being sought by the FAA. How-ever, the very measures that might ease capacity

problems or assure the safety of high-densityairspace are often controversial with some cate-gories of users because they are perceived as in-fringements on their freedom to use NAS. GAusers feel particularly threatened, but air carriersand commuter airline operators have also voicedconcern. The military services as well are waryof some new forms of ATC technology and theprocedures that may accompany their use be-cause they may interfere with military missionsor be incompatible with performance re-quirements for combat aircraft.

As the complexity of ATC technology has in-creased, so has the amount of equipment thatmust be carried on the aircraft and the amountof controlled airspace from which VFR flight isexcluded unless the aircraft is equipped with atransponder to allow identification and trackingby the ATC system. Restrictions on airport use,especially at large and medium hubs, have alsogrown more confining for VFR flights, and theairspace around many of the busiest airports isnow designated as a “terminal control area” inwhich all aircraft are subject to air traffic controland may operate only under rules and equip-ment requirements specified by FAA. GA, theprincipal user of the VFR system, finds itselfpressured in several ways. Uncontrolled airspaceis shrinking and may disappear altogether; it isbecoming increasingly difficult to use metropoli-tan airports because of equipment requirements;and the cost of equipping the aircraft with IFRavionics and acquiring an instrument rating areoften out of economic reach for the personal GApilot. Prospective technological improve-ments—such as the Traffic Alert and CollisionAvoidance System (TCAS), data link, or MLS—are viewed by many GA users as further restric-tions on their access to airports and airspace.Many of them feel that, while this new technol-ogy may be desirable or even necessary for aircarriers and larger business aircraft, it shouldnot be required of all GA users or made a pre-requisite for IFR services or access to commercialairports.

Commuter airline operators share some ofthese GA concerns. Virtually all commuter andair taxi operators are equipped for IFR operationand find their needs well served by the present

Ch. 2—Introduction and Overview ● 19—

ATC technology. They see little further advan-tage in new technology and are concerned aboutthe expense of having two sets of equipmentserving the same purpose—advanced avionicsneeded for a high-density terminal at one end ofthe flight and present-day equipment that maybe useful for many years to come at small com-munity airports. They are also concerned thatthe more advanced avionics might eventuallylead to more restrictive rules of operation or ac-cess to terminal areas. Thus, many commuterand air taxi operators would favor a dual-modesystem that allowed them to retain their presentIFR avionics even though more advanced formswere in use by other types of aircraft operators.

Military aviation operates under the civilATC system in all shared airspace and undermilitary control in areas restricted to militaryuse. In flying through civil airspace to and fromtraining areas, military aircraft must often fol-low circuitous routes or observe altitude andspeed restrictions that lengthen transit time. Themilitary services would prefer an arrangementthat allows more direct access to training areasand avoids operation in mixed airspace. Air car-riers have a different view: the most directroutes for trunk airlines are often blocked byrestricted military areas, and the air carriersargue for procedures that would allow them totraverse these areas in the interest of shorteningflight time and saving fuel.

Another issue has to do with new technologythat might be adopted for civil aviation, whichin most cases would be extra equipment for mili-tary aircraft. For combat aircraft, particularlyfighters, the space for avionics and antennas isoften at a premium. While careful coordinationof military and civil requirements can eliminatesome of these problems, certain basic incompati-bilities are likely to remain and to produce con-tinuing controversy.

The issues of freedom of airspace access anduse are discussed further in chapters in connec-tion with specific forms of new aviation technol-ogy.

Automation and Controller Functions

Despite the vast complex of ground-basedequipment and facilities for surveillance, com-munication, and data processing, ATC remainsa highly labor-intensive activity. FAA is keenlyaware of this and has sought for some time tofind ways to automate selected ATC functions.However, most of the automation that has beeninstituted so far has been to assist air traffic con-trollers rather than replace them. Decisionmak-ing and communication—two major elements ofcontroller workload—have not been automatedto any appreciable degree, and the ratio of con-troller work force to aircraft handled has re-mained relatively constant. In addition, thepresent method of backup to automated controlfunctions involves reversion to manual proce-dures used in the previous generation of ATCequipment; this method of assuring service inthe event of outages has tended to perpetuate theteam size and staffing patterns of the previousgeneration.

Plans for an advanced generation of ATC callfor automation of several manual controllerfunctions: conflict prediction and resolution,terminal area metering and spacing, flight planapproval and issue of clearances, and communi-cating routine control instructions to individualaircraft. Such forms of automation could lead tosubstantial increases in controller productivityand might eventually provide the basis for amore extensively automated system in whichmost routine control functions are carried out bycomputers, with the human controller acting inthe role of manager and overseer of machineoperation.

This path of evolution raises three importantgroups of issues. First, there are questions aboutthe feasibility and advisability of replacing thehuman controller to such an extent. ATC nowrelies heavily on judgment and awareness of thedynamics and subtleties of the air traffic situa-tion. Some observers doubt that all of thesecharacteristics could be dependably incorpo-rated into computer software in the foreseeable

20 . Airport and Air Traffic Control System

future. The proponents of automation arguethat much of the routine, repetitive, or predic-tive work of ATC is ideally suited to computers,and that an incremental approach to automationwill help solve many of the problems since eachnew step can build on successful previous ad-vances.

A second major set of issues is the reliabilityof automated systems and the backup methodsto be used when the inevitable equipmentfailures occur. Experience with the presentautomated ATC equipment indicates that com-puter failure rates are a cause for concern, andthe loss of computer-supplied data may meanthat ground personnel lose effective control oftraffic until manual backup procedures are in-stituted—a process that may take several min-utes to complete. Computer experts maintainthat equipment and software reliability can begreatly improved and that automated systemscan be designed to be more failure tolerant.These experts also contend that present ex-perience with manual procedures as backups tooutages of automated equipment indicates a fun-damental flaw in design philosophy because theproper backup to an automated system is notmanual operation, but another automated sys-tem. Critics of automation question the accept-ability of a system in which the human con-troller has no effective means of intervening indegraded states of operation.

A third issue is whether some of the respon-sibility that now resides with the ground-basedsystem ought not to be transferred to, or at leastshared with, the cockpit. A pilot in an aircraftequipped with an airborne collision avoidancesystem and a display of the immediately sur-rounding air traffic might be in a superior posi-tion to select the appropriate maneuver in caseof conflict; in effect, such an airborne systemwould create a mode of IFR operation similar tothe present VFR system. The chief disadvantageof this concept is that it could lead pilots to makea series of short-term tactical responses thatmight not be consistent with the overall schemeof managing traffic in congested airspace. In thiscase, the ground system would still have to actin the capacity of referee, and some contend that

it would be better to keep all control of individ-ual flight paths under one authority.

Chapter 5 contains a further examination ofthe issue of automation in connection with thediscussion of the proposed en route computer re-placement program and the mechanization ofthe Mode S data link and TCAS systems.

Funding and Cost Allocation

The expenditures that are likely to be requiredfor ATC system improvements over the comingyears could be considerably higher than those ofpast years. For the period 1971 to 1980, theamounts budgeted for facilities and equipment(F&E) and associated research, engineering, anddevelopment (RE&D) have averaged $397 mil-lion and $106 million respectively (in constant1980 dollars).3 Future improvements of the enroute and terminal area ATC system and relatedprograms for flight service station, navigation,and communication facility modernization maycall for spending at twice this annual level ormore. At the same time, operating and mainte-nance (O&M) costs are expected to rise, at leastuntil modern labor-saving equipment is installedand productivity gains begin to be realized.

Since creation of the Airport and AirwaysTrust Fund in 1970, FAA has had two sources offunding. F&E, RE&D, and airport grants-in-aidhave been covered wholly by appropriationsfrom the trust fund. In addition, the trust fundhas covered about 15 percent of O&M expenses,although this proportion has varied consider-ably from year to year. The balance of O&Mcosts, about $1.9 billion per year (1980 dollars),and all other FAA budget items have been fromgeneral fund appropriations. Overall, trust fundoutlays have met about 40 percent of annualFAA expenses. The major source of revenue forthe trust fund has been a tax levied on domesticand international airline passengers (see fig. 2).

In October 1980, the Airport and Airways De-velopment Act expired, and Congress declinedto pass reauthorizing legislation. At that timethe trust fund had an uncommitted balance of

30TA calculations based on FAA budget data, 1971-80.

Ch. 2—Introduction and Overview • 21—————————— — .

Figure 2.— FAA Budget and Funding Sources, 1971-80

Generalfund

SOURCE: Off Ice of Technology Assessment, based on FAA budget data, 1971-80.

$2.9 billion, the equivalent of about 2 years’ ex-penditure at the then prevailing rate. Since thattime some of the user taxes contributing to thetrust fund have still been collected (but at re-duced rates of taxation), and these revenueshave been deposited partly in the General Fund

and partly in the Highway Trust Fund. If theserevenues are included and if authorizations fromthe trust fund during fiscal year 1981 are de-ducted, the uncommitted trust fund balancestood at roughly $3 billion at the beginning offiscal year 1982.


In considering sources of funding for futureairport and ATC system improvements, Con-gress will encounter three broad and long-stand-ing areas of controversy. In the absence of atrust fund or some other form of user charges tosupport capital improvement programs, theseparts of the FAA budget would have to befunded from general revenues, which is certainto raise the issue of whether civil aviation andthe airport and ATC system should be subsi-dized by the general public. The argument thatthe recipients of a service should pay the costsfor the Federal Government to provide that serv-ice (a position strongly supported by the presentadministration), holds that capital improve-ments of facilities and equipment and the O&Mcosts of running the airport and ATC systemshould be borne by airspace users through vari-ous specific taxes. On the other hand, it can beargued that civil aviation, like other modes oftransportation, provides a general benefit andtherefore deserves support with public moneys.Other modes of transportation receive subsidyfrom the Government, and some members of theaviation community contend that there is no jus-tification for singling out civil aviation for fullrecovery of capital and operating costs.

The resolution of this issue that has prevailedfor the past 10 years has been a combination ofspecial users taxes and General Fund financing,with the former going for capital expendituresand a small share of operating costs and the lat-ter for the balance of FAA costs. A perpetuationof this scheme, through reestablishment of theAirport and Airways Trust Fund, could embroilCongress in another issue—what is the “fair”amount to be paid by various user classes. Mostpeople concede that each user should pay rough-ly in proportion to the cost that they impose onthe system, but there is violent disagreementwithin the aviation community as to what thesecosts are and how they are to be reckoned. Costallocation studies conducted by the Departmentof Transportation and the FAA have generallyconcluded that, under the tax structure that ex-isted before October 1980, commercial aviation

paid nearly all (88 percent) of the cost of servicesprovided to them. On the other hand, generalaviation taxes returned at almost one quarter ofallocated costs.4 GA representatives have disa-greed strongly with these findings, arguing thatthere is a substantial public benefit of aviationthat has been undervalued in these cost alloca-tion studies and that GA is charged for facilitiesand services that are neither required nor usedby a major part of GA operators. Congress hasshown little inclination to alter the user chargestructure, and most of the proposed legislationto reestablish the trust fund would have little ef-fect on the distribution of user charges that ex-isted previously.

The third area of controversy concerns howthe collected levies should be applied to costs.By congressional action, the use of trust fundmoneys is restricted largely to capital expendi-tures and research and development activities,with some contribution toward operating ex-penditures. There are two major points at issue:1) how should expenditures for capital improve-ments be allocated between airports and ATCfacilities and equipment (and among airportsand ATC facilities used by various types of avia-tion); and 2) should the allocation be broadenedto cover a substantial part (or perhaps all) ofO&M costs.

Resolution of these issues will become espe-cially important when FAA presents its long-range plan for ATC system improvement. In-creased expenditures for facilities and equipmentand associated R&D will be called for, and oper-ating expenses will probably remain high. FAAwill be seeking a long-term commitment and anassured source of funding, but it will face strongopposition from segments of the aviation com-munity if paying for FAA’s programs and oper-ating costs entails an increase in user taxes or areallocation of the share to be borne by variousclasses of airspace users.

‘J. M. Rodgers, Financing the Airport gnd Air-way System; CostAllocation and Recovery, FAA-AVP-78-14 (Washington, D. C.:Federal Aviation Administration, November 1978).

Chapter 3

THE NATIONAL AIRSPACE SYSTEM

Photo credit U S Department of Transportafton

Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .International Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Domestic Air Carrier Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Commuter Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reliever Airports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .General Aviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Air Traffic Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Landing Aids... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Flight Planning and Advisory Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Air Traffic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System Organization and Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ATC Sectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ATC Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airspace Users. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

List of Tables

Tablel. Airports Included in National Airport System Plan, 1980 . . . . . . . . . . . . . . . . . . .2. U.S. Pilot Population, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3. Summary of Aviation Activity, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

List of Figures

Figure3.4.5.6.7.8.

Airspace Structure . . . . . . . . .Typical Flight Service Station

. . .Communicat ionLinks. . . . . . . . . . . . . . . . . . . . . . : . . .

Air Route Traffic Control Center Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Connections of aTypical ARTCC With Other Facilities. . . . . . . . . . . . . . . . . . . . .ATC Activities for a Typical IFR Flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ATC Facilities and Equipment at a Typical Large Airport . . . . . . . . . . . . . . . . . . .

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Chapter 3

THE NATIONAL AIRSPACE SYSTEM

The National Airspace System (NAS) is a see how the system operates and to identify fac-large and complex network of airports, airways, tors that may shape its future development. Forand air traffic control (ATC) facilities that exists explanatory purposes, it first considers the goalsto support the commercial, private, and military of the system and then describes the systemuse of aircraft in the United States. This chapter under three major headings: airports, air trafficexamines the major parts of the system, both to services, and airspace users.

GOALS

NAS is designed and operated to accomplishthree goals with respect to civil aviation:

1. safety of flight;2. expeditious movement of aircraft; and3. efficient operation.

These goals are related hierarchically, with safe-ty of flight the primary concern. The use of air-port facilities, the design and operation of theATC system, the flight rules and procedures em-ployed, and the conduct of operations are allguided by the principle that safety is the firstconsideration.

Without compromising safety, the secondgoal is to permit aircraft to move from origin todestination as promptly and with as little inter-ference as possible. This involves preventingconflicts between flights, avoiding delays at air-ports or en route, and eliminating inefficient orcircuitous flight paths. It also entails makingmaximum use of airport and airway capacity inorder to satisfy demand, so long as safety is notcompromised. If safety and capacity utilizationare in conflict, the Federal Aviation Adminstra-tion’s (FAA) operating rules require that the vol-ume of traffic using the system be reduced to alevel consistent with safety.

The third goal is to provide airport and ATCservices at low cost. This entails minimizing thecosts to users—not only monetary costs but alsothe penalties of delay, inconvenience, or unduerestriction. It also entails operating the system asefficiently as possible so as to reduce transactioncosts and to increase productivity, i.e., to han-

dle more aircraft or to provide better service tothose aircraft with a given combination of run-ways, controllers, and ATC facilities.

Whereas safety cannot be compromised in theinterest of cutting costs, capacity and cost maybe traded off for the sake of safety. The specialmeasures adopted to deal with disruption of thesystem as a result of the air traffic controllers’strike in August 1981 illustrate the hierarchal re-lationship of safety, capacity, and efficiency. Inorder to continue safe operation in the face ofwork force reductions, the number of aircraft al-lowed to use certain crowded airports and airways at peak demand hours was reduced to alevel that could be handled safely. These meas-ures reduced capacity (the number of aircraftthat the system could accommodate) and in-creased cost (delays, canceled flights, adherenceto quotas), but an effort was made to allow theremaining capacity to be used effectively andkeep costs within reasonable limits. For exam-ple, limits on the number of air carrier flightswere imposed only at the 22 busiest airports,and restrictions were later eased at those airportswhere more operations could be accommodated.Airlines were allowed to use larger aircraft so asto provide as much seat capacity as possible butwith fewer flights, and wherever possible flowcontrol procedures were employed to ensurethat aircraft were delayed on the ground ratherthan in flight, so as to minimize waste of fuel.Other restrictive measures were applied to cutback on general aviation (GA) flights. The mili-tary services voluntarily reduced flight oper-ations.

25

26 • Airport and Air Traffic Control System—.—

The anticipated growth of air traffic and the capacity. Before turning to examination of thesedemand for ATC services over the next two dec- problems, however, it is first necessary to lookades poses several problems, and the need to at the major parts of the NAS and to considermaintain a dynamic balance among system goals the factors that could shape their course of re-motivates the search for improved methods of velopment.ATC and better utilization of airway and airport

AIRPORTS

Airports are the first major part of NAS. Theyare any place designed, equipped, or commonlyused for the landing and takeoff of aircraft. Thisdefinition covers a broad variety of sites: manyof the sites designated as airports by the FAA aremerely dirt strips or seaplane moorings nearopen water; at the opposite end of the spectrumare complex air terminals serving major metro-politan areas, like the 5,000-acre JFK Interna-tional Airport in New York. About 60 percent ofthe 15,000 U.S. airports are private or militaryfields and not available for public use. Of theroughly 6,500 civil airports open to the public,almost 90 percent are used exclusively by smallGA aircraft. The remaining 780 airports (about 5percent of all U.S. airports) are served either byscheduled air carriers or by commuter and airtaxi operators (see table 1).

FAA, in compliance with the Airport and Air-way Development Act of 1970, maintains a mas-ter list of airport development needs for the nextdecade. This compilation, which is periodicallyrevised, is known as the National Airport Sys-

tem Plan (NASP). It identifies categories of air-ports that are of Federal interest and that areeligible for Federal funds under the Airport De-velopment Aid Program (ADAP), and the Plan-ning Grant Program administered by FAA.NASP categorizes public use airports accordingto the type of aviation activity they accommo-date: international, domestic air carrier, com-muter, reliever, and general aviation. This doesnot imply that GA aircraft use only GA airports;in fact, there are GA operations at all categoriesof airports. Rather, the GA classification de-notes that such airports serve only GA and notother types of users.

International Airports

An international airport regularly serves aircarrier flights operating between the UnitedStates and foreign countries. International air-ports tend to be among the best equipped air-ports in terms of runways, landing aids, andATC facilities. In 1980 there were 76 such air-ports.

Table 1 .–Airports Included in National Airport System Plan, 1980a

Type of service Conventional Heliport Seaplane Total

Air carrierb . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603 1 31 635Commuter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 — 6 145Reliever. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 — 155General aviation. . . . . . . . . . . . . . . . . . . . . . . . 2,198 4 22 2,224

Total NASP airports. . . . . . . . . . . . . . . . . . . 3,095 5 59 3,159Total public-use airports not in NASPc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3,360

Total. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.519alncludes airports in Hawaii and Alaska.blnclude5 76 airports designated as ports of entrY.cEntirely general aviation.

SOURCE: Federal Aviation Administration, National A/rPort Sysfern Plan, 1980-89, 1980,

Ch. 3—The National Airspace System . 27

Domestic Air Carrier Airports

In 1980, NASP included 603 airports servedby domestic air carriers, a figure that includes allof the international airports described above butexcludes 1 heliport and 31 seaplane facilitiesserved by scheduled air carriers. These airportsare classified by FAA according to the size of thetraffic hub they serve, where a hub is definedas a Standard Metropolitan Statistical Area(SMSA) requiring air service. The hub classifica-tions are:

Percentage of totalHub classification: airline passengers *

Large (L) 1.00 or moreMedium (M) . 0.25 to 0.99Small (S) . . . . . . . . . . . . . . . . 0.05 to 0.24Nonhub (N) . . . . . . less than 0.05

*Passengers eplaned by domestic and foreign carriers at U S airports

A hub may have more than one air carrier air-port, and the 25 SMSAs presently designated aslarge hubs are served by a total of 38 air carrierairports. The distribution of aviation activity atdomestic air carrier airports is highly skewed,with progressively greater percentages of flightsand passengers concentrated at fewer and fewerairports. In 1980, for example, the 486 nonhubshandled only 3 percent of all passenger enplane-ments; the 76 small hubs handled 8 percent; the41 medium hubs handled 18 percent; and the 25large hubs handled 70 percent. To carry thispoint one step further, the top five air carrier air-ports (Chicago, Atlanta, Los Angeles, Denver,


All filled up


Room to grow

and Dallas/Fort Worth) handled about one-quarter of all passenger enplanements and one-fifth of all airline departures. This means that airtraffic congestion tends to center at a very smallfraction of airports; but because of the volumeof traffic handled at these airports, it affects alarge percentage of all aircraft and passengers.

Commuter Airports

Until the Airline Deregulation Act of 1978,many commuter and air taxi airlines were notcertificated as scheduled air carriers by the CivilAeronautics Board (CAB), and NASP classifiedairports served exclusively by commuter and airtaxi in a separate category. Since airline deregu-lation, the number of airports in this categoryhas fluctuated widely, showing sharp increasesin 1979 and 1980 as commuter airlines sought toopen up new markets and an almost equallysharp drop in 1981 as these markets failed tomaterialize. Commuter airports, typically lo-cated in small communities, handle a very lowvolume of traffic, 2,500 to 5,000 passenger en-planements per year. The major concern aboutthis category is not capacity but keeping the air-port in operation so as to provide essential airservice for the small communities in which theyare located.

Reliever Airports

Reliever airports are a special category of GAairport whose primary purpose is to reduce con-gestion at air carrier airports in large and medi-


urn hubs by providing GA users with alternativeoperational facilities and aircraft services ofroughly similar quality to those available at hubairports. The criteria for classification as a re-liever airport in NASP are 25,000 itinerant oper-ations or 35,000 local operations annually,either at present or within the last 2 years. Thereliever airport must also be situated in a SMSAwith a population of at least 500,000 or wherepassenger enplanements by scheduled airlinesare at least 250,000 annually. There were 155airports designated as relievers in the 1980-89NASP.

General Aviation

GA airports are either private use or publicuse, but only the latter are eligible for Federal

development or improvement funds underNASP. There were approximately 2,200 GApublic-use airports in the 1980 NASP. Capacityis usually not a concern except at the largest GAairports, such as Long Beach, Van Nuys, Teter-boro, or Opa-Locka, which may require im-provements similar to those contemplated atmajor hub airports. For most GA airports thechief concern is upgrading and extending airportfacilities and ATC services so as to accommo-date larger and more sophisticated aircraft andto allow operation under adverse conditions.These improvements are being sought both tosupport the expected growth of GA and to pro-vide facilities comparable to air carrier airports,thereby permitting diversion of some GA opera-tions from congested hubs.

AIR TRAFFIC SERVICES

The ATC system— the second major part ofthe National Airspace System—offers threebasic forms of service: navigation aid (includinglanding), flight planning and in-flight advisoryinformation, and air traffic control.

Navigation

Aid to navigation was the first service pro-vided to civil aviation by the Federal Govern-ment. At the end of World War I, the PostOffice undertook to set up a system of beaconsalong the original airmail routes to guide avia-tors at night and in times of poor visibility. By1927, this airway extended from New ‘fork toSan Francisco, with branches to other majorcities.

In the 1930’s, ground beacons for visual guid-ance were replaced by two types of low-fre-quency radio navigation aids—nondirectionalbeacons and four-course radio range stations.The nondirectional beacon emitted a continuoussignal that allowed the pilot to navigate, in amanner analogous to using a light ground bea-con, by homing on the signal with an airbornedirection finder. The radio range station was afurther improvement in that it emitted a direc-

tional signal, forming four beacons alined withrespect to the compass, each defining a course.Pilots listened to a radio receiver and followedthese radio beams from station to station alongthe route. The four-course radio range systemwas phased out beginning in 1950, after reachinga maximum deployment of 378 stations. Low-frequency nondirectional radio beacons are stillin limited use in the United States and wide-spread use in other parts of the world. *

The technology that supplanted the low-fre-quency four-course range as the basic navigationsystem for civil aviation was very high fre-quency omnirange (VOR) transmitters, whichwere first put in service in 1950. This system hadseveral advantages over low-frequency radio.VOR is less subject to interference and aberra-tions due to weather; it is omnidirectional, per-mitting the pilot to fly on any chosen radialrather than only the four courses possible withthe radio range station; and the addition of acockpit display freed the pilot from the need tolisten to radio signals continuously. The majordisadvantage of VOR is that signals are blocked

● In 1981, there were 1,095, nondirectional radio beacons inservice in the United States, including 54 military and 734 non-Fed-eral installations.

Ch. 3—The National Airspace System . 29

at the horizon, and navigational signals from astation can be received over a much smaller areathan low-frequency radio. To provide the samegeographical coverage as the older low-fre-quency radio system, therefore, a great manymore VOR stations were required. At present,there are 1,039 VOR stations in operation (930FAA, 42 military, 67 non-Federal), providing ex-tensive but not complete coverage of the con-tiguous 48 States and Hawaii and limited cover-age of Alaska.

In the 1960’s, the basic VOR system was sup-plemented by distance measuring equipment(DME) that permitted measurement of range aswell as direction to a station. The DME used thedistance-measuring portion of a military Tac-tical Control and Navigation System (TACAN),colocated with a VOR station to create what iscalled a VORTAC. This is the standard airwaynavigation aid in use today, and at present allcommercial air carriers have VOR/DME equip-merit. ’ Over 80 percent of GA aircraft are alsoequipped with VOR receivers, and over one-third of these also have DME. In addition to theFederal investment in VORTAC facilities (on theorder of $250 million), there is a very large pri-vate investment (roughly $300 million) in air-borne navigation equipment to use the presentVORTAC technology. As a result, both the Fed-eral Government and the aviation communityhave a strong incentive to protect this invest-ment by prolonging the operational life of theirVORTAC equipment and the airway routestructure based on it.

Nevertheless, VOR—which relies on 30- or40-year-old technology-has some inherent dis-advantages. Because it is a ground-based sys-tem, it does not provide coverage of oceanicareas. Because it is a line-of-sight system, VOR isof limited usefulness at low altitudes or in moun-tainous areas. The VOR route structure concen-trates traffic along rather narrow channels andproduces a potential for conflict at intersectionswhere airways cross. Further, navigation fromone fix (intersection) to the next does not always

● Military aircraft are equipped with TACAN, VOR/DME, orboth.

produce the most direct routing from origin todestination.

Several alternative navigational systems (de-veloped principally for military aviation) areavailable, and some are already used in auxiliaryapplications by civil aviation. The Omega sys-tem, developed by the U.S. Navy, is a low-fre-quency radio system that provides global cover-age. It has been purchased by some airlines fortransoceanic flights. Loran-C (also low-freq-uency radio), operated by the Coast Guard, is amaritime navigation system that also coversmost of the continental United States; it affordsvery good accuracy and low-altitude coverage,even in mountainous areas. Some airline andcorporate jet aircraft have self-contained air-borne navigation systems such as Doppler radaror Inertial Navigation System (INS), which areaccurate and are usable worldwide. All of thesenew systems permit “area navigation” (RNA V),whereby the pilot can fly directly between anytwo points without restriction to a VOR airway.There are also available RNAV systems that per-mit the aircraft to follow direct routings usingVOR as a reference.

Many commercial air carriers and more than 7percent of GA aircraft (largely business and cor-porate aircraft) have RNAV capability. Since1973, FAA has been gradually implementingRNAV routes in the upper airspace and insti-tuting approach procedures at selected airportsto accommodate aircraft equipped with suchsystems. Phasing out the current airways struc-ture and converting to a more flexible system ofarea navigation is a process that will requiremany years to complete. At present, FAA iscommitted to upgrading VORTAC stations tosolid-state equipment at a cost of roughly $210million (fiscal year 1980 dollars) over the next 10years. At the same time, FAA must face thequestion of adopting new navigation technology

to conform to new international standardsscheduled for consideration by the InternationalCivil Aviation Organization in 1984. The issue isnot so much selection of a single new navigationsystem to replace VORTAC as it is a question ofadopting procedures for worldwide navigation


(especially RNAV) that will be compatible withseveral possible technologies.

Landing Aids

A guidance system for approach and landingis simply a precise, low-altitude form of naviga-tion aid with the additional accuracy and relia-bility needed for landing aircraft in conditions ofreduced visibility. The standard system now inuse, the Instrument Landing System (ILS), wasfirst deployed in the early 1940’s although a pro-totype system was first demonstrated by JamesDoolittle in 1929.

ILS provides guidance for approach and land-ing by two radio beams transmitted from equip-ment located near the runway. One transmitter,known as the localizer, emits a narrow beamalined with the runway centerline. The othertransmitter, the glide slope, provides verticalguidance along a fixed approach angle of about3°. These two beams define a sloping approachpath with which the pilot alines the aircraft,starting at a point 4 to 7 miles from the runway.Because the ILS is generally not accurate or relia-ble enough to bring the aircraft all the way ontothe runway surface by instrument referencealone, the pilot makes a transition to externalvisual reference before reaching a prescribedminimum altitude on the glide slope (the deci-sion height). The decision height varies accord-ing to the airport and the type of ILS installa-tion: 200 feet for most airports (category I), but100 feet on certain runways at some airports(category II). At present there are 708 category Iand 44 category II ILS installations in commis-sion in the United States. * FAA plans call for in-stallation of ILS at additional sites, primarilycommuter airports, and for modernization ofsome 250 existing sites by converting to solid-state equipment and, in the process, upgrading69 of them to category II capability.

ILS has two major limitations, both of whichaffect airport capacity. First, since the ILS doesnot provide reliable guidance all the way totouchdown, there are times and conditions when

the airport must be closed. Such severely re-duced visibility occurs less than 1 percent of thetime for U.S. airports as a whole, but when thishappens at a busy airport, traffic can be backedup not only at the affected airport but also atalternate landing sites and at airports where traf-fic originates. The other limitation is that it pro-vides only a single fixed path to the runway—ineffect, a conduit extending 4 to 7 miles from therunway threshold through which all traffic mustflow. This has an even greater affect on capac-ity. When visibility is such that the ILS approachmust be used, traffic must be strung out along asingle path and the rate at which landings can beeffected is constrained by the speed and spacingof aircraft in single file.

The Microwave Landing System (MLS),which has been under development by FAA forseveral years and is now ready for initial de-ployment, could overcome these limitations ofILS, which in turn could help improve the flowof traffic in terminal areas by allowing moreflexibility in segregating and sequencing the ar-rival of aircraft on the runway. The magnitudeof the resulting capacity gains is subject to somedispute, however, and not all agree that MLSwould play a major part in reducing terminalairspace congestion. The MLS is discussed fur-ther in chapter 5.’

Flight Planning andAdvisory Information

Timely and accurate information aboutweather and flight conditions is vital to airmen,and FAA perceives this aspect of system opera-tion to be a prime benefit, particularly to the GAcommunity. Flight planning and informationservices take several forms and are providedpartly by FAA and partly by the National Oce-anic and Atmospheric Administration (NOAA)of the Department of Commerce. NOAA pub-lishes maps, aeronautical charts, and relateddocuments from information furnished by theFAA. The National Weather Service of NOAAprovides weather maps and reports. FAA pub-

● In addition, there are 48 non-FAA facilities that have category IILS installations.

‘Microwave Landing Transition Plan, APO-81-1 (Washington,D. C.: Federal Aviation Administration, 1981).

Ch. 3—The National Airspace System • 31

lishes manuals, instructions, and notices to air-men (NOTAMs) to help pilots in planning andexecuting flights. FAA operates a nationalweather teletype network, disseminates weatherinformation by radio broadcast and recordedtelephone messages, and provides weather brief-ings. FAA also disseminates to airmen, both pre-flight and in flight, information concerning thestatus of navigation aids, airport conditions,hazards to flight, and air traffic conditions. FAApersonnel are also available to help pilots in pre-paring and filing flight plans and to disseminatethese flight plans to other ATC facilities alongthe intended route and at the destination.

All of these planning and advisory services areintended to guide the airman in making use ofthe airspace under either of two basic sets ofrules—Visual Flight Rules (VFR) and InstrumentFlight Rules (IFR)—which govern the movementof all aircraft in the United States. * In general, apilot choosing to fly VFR may navigate by anymeans available to him: visible landmarks, deadreckoning, electronic aids (such as VORTAC),or self-contained systems on board the aircraft.If he intends to fly at altitudes below 18,000 ft,he need not file a flight plan or follow prescribedVOR airways, although many pilots do both forreasons of convenience. The basic responsibilityfor avoiding other aircraft rests with the pilot,who must rely on visual observation and alert-ness (the “see and avoid” concept).

In conditions of poor visibility or at altitudesabove 18,000 ft, pilots must fly under IFR. Manyalso choose to fly IFR in good visibility becausethey feel it affords a higher level of safety andaccess to a wider range of ATC services. UnderIFR, the pilot navigates the aircraft by referringto cockpit instruments and by following instruc-tions from air traffic controllers on the ground.The pilot is still responsible for seeing and avoid-ing VFR traffic, when visibility permits, but theATC system will provide separation assurancefrom other IFR aircraft and, to the extent prac-tical, alert the IFR pilot to threatening VFR air-craft.

● Similar visual and instrument flight rules are in force in foreigncountries that are members of the International Civil Aviation Or-ganization (ICAO). In many cases, ICAO rules are patterned onthe U.S. model,

Photo credit Federal Aviation Administration

A display of air traffic as it appears to a controller

The distinction between VFR and IFR is basicto ATC and to the safe and efficient use ofairspace, since it not only defines the servicesprovided to airmen but also structures theairspace according to pilot qualifications and theequipment their aircraft must carry. VFR flightsover the contiguous 48 States may not operate ataltitudes above 18,000 ft, which are reserved forIFR flights. The altitudes between 18,000 and60,000 ft are designated as positive controlairspace; flights at these levels must have an ap-proved IFR flight plan and be under control ofan ATC facility. Airspace above 60,000 ft israrely used by any but military aircraft. Most ofthe airspace below 18,000 ft is controlled, butboth VFR and IFR flights are permitted.

The airspace around and above the busiestairports is designated as a terminal control area(TCA) and only transponder-equipped aircraftwith specific clearances may operate in it regard-less of whether operating under VFR or IFR. Allairports with towers have controlled airspace toregulate traffic movement. At small airportswithout towers, all aircraft operate by the see-and-avoid principle except under instrumentweather conditions. Figure 3 is a schematic rep-


Figure 3.—Airspace Structure

*

45,000 ft.

Continental(FL450)

controlarea

Positive Transponder

I Jet control with altitude

routesI

area encodingI required

1—18,000 ft MSL 1

A14,500 f’ MSL

v—.—— ——..—— --— - 1 2 , 5 0 0 f t M S L- --

Control areas and transition areas

--3,000 f’ AGL ~v-

7

– – 1,200 ft AGLAirport 1-transition area ‘trafficarea r 700 ft AGL

. . -. ,>. !

AGL - Above ground levelMSL - Mean sea levelFL - Flight level

SOURCE: Federal Aviation Administration.

resentation of the resulting airspace structure; as have a radio if he elects to file a VFR flight planthe general rule, VFR flights are permitted every-where except in positive control airspace al-though clearances are required to operate withinTCAs and at airports with control towers.

The IFR/VFR distinction also governs avi-onics and pilot qualifications. A VFR flight tak-ing off and landing at a small private field andflying only in uncontrolled airspace needs littleor no avionic equipment, although a pilot must

or land at an airport with a control tower. Air-craft flying under IFR, on the other hand, are re-quired to have radio and avionics equipmentthat will allow them to communicate with allATC facilities that will handle the flight fromorigin to destination. They must also be instru-mented to navigate along airways and to executean IFR approach at the destination airport.These requirements apply to all IFR aircraft, andFederal Air Regulations also specify additional

Ch. 3—The National Airspace System ● 33

equipment requirements and pilot qualificationsfor various classes of air carrier aircraft. In addi-tion, both IFR and VFR aircraft must have trans-ponders that automatically transmit their iden-tity and altitude when they are in TCAs* or ataltitudes above 12,500 feet.

The VFR/IFR distinction also determines thetype of ATC facility that will provide service toairspace users. There are three general types offacilities operated by FAA: air route traffic con-trol center (ARTCC), which serve primarily IFRtraffic; airport traffic control towers, whichserve both IFR and VFR aircraft; and flight serv-ice stations (FSS), which primarily serve VFRtraffic.

FSS serves three primary purposes: flightplanning and advisory information for all GAaircraft; the dissemination of flight plans (VFRand IFR) to other facilities along the intendedroute; and operation of teletype networks to fur-nish information on weather and facility statusto civil and military users. FAA encourages butdoes not require pilots flying VFR to file a flightplan; IFR flights must file a flight plan and ob-tain clearance to use the airspace. Personnel areon duty to provide direct briefings and assist-ance in filing flight plans (counter service), butmost FSS contacts are by telephone or by radio.If a VFR flight encounters weather or restrictedvisibility en route, the pilot (provided he is ratedfor instrument flight) can change to an IFR flightplan while in the air and be placed in contactwith the ATC system. The FSS handles these re-quests and coordinates changes with towers orARTCCs. * *

FSS personnel are also ready to aid VFR pilotswho experience in-flight emergencies. If a pilot islost, the FSS will assist him by means of direc-tion-finding equipment or arranging for trackingby an ATC radar facility. FSS personnel provideweather reports to pilots aloft and receive andrelay pilot reports on weather and flight condi-tions. In more serious cases, such as engine trou-ble or forced landing, the FSS will attempt to

*Altitude-encoding transponders (Mode C) are required only inGroup I TCAs, of which there are nine at present.

● *In the interest of reducing controller workload, this servicewas suspended following the controllers’ strike in August 1981.

pinpoint the location and coordinate search andrescue operations. Flight service stations alsomake periodic weather observations and trans-mit this information by teletype network toother ATC facilities and U.S. weather reportingservices. Thus, FSS is essentially a communica-tions center, serving general aviation directlybut also providing information services for allairspace users. Figure 4 illustrates the communi-cation links and the types of facilities that are incontact with a typical FSS.

FAA operates 317 FSSs, mostly at airportswith VORTAC installations. Since traffic oper-ates out of thousands of airports, much of FSS’swork is done by means of transcribed messagesand standardized briefings. The importance ofFSS as an onsite facility at airports may thus bediminishing, and FAA has plans to consolidateFSSs into about 60 centralized locations. Con-current with the reduction in the number ofFSSs, FAA plans to increase the amount andtype of on-call and remote services, includingmethods for semiautomatic filing of flight plans.FSS personnel would, however, be available—but usually at a remote location—to provideemergency services or to provide direct assist-ance to airmen. This proposed consolidation ofFSS facilities has been the subject of controversyin the aviation community because it is fearedthat the quality and extent of services might bediminished and that observations for the Na-tional Weather Service might be curtailed.

Air Traffic Control

The essential feature of air traffic control serv-ice to airspace users is separation. The need forthis service derives from the simple fact that,under IFR conditions, the pilot may not be ableto see other aircraft in the surrounding airspaceand will therefore need assistance to maintainsafe separation and reach his destination. His-torically, this need came about gradually withthe increasing use of the airspace as the airlinesbegan to operate under instrument flight condi-tions in the 1930’s. In 1934 and 1935, the airlinesorganized a system for controlling traffic withinroughly 100 miles of Newark, Chicago, and


Cleveland. In 1936, the U.S. Government as-sumed responsibility for these centers and estab-lished five more “airway” centers within the fol-lowing year.

This “first generation” of separation servicerelied solely on radio and telephone communica-tion. At established points along the airways,pilots were expected to report their time of ar-rival and altitude and their estimated time of ar-rival over the next checkpoint. In the ATC cen-ter controllers wrote the message on a black-board and tracked flights by moving a markeron a tabletop map. In a later improvement,paper strips marked with flight data were postedin the order of their estimated arrival at eachreporting point or airway intersection. Thisflight-strip system is still available as a backupsystem in the event of radar surveillance equip-ment failure, since it requires only radio commu-nication between the pilot and the controller. Toprovide direct pilot-controller contact, espe-cially as traffic density grew, it became neces-sary in the 1950’s to establish remote communi-cation air-ground stations at distances over 100miles from ATC centers to relay messages from

pilots to the controller handling their flights.This greatly improved the safety, capacity, andefficiency of the control process. In the firstgeneration system, aircraft flying in the samedirection and altitude were kept 15 minutesapart in their estimated arrival times at reportingpoints. This separation standard depended onthe accuracy of position information and—equally important—on the speed and reliabilityof communicating instructions to resolve poten-tial conflicts. Since the capacity of the ATC sys-tem increases as separation standards are re-duced, progress therefore depended on furtherimprovements in both communications and sur-veillance equipment as the ATC system devel-oped.

The second generation of separation servicecame with the introduction of radar after WorldWar II. In the 1950’s, airport surveillance radars(ASRs) were introduced at major airports toprovide data on arriving and departing aircraftwithin roughly 50 miles* At about the sametime, the Civil Aeronautics Authority (predeces-

● FAA now operates 195 ASRs.

Ch. 3—The National Airspace System ● 3 5

sor to FAA), in coordination with the Air Force,began purchasing long-range (200-mile) radarsfor the en route centers with a view to establish-ing complete radar coverage of the continentalUnited States. This was completed in 1965, withthe exception of some gaps in low-altitudecoverages, and today data from multiple radarsites are relayed to ATC centers, so that radarcontact can be kept with almost every IFR flight.The introduction of radar allowed continuousmonitoring of actual aircraft progress and thedetection of potential conflicts or hazard situa-tions. The controller, under a process known as“radar vectoring, ” could direct aircraft awayfrom thunderstorms, around slower aircraft ordownwind for spacing in the approach area. Inso doing, however, the controller began topreempt control of heading and altitude fromthe pilot for short periods of time. Radar separa-tion standards were greatly reduced from thoseof the first generation: 3 miles on approach orabout 2 minutes at piston aircraft speeds.

Despite these improvements, there were stilltwo major deficiencies in a surveillance systemthat relied on raw radar return: the altitude ofthe aircraft was not measured; and the identityof the aircraft could not be established fromradar return alone. In 1958, the newly formedFAA began development of a so-called “second-ary” radar surveillance system in which theradar beam, as it rotated in the scan of azimuth,triggered a positive, pulsed-code reply from a“transponder” (or beacon) on board the aircraft.This pulse contained information on the identityand altitude of the aircraft which could be cor-related with primary radar return. This develop-ment program, known as Project Beacon, led toadoption of the secondary radar system in 1961,and it is the standard surveillance method in usetoday for separation assurance. All commercialair carriers and about two-thirds of GA aircraftare now equipped with transponders* and theprimary radar system has become a backup foruse in the event of equipment malfunction. Theintroduction of transponders and the simul-taneous development of digitized informationsystems and computer-driven traffic displays led

● Slightly less than 30 percent of GA aircraft have altitude-encoding (Mode C) transponders.

to a reduction of controller workload. Auto-mated flight plan processing and dissemination,introduced at about the same time, furtherreduced controller workload by facilitatinghandoffs of aircraft from one en route sector toanother and between en route and terminal areacontrollers. Collectively, these technologicalchanges constitute the third generation of airtraffic control.

All of these improvements have simplified andspeeded up the acquisition of informationneeded to provide separation service, but theyhave not substantially altered the decisionmak-ing process itself, which still depends upon thecontroller’s skill and judgment in directing air-craft to avoid conflicts. In recent years, attemptshave been made to automate the decisionmakingaspects of separation assurance or to provide abackup to the controller in the form of com-puter-derived conflict alerts. Computers cannow perform a simplistic conflict alert functionby making short-term projections of aircrafttracks and detecting potential conflicts that thecontroller may have missed. Since the techniquedepends upon all aircraft being equipped withtransponders, however, it does not provide sep-aration assurance between unequipped aircraft.

The introduction of two-way digital commu-nication rather than voice would mark the be-ginning of a new generation of separation serv-ice. In 1969, the Air Traffic Control AdvisoryCommittee recommended the introduction of animproved form of radar known as the DiscreteAddress Beacon System (DABS). This systemprovides selective identification and address anda two-way, digital data link that allows im-proved transmission of data between groundand aircraft, so that much of the routine ATCinformation can be displayed in the cockpit forthe pilot. DABS would thus provide more com-plete and rapid exchange of information than thepresent voice radio method. DABS would im-prove separation service in other ways as well. Itcould provide more accurate position and trackdata and could lead to more comprehensiveforms of automated conflict detection and reso-lution. Further, because DABS can interrogateaircraft selectively it can avoid the overlap ofsignals in areas of high traffic density.


Another method for providing improved sep-aration assurance is by means of collision avoid-ance systems on board the aircraft, which wouldalert the pilot to converging aircraft and directan avoidance maneuver. Airborne collisionavoidance systems, while conceived as a backupto ground-based separation service, would effec-tively transfer back to the IFR pilot some of thesee-and-avoid responsibility that now governsVFR flight. Still another approach to separationassurance is the use of techniques to meter orspace the movement of aircraft traffic into ter-minal areas from the en route portion of the sys-tem. These are strategic rather than tacticalmeasures, in that they are directed not at avoid-

ing conflicts per se but at preventing the con-gested conditions in which conflicts are morelikely to occur. Traffic metering, spacing, andsequencing techniques are now used by control-lers to prevent traffic buildup or undesirablemixes of aircraft, but for some time FAA hasbeen seeking to develop automated methods thatwill accomplish this smoothing and sorting oftraffic flow without intervention by controllers.Success of these efforts will depend upon devel-opment of computer prediction and resolutionroutines that will detect conflicts among flightplans (rather than flight paths) and issue appro-priate instructions before actual conflict occurrs.

SYSTEM ORGANIZATION AND OPERATION

The third major part of the National AirspaceSystem is the facilities and operational proce-dures for managing air traffic.

ATC Sectors

From the controller’s viewpoint, the ATC sys-tem is made up of many small sectors of air-space, each defined in its horizontal and verticalextent and each manned by a controller with oneor more assistants. Each sector has one or moreassigned radio frequencies used by aircraft oper-ating in the sector. As the flight moves from sec-tor to sector, the pilot is instructed to changeradio frequencies and establish contact with thenext controller. On the ground, the controllermust perform this “hand off” according to strictprocedures whereby the next controller must in-dicate willingness to accept the incoming aircraftand establish positive control when the pilotmakes radio contact before relieving the firstcontroller of responsibility for the flight.

Since the number of aircraft that can be undercontrol on a single radio frequency at any onetime is limited to roughly a dozen, sector bound-aries must be readjusted to make the sectorssmaller as traffic density grows. At some point,however, resectorization becomes inefficient;the activity associated with handing off and re-

ceiving aircraft begins to interfere with the rou-tine workload of controlling traffic within thesector. To help manage this workload, the sec-tors around busy airports are designed in such away that arriving or departing traffic is chan-neled into airspace corridors, in which aircraftare spaced so as to arrive at sector boundaries atregular intervals. While this procedure facilitatesthe task of air traffic control, it results in longerand more fuel-consuming paths for aircraft,which have to follow climb and descent pathsthat are less than optimal. To this extent, theperformance characteristics of the ATC systemaggravate the effects of congestion in busyairspace and detract from the overall efficiencyof airspace use.

ATC Facilities

Organizationally, the facilities that control airtraffic are of three types: en route centers, ter-minal area facilities (approach/departure con-trol and airport towers), and flight service sta-tions. The first handles primarily IFR traffic; ter-minal area facilities and flight service stationshandle both IFR and VFR flights. In addition,flight service stations perform information col-lection and dissemination activities that are ofsystemwide benefit.


The en route portion of the ATC system con-sists of 20 ARTCCs, * each reponsible for a ma-jor geographic region of the continental UnitedStates (see figs. 5 and 6). An ARTCC containsbetween 12 and 25 sectors which control trafficon the airways within the region, and ARTCCairspace is further divided into low-altitude sec-tors primarily used by propeller aircraft andhigh-altitude jet sectors. When aircraft are inlevel cruise, management of traffic is relativelysimple and problems are infrequent. The sectorsthat are difficult to control are those whereflights are climbing or descending around a ma-jor airport. Since these en route sectors are feed-ing aircraft into and out of terminal areas, thetask of control also becomes complicated if theairport is operating near capacity. En route con-trollers may be required to delay the passage ofaircraft out of their sector in order to meter traf-fic flow into terminal areas.

At smaller airports, aircraft leaving control ofan ARTCC pass directly to control by the air-

*In addition, there are two ARTCCs located outside the con-tinental United States, in Hawaii and Puerto Rico.

port tower. At major hubs, however, there is anintermediate ATC facility called terminal radarapproach control (TRACON) located at the air-port. The TRACON (or “IFR room”) handles ar-riving and departing traffic within roughly 40miles of the airport—sequencing and spacing ar-rivals for landing on one or more runways, andsometimes at more than one airport. TheTRACON also vectors departing aircraft alongclimbout corridors into en route airspace. Theapproach and departure controllers at aTRACON exercise a high degree of control overaircraft and must monitor the progress of eachaircraft closely, as well as coordinate their ac-tivities with the ARTCCs from which they arereceiving traffic and with the towers that arehandling the takeoffs and landings at the airportitself.

Tower personnel control the flow of traffic toand from the runways and on ramps and taxi-ways connecting to the terminal. Tower control-lers are the only ATC personnel that actuallyhave aircraft under visual observation, althoughat larger airports they rely heavily on radar forsurveillance. Figure 7 illustrates the activities of

Figure 5.— Air Route Traffic Control Center Boundaries

n



Figure 6.—Connections of a Typical ARTCC With Other Facilities


ATC terminal and en route facilities handling atypical IFR flight.

There are currently 431 airports with towersoperated by FAA, of which 234 are approachcontrol towers and the remainder are nonap-proach control towers. An approach controltower, with its associated TRACON, providesseparation and instrument landing services forIFR traffic and is also responsible for integratingVFR traffic into the approach Pattern. Figure 8

available at a large airport with an approachcontrol tower. A nonapproach control tower isresponsible for assisting traffic by providingweather, traffic, and runway information for allarrivals (VFR or IFR), but does not provide ILSor separation assurance.

Airspace UsersThe users are the fourth major part of the Na-

tional Airspace System. They cover a wide spec-illustrates the equipment and facilities typically trum in skill and experience, types of aircraft


Figure 7.—ATC Activities for a Typical IFR Flight

Chicago O’HareInternational Airport

At the departure gate, pilot con-firms altitude, speed, route andestimated flight time with con-troller in the Chicago tower atO’Hare. After flight clearance,pilot contacts Chicago groundcontrol for taxiing instructionsand proceeds to runway.

Thirty miles farther in the flight,the departure controller transfersresponsibility by instructing thepilot to contact a particular con-troller at the en-route ChicagoCenter, located in Aurora, Ill.

The controller at Chicago Centertracks the plane as it climbs to ap-proximately 23,000 feet, thenhands over the flight to anothercontroller at the center whohandles flights above that height.The airplane reaches cruising alti-tude of 33,000 feet about 100miles east of Chicago.

The plane continues its descentand New York Center hands offresponsibility for the flight to thelocal New York approach-controlfacility at Garden City, N. Y.,where a controller lines up theplane for its final approach to LaGuardia Airport.

When ready for takeoff, pilot onceagain contacts controller in theChicago tower who, using radarand his own view from the tower,clears airplane for takeoff.

One mile away from takeoff point,the controller in the Chicagotower transfers responsibility forthe fright to a departure control-ler, also at O’Hare airport, whodirects the pilot to the propercourse for the first leg of theflight.

The next handoff takes place asChicago Center passes responsi-bility to the en-route ClevelandCenter in Oberlin, Ohio. One con-troller tracks the airplane andtransfers responsibility to a col-league as the fright passes fromone sector to another.

About 6 miles from the runway,responsibility passes to the towerat La Guardia, where a controllermonitors the aircraft’s instrumentlanding. The last handoff of theflight is made from tower toground control, which directs theplane to its assigned gate.

SOURCE Newsweek

flown, and demands for air traffic services. Theycan be grouped in three categories—commercial,GA, and military—with GA exhibiting thegreatest diversity. Table 2 is a summary of theU.S. pilot population in 1980 according to thetype of license held and the percentage with in-strument ratings, i.e., those qualified to use theairspace under IFR. The table shows that about42 percent of all pilots are now IFR qualified; 10years ago the percentage was about 30 percent.Almost all of this growth has occurred in theprivate (GA) category.

Table 3, which is a breakdown of aviation ac-tivity according to type of aircraft and hours

Table 2.-U.S. Pilot Population, 1980

InstrumentPilot group Number rated Percent

Private (GA):Student. . . . . . . . . . . 199,833 0 0Private license ... , . 357,479 39,347 11

Commercial:Commercial a . . . . . . 183,422 147,741 81Airline transport

Iicense b. . . . . . . . . 69,569 69,569 100Total (excluding

students) . . . . . 610,490 256,547 42‘A cO~mercla\ license allows the holder to work aS a pilot and operate on air

craft providing passenger service for hire.bA more advanced rating required of pIlols for air Carrier airlint3S.

SOURCE: FAA Statist/ca/ I-/arrdbook of Aviation, 7980.


Figure 8.—ATC Facilities and Equipment at a Typical Large Airport


Table 3.–Summary of Aviation Activity, 1980

Number of Percent Estimated hours flown (millions)

User group aircraft IFR-equipped a Total IFRa Percent IFRa

Commercial air carrier:Piston. . . . . . . . . . . . . . . . . 595 100 0.48 0.48 100Turboprop . . . . . . . . . . . . . 682 100 1.11 1.11 100Turbojet . . . . . . . . . . . . . . . 2,526 100 6.63 6.63 100Rotorcraft . . . . . . . . . . . . . 2 100 <.01 <.01 100

Total . . . . . . . . . . . . . . . . 3,805 100 8.22 8.22b 100

General aviation:Piston (single-engine). . . . 168,435 34 28.34 2.83 10Piston (multiengined) . . . . . 24,578 91 6.41 2.82 44Turboprop . . . . . . . . . . . . . 4,090 99 2.24 1.66 74Turbojet . . . . . . . . . . . . . . . 2,992 100 1.33 1.22 92Rotorcraft . . . . . . . . . . . . . 6,001 2 2.34 <.01 0

Total . . . . . . . . . . . . . . . . 206,096 42 40.66 8.53 21

Military (all types) . . . . . . . . . 18,969 N.A. 5.26 N.A. N.A.aEStirnateS based on 1979 survey of general aviation aircraft.blncludes 7,00 million hours for air carriers (all classes); 0.09 million hours for air taxi; 0.99 dlliOn hours fOr COmmuWrS; and

0.14 million hours for air cargo.

SOURCES: FAA Statistical Handbook of Aviation, 1980; General Aviation Activity and Avionics Survey, 1979, FAA-MS-B1-1,January 1981.

flown, indicates the relative airspace use and de- a class, general aviation aircraft (98 percent ofmand for IFR services among user categories. the civil fleet) fly only about 1 hour in 5 underCommercial air carrier aircraft (including com- IFR, but this figure is deceptive. Turboprop andmuters and air taxis) make up less than 2 percent turbojet GA aircraft (those with performanceof the civil aviation fleet, but they account for characteristics and usage most like air carrier air-about 17 percent of hours flown and almost half craft) are virtually all IFR-equipped and log aof the total IFR hours flown in civil aviation. As very high percentage of their flight hours under


IFR. The growing numbers and increasing tend-ency of these more sophisticated GA aircraft tooperate under IFR has caused the general in-crease in ATC system workload over the past 10years. At present, GA aircraft account for 51percent of all IFR flight hours, 30 percent of IFRaircraft handled by ARTCCs and 45 percent ofinstrument approaches at FAA control facilities.

Commercial air carriers are the most homo-geneous category of airspace users, althoughthere are some differences between trunklineoperators and commuter or air taxi operators interms of demand for ATC services. Certificatedroute air carriers follow established schedulesand operate in and out of larger and betterequipped airports. They have large, high-per-formance aircraft that operate at altitudes above18,000 feet en route, where they have onlyminimal contact with aircraft not under the posi-tive control of the ATC system. In terminalareas, however, they share the airspace and fa-cilities with all types of traffic and must competefor airport access with other users. Airline pilotsare highly proficient and thoroughly familiarwith the rules and procedures under which theymust operate. All air carrier flights are con-ducted under IFR, regardless of visibility, inorder to avail themselves of the full range ofservices, especially separation assurance.

Commuter airlines also follow establishedschedules and are crewed by professional pilots.However, they characteristically operate smallerand lower performance aircraft in airspace thatmust often be shared with GA aircraft, includingthose operating under VFR. As commuter opera-tions have grown in volume, they have createdextra demands on the airport and ATC systems.At one end of their flight they use hub airportsalong with other commercial carriers and so maycontribute to the growing congestion at majorair traffic nodes. Their aircraft are IFR-equippedand can operate under IFR plans like otherscheduled air carriers, but this capability cannotbe used to full advantage unless the airport atthe other end of the flight, typically a small com-munity airport, is also capable of IFR operation.Thus, the growth of commuter air service cre-ates pressure on FAA to install instrument land-

ing aids and control facilities towers at moresmaller airports.

GA aircraft include virtually all types, rangingfrom jet aircraft like those used by scheduled aircarriers to small single-engine planes that areused only for recreation. Most are small, low-performance aircraft that operate only at low al-titudes under VFR, and many use only GA air-ports and never come into contact with the enroute and terminal control facilities of the ATCsystem. However, there is increasing use of moresophisticated, IFR-equipped aircraft by busi-nesses and corporations, many of whom operatetheir fleets in a way that approximates that ofsmall airlines. By using larger aircraft and equip-ping them with the latest avionics, the businessportion of the GA fleet creates demands forATC services that are indistinguishable fromcommercial airspace users.

It is the disparate nature of GA that makes itincreasingly difficult to accommodate this classof users in NAS. The tendency of GA aircraftowners at the upper end of the spectrum to up-grade the performance and avionic equipment oftheir aircraft increases the demand for IFR serv-ices and for terminal airspace at major airports.In response, FAA finds it necessary to increasethe extent of controlled airspace and to improveATC facilities at major airports. These actions,however, tend to crowd out other types of GA,typically VFR users who would prefer not toparticipate in the IFR system but are forced to doso or forego access to high-density terminalareas. The safety of mixed IFR-VFR traffic is themajor concern, but in imposing measures to sep-arate and control this traffic, the ATC systemcreates more restrictions on airspace use andraises the level of aircraft equippage and pilotqualification necessary for access to the air-space.

Military operations can be placed in twobroad categories. Many operations are similar toGA, but others involve high-performance air-craft operating in airspace where they are sub-ject to control by the ATC system. Front an op-erational point of view, military flight activitiescomprise a subsystem that must be fully inte-


grated within NAS; but military aviation hasunique requirements that must also be met, andthese requirements sometimes conflict with civilaviation uses. Training areas and low-levelroutes that are used for training by military air-craft are set aside and clearly indicated on thestandard navigation charts. The military serv-ices would like to have ranges located near theirbases in order to cut down transit time and max-

imize the time aircrews spend in operational ex-ercises. Civilian users, on the other hand, areforced to detour around these areas at consider-able expense in both time and fuel. FAA ischarged with coordinating the development ofATC systems and services with the armedforces, so that a maximum degree of compati-bility between the civil and military aviation canbe achieved.

Chapter 4



A busy airport terminal

Contents

PageIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

FAA Aviation Forecasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Baseline Scenarios: Procedures and Assumptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Alternative Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Other Aviation Forecasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Forecast Structures and Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Comparison and Critique of Forecasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Factors Affecting Traffic Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55U.S. Economic and Regulatory Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Deregulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Industry Maturity and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Fuel and Labor Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Financing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Substitution for Air Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Strike Impacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Implications for Airport Congestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58Continued Growth and Airport Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58Redistribution of System Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Expanded Capacity and Improved Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

List of Tables

Table Page4. Comparison of Selected Economic Assumptions and Aviation

Growth Predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545. Aviation Growth Assumptions for ’’Redistribution” Scenarios, Domestic

Service, 48 States. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

List of Figures

Figure Page9.

10.11.12.13.14.

15.16.17.18.19.20.21.22*

FAA Tower Workload, Actual and Forecast 1960-93 . . . . . . . . . . . . . . . . . . . . . . 46FAA En Route Workload, Actual and Forecast 1960-2000 . . . . . . . . . . . . . . . . . . 47FAA Flight Service Workload, Actual and Forecast 1960-2000. . . . . . . . . . . . . . . 48Tower Operations, Actual and Forecast 1974-93 . . . . . . . . . . . . . . . . . . . . . . . . . . soInstrument Operations, Actual and Forecast 1974-93 . . . . . . . . . . . . . . . . . . . . . . 51IFR Aircraft Handled by En Route Centers, Actual and Forecast,1974-93 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Total Flight Service Station Activities, Actual and Forecasq 1943-93 . . . . . . . . . . 52Projected U.S. Certificated Air Carrier Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Possible Long-Term Impacts of PATCO Strike on ATC Workload Levels . . . . . 58Activity at Top 50 Commercial Airports-48 States, 1978. ......, . . . . . . . . . . . 60Airport Airside Capacity Perspectivte-Low Economic Growth Scenario . . . . . . 60Airport Airside Capacity Perspective-Average Economic Growth Scenario . . . 61Airport Airside Capacity Perspective-High Economic Growth Scenario . . . . . . 61Number of Commercial Airports Overcapacity-Year2000,48 Contiguous States. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Chapter 4


INTRODUCTION

There is a general consensus that domesticaviation activity will increase over the next 10to 20 years, and with it the demands placed onthe Nation’s airports and air traffic control(ATC) system. There is far less agreement, how-ever, about how much growth there will be,how it will be distributed, and how it will affectthe future characteristics of the National Air-space System (NAS). As a result, there is uncer-tainty about where system improvements will beneeded, and how soon.

Federal Aviation Administration (FAA) plansfor the modernization and expansion of the NASare predicated on the continued rapid growth ofair traffic and ATC workloads. Preliminary fig-ures for the most recent FAA “Aviation Fore-casts” indicate that the number of aircraft usingthe system will double by 2000 and that, be-tween 1981 and 1993, total operations will in-crease by 56 percent at en route ATC centers, by60 percent at FAA-towered airports, and by 88percent at flight service stations.

Accommodating this anticipated demandgrowth has been a primary justification for pro-posed investments in system improvements, butFAA’s forecasts have consistently proven to betoo high in the past. In part, this is due to theway in which they are made: FAA makes itsforecasts on the assumption that present trendswill continue, that there will be no constraintson growth, and that proposed improvementswill in fact be made.

Comparison with other aviation forecasts isdifficult, since only FAA projects ATC work-loads, but it is of interest that some recent fore-casts of other measures of demand have been

higher than FAA’s. In all such projections, how-ever, there is considerable uncertainty about anumber of factors that might affect futuregrowth and system requirements, such as U.S.economic growth, fuel prices and availability,airline profitability, new technology, and thepossibility of significantly higher aviation userfees. Industry maturity may lead to a leveling-off of airline operations, and changes in routestructure may lead to a more even distributionof these operations throughout the system. Evengreater uncertainty surrounds the effects ofairline deregulation and the long-term impactsof the Professional Air Traffic Controllers(PATCO) walkout.

As a result of these uncertainties, there arevalid questions about the accuracy and useful-ness of any projection of aviation activity over10 or 20 years. At present, no individual projec-tion—including FAA’s-should be consideredmore than a broad estimate. Collectively, suchprojections indicate a likely range of possiblefutures for NAS and its ATC requirements; butbecause they are based on similar assumptionsand similar forecasting procedures, they mayalso be subject to similar errors.

This chapter examines and compares a num-ber of projections, but its main focus is on theprocedures and assumptions underlying theaviation forecasts on which FAA will base its1982 system plan. The purpose of this examina-tion is to provide some sense of the range of pos-sible future demand for aviation facilities andservices, in order to assist Congress in making itsdecisions about long-lived investments in bothairports and ATC equipment.

FAA AVIATION FORECASTS

FAA is the most continuous, comprehensive,and detailed source of aviation projections. Its“Aviation Forecasts” are made annually by the

Office of Aviation Policy and Plans (OAPP) insupport of current operations and as a basis forlong-range planning. Many other organizations

45


also use FAA’s forecasts as the basis for theirown long-range planning activities.

However, FAA has a poor forecasting record:over the past 15 years its predictions have con-sistently been too high, often by 50 percent ormore. Figures 9, 10, and 11 compare past fore-casts with actual levels of operations at FAAtowers, en route centers, and flight service sta-tions. They show that the workloads originallyforecast for fiscal year 1981 were between 50 and180 percent higher than what actually occurred;in more recent forecasts this level of demand onthe ATC system is not expected until the 1990’sor later.

Several unforeseeable events combined tocause these errors, including the 1973 oil em-

bargo, sharp increases in fuel prices, rising infla-tion and interest rates, and airline deregulation.These factors and other pertinent changes in his-torical trends are now reflected in FAA fore-casts, but current expectations may once againbe betrayed by unanticipated developments inthe future. If key assumptions are overly op-timistic, the resulting projections will once againbe too high.

Three sets of FAA forecasts were compared indetail for this review: those of September 1978,which predate the Airline Deregulation Act, andthose of 1979 and 1980. The year-by-year fore-casts for 1982-93, due in October 1981, were“sent to the shredders instead of the printers” (inthe words of the Director of OAPP) because the

Figure 9.— FAA Tower Workload, Actual and Forecast, 1960-93

I i *1971a

160

140 - 1967 *

*1975120 -

Difference betweenactual and forecast 1979 *

100 - 1977* * * *1978 1980

60 -

60 -

40 “

1981

20t

III

o I I I I I 1 I1960 1965 1970 1975 1980 1985 1990 1995 2000

Source. Off Ice of Technology Assessment, from Federal Aviation Administration data.

Ch. 4—Aviation Growth Scenarios ● 4 7

Figure 10.— FAA En Route Workload, Actual and Forecast, 1960-2000

I

D i f f e r e n c e b e t w e e n ~actual and forecast ~

*

1 9 7 3 * *1 9 7 5

I

III

iII

NASP*1982

*1961

*1980

I

1 I II

1 I I I1960 1965 1970 1975 1980 1985 1990 1995 2000

Source Office of Technology Assessment, from Federal Aviation Admlnistratlon data

uncertain impacts of the PATCO walkout hadinvalidated the short-term projections. Prelim-inary long-term figures only are used in the fol-lowing discussion and accompanying graphics,but these projections are somewhat higher thanthose of 1980 despite a decline in overall activitysince 1979. Forecasting procedures, assump-tions, and scenario specifications are based onthe last published forecast, that of September1980.

Baseline Scenarios: Proceduresand Assumptions

As described in the 1980 “Aviation Forecasts, ”FAA predictions are based on a combination ofeconometric modeling, trend extrapolation, andexpert judgment. Forecasts of key economic in-

dicators are prepared by Wharton EconometricForecasting Associates, Inc., using their long-term industry and economic forecasting model.In the withdrawn 1981 forecasts, however, thebaseline scenario is based on economic projec-tions supplied by the Office of Management andBudget (OMB) rather than the Wharton model.Aviation activity levels and ATC workloads arederived from these economic indicators bymeans of aviation submodels designed and runby FM itself.

The baseline (or most probable) projectionsare based on the general assumption of uncon-strained growth—that past trends will continueand that there will be no change in the relation-ships between economic activity and aviationvariables. Specific assumptions about the var-ious user groups include the following:


Figure 11.— FAA Flight Service Workload, Actual and Forecast, 1960-2000

150

140

F130

r120

110[

Di

975

*1978

‘1977

979* *

NASP1982

*

*1981

*1980

1960 1965 1970 1975 1980

Source. Of ftce of Technology Assessment, from Federal Aviation Admlnlstratlon data.

●

●

●

Federal policy—no change in Governmentpolicy toward the aviation industry (i.e.,airline deregulation goes forward, existing

●

noise and pollution standards are imple-mented, but no new environmental or pol-icy constraints—such as higher user fees—are imposed).General aviation —continued rapid growthof business and commercial GA (i. e., largerturboprops and jets used as corporate air- ●

craft or air taxis) and continued availabilityof aviation fuel, although prices rise morerapidly than the consumer price index.Air carriers—additional mergers, resultingin route optimization and more efficientfleet utilization, and continued replacement

1985 1990 1995 2000

of older equipment with larger, quieter,more fuel-efficient aircraft.

Commuter carriers-a decrease in the num-ber of carriers as competition leads to mer-gers, no loss of competitiveness with thepersonal automobile, increases in averageaircraft size and stage length, and a relative-ly stable, mature industry after 1984.

FAA workloads—increases in the numberof FAA-towered airports and terminal con-trol areas, which will tend to increase thenumber of IFR operations and flight planfilings, and greater utilization of flight serv-ices due to increased convenience and im-proved services.


Photo credit: Business and Commercial Aviation Magazine

Business and commercial aviation—a growing sector

Alternative Scenarios

Because of the uncertainties involved in tryingto predict the future, FAA forecasts include notonly a baseline scenario (the most likely foresee-able outcome) but also alternative scenarios thatreflect what might happen if there were majorchanges in the driving economic, societal, orpolitical factors. Higher and lower economicprojections from the Wharton model are runthrough FAA aviation submodels, and the for-mal techniques of trend-impact analysis andcross-impact analysis are used to determine thefurther effects of other events or changes.

Because FAA varies several factors at once,however, it is difficult to assess the sensitivity ofthe projections to changes in any specific var-iable. In some cases, moreover, the scenario spe-cifications are so extreme that they underminethe credibility of the resulting projections. Final-ly, the resulting range of possible outcomes overan 12-year projection is so wide that the alterna-tive scenarios may be of little value for long-range planning purposes. In the 1980 forecasts,for example, the alternative projections of FAAworkloads in 1993 were as much as 40 percenthigher or 25 percent lower than the baseline.This “range of uncertainty” has increased in re-cent forecasts (see below).

In 1978 and 1979 there were two alternatives,“high prosperity/slow growth” and “rapid

growth/stagflation, ” respectively. In 1980 therewere three alternatives, with the following sce-nario specifications:

●

●

●

“Economic expansion’’—rapid economicgrowth accompanied by a resurgence of thework ethic, attempts to reestablish U.S.military and economic preeminence in theworld, easing of Federal environmental re-strictions and market intervention, “tre-mendous increases” in user fees (especiallyGA) for airports and ATC services as Fed-eral subsidy of system costs is eliminated,but strong growth in corporate and per-sonal flying due to continued business dis-persal and mobile lifestyles.“Energy conservation’’—aviation becomesa “special target” of Federal efforts toachieve energy independence through regu-lation and taxation, U.S. lifestyle shiftstoward that of “a more slow-paced cul-ture, ” increasingly stringent environmentalstandards and the closing of some metro-politan airports, reestablishment of Federalcontrol over airline routes and fares, andsevere constraints on GA (including higheruser fees, fuel rationing, and banning fromhub airports).“Stagflation” -prolonged worldwide reces-sion, strong Federal intervention throughnationalization and reorganization of avia-tion and other industries, severe rationing


and high prices to encourage energy conser-vation, increased defense spending and wel-fare costs, Federal aid keeps major hubsopen but many GA airports close and airservice to small communities deteriorates,and both business and government makemore use of teleconferencing and other sub-stitutes for personal travel.

Preliminary projections for the 1981 “Avia-tion Forecasts” also include three alternative sce-narios: “economic expansion, ” “Wharton Econ-ometric Model, ” ‘4 stagflation. ” The middle sce-nario reflects the baseline Wharton economicindicators and would have been called the “base-line” scenario in past years; the 1981 baseline,however, is based on OMB’s economic projec-

tions, which are closer to those of 1980 “eco-nomic expansion” scenario (3.6 and 3.9 percentaverage real GNP growth per year, respec-tively). “Energy conservation” was dropped; thespecifications for the other scenarios remain thesame as for 1980.

FAA projections of ATC workloads from re-cent “Aviation Forecasts” are presented in fig-ures 12 through 15. Several features of these pro-jections are worth noting:

●

●

Figure 12.—Tower Operations, Actual

the spread between high and low projec-tions has increased dramatically, suggestinggreater uncertainty about future trends;the overall range of the projections is lower,suggesting less-confidence ‘about the prob-ability of rapid growth;

and Forecast, 1974-93

I

x

xX

1981

50 I I 1 I 1 I I I I i I 1 1 I I I I I I I1975 1978 1979 1980 1981 1985 1990 1991 1992 1993

SOURCE: Office of Technology Assessment, from Federal Aviation Administration data.


Figure 13.— Instrument Operations, Actual and Forecast, 1974-93

A = Historical data* = Baseline scenarios● = Alternative scenarios

xx

xx

I I I I I I I I I i I I I20

I I I I I 1 11975 1978 19791980 1981 1985 1990 1991 19921993


Figure 14.— IFR Aircraft Handled by En Route Centers, Actual and Forecast, 1974-93

.

.

J I I I 1 I I I I I 1 I I I I I I I I f1975 1978 19791980 1981 1965 1990 1991 1992 1993



Figure 15.—Total Flight Service Station Activities, Actual and Forecast, 1974-93

I

● the baseline projections, on which FAAbases its system plans, have neverthelessmoved from the middle of the overall rangetoward the upper end; and

. the baseline projections are higher in 1981than in 1980, despite changes in the histor-ical data that would seemingly have causedthem to be lower.

The reason for the growing uncertainty in recent“Aviation Forecasts” is not immediately clear.However, in combination with FAA’s poor fore-casting record in the past (see figs. 9, 10, and11), it raises questions about the usefulness ofFAA forecasts as a guide to decisions aboutlong-term investments in system improvementsand expansion.

OTHER AVIATION FORECASTSLong-range forecasts of aviation activity are

also made by a number of organizations otherthan FAA, including airlines, aerospace manu-facturers, investment firms, and private consult-ants. The scope and emphasis of these forecastsdiffer according to the purposes and interests ofthose who make them; understandably, onlyFAA projects FAA workloads. Nevertheless,they follow the same general approach and em-ploy the same general techniques of analysis andprojection. In some cases, however, there are

significant differences in their assumptionsabout the specific variables, trends, or eventsrelevant to the future growth of domestic avia-tion.

OTA reviewed several forecasts about whichthe available documentation was sufficiently de-tailed to permit comparison with FAA projec-tions:

• Boeing Commercial Aircraft Co, —Theseforecasts aim primarily at identifying the


world market for aircraft in the commercialfleet, rather than the level or patterns of air-line operations. Two sets of projectionswere reviewed: “Dimensions of AirlineGrowth” (March 1980) and “Current Mar-ket Outlook” (November 1981); both arebased on economic projections from CaseEconometrics.

• Transportation Research Board (TRB). —This is not a regularly published forecastbut rather a result of the ongoing activitiesof the Aviation Forecasting Committee ofTRB, which is part of the National ResearchCouncil of the National Academy of Sci-ences. Published in August 1981 as “As-sumptions and Issues Influencing the FutureGrowth of the Aviation Industry,” the fore-cast represents the consensus of forecastingworkshop participants representing mostsegments of the aviation community.

● Office of Technology Assessment (OTA). —These projections were commissioned byOTA to provide different kinds of informa-tion than was provided by the other majorforecasts. In particular, its structure and as-sumptions are designed to project the distri-bution as well as the volume of future avia-tion activity, in order to determine its im-pact on airport congestion and ATC capac-ity (see below). It is thus a “conditional”forecast, since its different assumptions re-quire a change in current traffic patternsand industry structure.

● Other Aviation Forecasts. —Recent updatesto the 1975 Air Transport Association(ATA) forecast became available during thecourse of this study, as did the most recentedition of Lockheed-California Co. ’s reg-ularly published “World Air Traffic Fore-cast.” The ATA forecast focuses on the fi-nancial performance and capital needs ofthe airline industry, while the Lockheed re-port emphasizes international rather thandomestic traffic. However, neither reportpresents its forecast on a level of detail con-sistent with the above forecasts, and as a re-sult they are given only cursory treatmentin the discussion that follows. The judg-ments and informal forecasts of a number

of other sources have also been consideredin OTA’s analysis.

Forecast Structures and Assumptions

Table 4 presents the specific features and re-sults of the six forecasts that have been studiedin detail. In each case, the forecast begins by as-suming the macroeconomic indicators that arebelieved to be the driving force behind air trafficgrowth, and then uses these variables to gener-ate the growth rates and absolute levels of avia-tion activity at the end of the forecast period.Although disposable personal income (DPI) ap-pears to be the most important driving variablein most of the forecasts, the direct link betweenmacroeconomic forecasts and traffic forecasts isseldom explictly given.

On the basis of their economic projections,the forecasts then derive growth rates and actuallevels of commercial air traffic in terms of reve-nue passenger miles (RPMs). FAA and OTAforecasts are the only ones that include explicitreference to GA operations; given the increasingimportance of GA activity, its absence is a majorshortcoming in the other forecasts. Similarly,only FAA’s “Aviation Forecasts” proceed fromtraffic levels to FAA workloads; lacking this fur-ther analysis, the other major forecasts (includ-ing OTA’s) are useful only for purposes of com-parison in evaluating the traffic growth and air-craft fleet mixes that the ATC system wouldneed to accommodate.

All of the projections include alternative sce-narios that reflect different assumptions abouteconomic growth, typically referred to as low,medium, and high. The most recent FAA fore-casts contain four scenarios, but only the base-line scenario is described in detail. Beyond thesescenario specifications, none of the forecastspostulates specific events that might affect trafficgrowth of system evolution; all of them as-sume —explicitly or implicitly—that no “majorcatastrophe” will occur. (The PATCO strike andsubsequent traffic restrictions may not consti-tute such a catastrophe, but they do affect theshort-term prospects of growth and may affectlong-term patterns. This has created sufficient


Table 4.-Comparison of Selected Economic Assumptions and Aviation Growth Predictions

Real GNPa growth Real DPlb growth RPMc growth(percent/year) (percent/year) (percent/ year) Load factor

RPMs 1991 1991Forecast 1979-86 1986-91 1979-86 1986-91 1979-86 1986-91 (millions) (percent)

FAA 1978 . . . . . . . . . . highmedlow

FAA 1979 . . . . . . . . . . highmedlow

FAA 1980 . . . . . . . . . . highmedaltlow

FAA 1981 . . . . . . . . . . highOMBmedlow

Boeing 1980 . . . . . . . . highlow

Boeing 1981 . . . . . . . . highlow

TRB 1981 . . . . . . . . . . highmedlow

OTA 1981 . . . . . . . . . . highmedlow

Range of allforecasts . . . . . . . . high

medlow

aGross national product.bDlsposable personal income.cRevenue passenger miles.

4.43.32.84.02.82.5

2.3

3.6

3.02.4

4.33.22.4

3.7

2.92.1N/A

N/AN/A

3.02.6

4.33.42.5

3.0-4.52.7-3.62.0-2.8

4.63.22.54.92.82.1

2.9

4.43.72.93.92.51.8

2.3

3.3

3.13.0

3.52.82.2

4.33.22.4

3.8

2.81.9NIA

NIAN/AN/AN/AN/AN/A

4.33.42.5

3.8-4.62.7-3.41.7-2.5

4.43.12.25.72.81.6

3.0

3.52.82.2

6.85.42.86.05.54.4

4.8

4.9

6.54.6

7.0

5.8

4.33.6N/A

N/AN/A

7.34.6N/A

N/A7.55.54.1

5.8-7.54.3-7.03.6-4.6

uncertainty that FAA has delayed publication ofthe 1981 forecasts until the impacts can be as-sessed. )

Comparison and Critique of Forecasts

All of the major forecasts assume roughly sim-ilar economic growth rates. FAA’s projectionshave tended to be lower than the others and hadbecome more so in recent years, although thepreliminary figures for the withdrawn 1981 fore-cast reflect OMB’s optimism about future eco-nomic growth. Nevertheless, given the range offorecast growth rates, the differences betweenthe individual economic assumptions are prob-ably not significant. In terms of aviation-specificfactors, there also seems to be general agreementamong the projections about variables such asload factors, aircraft size, and stage length.

Not surprisingly, the resulting growth ratesfor domestic RPMs are also quite similar. OTA’sprojections for RPMs tend to be at the upper endof the range for all the forecasts. The 1980 FAA

4.64.54.46.74.24.0

3.7

5.53.9

7.0

406369308426365336405341342314N/A346N/AN/A434354358336N/A450N/A443360311

405-600341-460311-450

60.060.060.062.062.062.063.363.363.363.3N/AN/AN/AN/A66.266.2N/AN/AN/A63.0N/A60.060.060.0

60.0 -66.2

forecasts are slightly but not significantly lowerthan the others. Despite the more optimistic eco-nomic assumptions, the 1981 FAA forecasts (ifand when published) will probably be somewhatlower as well. Lockheed’s corresponding fore-cast, a single figure of 307 billion RPMs in 1990,is somewhat lower than any of the forecasts in-cluded in table 4.

Only the FAA and OTA-commissioned fore-casts break down these RPM figures into projec-tions of air carrier operations by type. FAA’soperations forecasts are considerably lower thanOTA’s, particularly in the 1980 forecast. Wherethe OTA “low” scenario translates 4.1-percentRPM growth into 1.5-percent annual growth inair carrier operations, the 1980 FAA “baseline”scenario shows 4.3-percent RPM growth but nooperations growth, and the FAA “stagflation”scenario translates 3.6-percent RPM growth intoa 0.8-percent decline in operations. As a result,OTA’s forecast range for air carrier operationsin 1991 is 12.1 million to 19.6 million, while theFAA’s is 9.2 million to 15.5 million. The corre-

Ch. 4—Aviation Growth Scenarios ● 55

sponding projection from the Air Transport As- ●

sociation, reflecting the judgments of its airlinemembers, is for 10.4 million air carrier opera-tions in 1990. The overlap between these projec-tions is sufficiently wide that the differences areprobably not significant, particularly whenstructural differences between the models areconsidered. However, because the forecasts rely ●

on common assumptions, they produce similarresults all of which may be in error for the samereasons. ●

The TRB Aviation Demand Forecasting Com-mittee’s 2-day workshop on FAA aviation fore-casts resulted in four principal recommenda-tions, all of which also apply to the other fore- ●

casts considered here. In the opinion of theworkshop participants, the following featuresare needed by planners and decisionmakersalike:

high and low estimates of key assumptionsto measure the extent of uncertainty aboutdriving variables, and consequently an in-crease in the number of alternative sce-narios (at present the FAA provides com-plete results only for its “baseline” sce-nario);a variety of techniques rather than a singletechnique, in order to produce better fore-casts or competing scenarios;in particular, less reliance on econometricmodels and more on expert judgment (espe-cially industry experts), taking account ofnonlinear economic relationships and non-economic factors; andforecasts of components rather than aggre-gates alone—regional and local activityrather than national, for instance, andpoint-to-point traffic levels rather than onlytotal volumes.

FACTORS AFFECTING TRAFFIC GROWTH

The future growth of aviation activity in theUnited States will be affected by a number offactors that are not or cannot be anticipated ade-quately or with certainty in the models used forthe forecasts discussed above. In some casesthese factors may constitute “levers” throughwhich the rate or pattern of growth might be in-fluenced through appropriate policies or pro-grams. In most cases, however, neither the di-rection nor the impact of these factors can be ac-curately foreseen. These factors include but arenot limited to those discussed below.

U.S. Economic and Regulatory Policy

The preliminary figures for FAA's 1981 fore-casts reflect considerable optimism about the im-plementation and success of the present adminis-tration’s economic recovery plan. The growthand structure of the aviation system will be in-fluenced significantly by the speed and strengthwith which the Nation recovers from the currentrecession. The growth of aviation will also con-tinue to be influenced by air safety and air trafficregulations, by the way in which ATC system

costs are apportioned through user fees and avi-ation taxes, and by the constraints imposed bypresent and future noise and environmental reg-ulations. The potential impact ofand policy factors is uncertainfuture changes.

Deregulation

these economicand subject to

Airline deregulation has destabilized the in-dustry’s price and market structures. Some ana-lysts believe that the transition toward a freemarketplace is causing overcompetition, whichin turn is undermining major airline profitabilityand reducing their ability to finance badlyneeded new equipment. Termination of Section406 and 419 subsidies in 1985 and 1988 will alsoaffect commuter airline profits and may affectair service to as many as 100 small- and medium-size cities. Some analysts feel that the demise ofsome carriers may be a natural and indeed desir-able result of complete deregulation, since theelimination of financially ailing carriers wouldrelieve the overcapacity that currently hindershealthier competitors. Some analysts predict the


bankruptcy of a major carrier by mid-1982, andthat by 1990 the industry will probably witnessconsiderable consolidation through mergers, ac-quisitions, and outright failures. The survivors,however, may be in a far stronger financial andcompetitive position.

Industry Maturity and Structure

Rolls Royce, a major aerospace manufacturer,has suggested that even if positive steps aretaken to reduce costs and increase efficiency, theU.S. airline industry has already reached about60 percent of its mature size (see fig. 16). Othersput the figure at closer to 80 percent. If this is so,then major air carrier passenger traffic maybegin to level off before the end of the century,and tower operations might actually decline.The continued growth of commuter carriers andGA traffic might nevertheless result in a con-tinued increase in the number of airport and

ATC operations beyond 2000, but FAA expectscommuters too to become a “stable, mature in-dustry” after 1985 and GA may face growth con-straints. It seems likely, in any case, that by 1990there will be a smaller number of trunk carriers,offering primarily long-haul service; a decliningnumber of specialized carriers, offering low-costservice in major hubs and major markets; and alarge number of commuters of various sizes, in-cluding some that offer “regional” service.

Fuel and Labor Costs

The greatest uncertainty facing domestic avia-tion in both the short and the long term is the fu-ture price and availability of aviation fuels. Thisfactor is crucial to the continued profitability ofthe airlines, which depends in a major way ontheir ability to absorb any differences betweenthe increase in fuel prices and the increase in theCPI. The future course of fuel prices can only be

Figure 16.—Projected U.S. Certificated Air Carrier Growth

400

350

300

–4 –3 –2 -1 0 1 2 3 4 5 6

SOURCE: Rolls Royce, Inc., U.S. Air/ines /ndlcators and Pro/ecflons, July 1981,


guessed at, particularly in view of uncertaintyabout future OPEC policy and the inherent in-stability of the Middle East. However, the cur-rent “oil glut” and price decreases are probably atransient event in the long-term price trend,although it is less certain whether or how rapidlythe real price of fuel will rise in the future. Nolong-term shortage is expected. There are indica-tions, however, that aviation gasoline (used bysmaller piston-engined GA aircraft) may be in-creasingly difficult to obtain. GA activity is par-ticularly sensitive to fuel prices, but rapid in-creases are more likely to reduce personal GAtraffic than business and commercial GA (cor-porate and air taxi users, who generate greaterdemand for ATC services).

Labor costs are also a major factor in air car-rier profitability, and airlines can be expected toseek long-term wage and benefit concessionsfrom their unions during the 1982 round of con-tract negotiations. Financing costs may also be-come an increasingly important factor in thefuture.

Technology

Considerable optimism remains about the fu-ture impact of advanced air transport technol-ogy, but such improvements are likely to be in-troduced more slowly in the future than over thelast 20 or 30 years. Recent improvements inairline efficiency and productivity have comethrough higher utilization and economies ofscale (aircraft size and seating density) ratherthan technology (aircraft speed or fuel efficien-cy). Several promising new developments ap-pear to be possible in the near future, but there isa considerable amount of aviation technologycurrently “on the shelf” that is only beginning toappear in the U.S. fleet. Whether the aerospaceindustry will continue to develop a new genera-tion of advanced-technology aircraft will de-pend on the potential market, and this in turndepends on the ability of the airlines to generateprofits and/or obtain financing. Severalmanufacturers have announced plans for a new150-passenger aircraft for the late 1980’s; severalnew commuter aircraft will be available evensooner. Some near-term increases in fleet effi-

ciency could, however, be achieved by retrofit-ting engines and making other modifications toexisting aircraft.

Financing

Reports by various airline and bankingsources indicate that the equipment needs of theU.S. airline industry will impose capital require-ments of $50 billion to $100 billion by 1990,compared to total capital additions of only $30billion between 1960 and 1979 (current dollars).This capital requirement would demand an aver-age annual corporate return on investment(ROI) of 13 to 15 percent for the entire decade.Industry ROI averaged 6.4 percent during the1970’s, and only once—in 1978—has it risen ashigh as 13 percent. There are signs of increasingreluctance on the part of insurance companiesand even banks to provide long-term debt, evenwhen secured by the leveraged-lease financing orequipment trust certificates that were used in the1970’s. Deregulation has further increased therisks and uncertainties of airline financing, al-though a restructuring of the industry throughbankruptcies or mergers (see above) might alterthis situation in the future. Without a firm mar-ket, furthermore, aerospace manufacturersmight be less willing to develop and introducemore advanced aircraft in the future.

Substitution for Air Transport

Very little can be said with any certaintyabout the future impacts of developments ineither substitute transportation modes (such ashigh-speed trains or, with higher speed limitsand gas mileage, the personal automobile) or al-ternatives to travel (such as advanced telecom-munication technologies and corporate telecon-ferencing). Neither is likely to cut into aviation’slong-haul markets, although the industry mayfind it increasingly difficult to compete with theautomobile and train in short-haul markets(under 200 or perhaps even 300 miles).

Strike Impacts

Ironically, the PATCO strike has in effect de-regulated the industry by imposing traffic re-


striations on the 22 busiest hubs and by placingsevere constraints on GA traffic. Some obser-vers feel that the strike may actually have helpedairline profits by removing overcapacity andenabling major carriers to ground inefficient air-craft, lay off personnel, and reduce other costs.On the other hand, these same restrictions im-pose constraints on GA traffic and on the expan-sion of commuter carriers and new entrants.

Strike-related traffic restrictions will probablycontinue for at least 2 more years, and adjust-ments made by users during this period may per-manently change aviation growth trends andtraffic distribution. As a result, there is little cer-tainty about the long-term impact on the level ofoperations: traffic might rebound rapidly, butpreviously projected levels might not be reacheduntil later than anticipated, if at all (see fig. 17).In addition, these traffic restrictions (particu-larly at major hubs) could be extended or reim-posed in the future as a means of addressing air-port congestion and encouraging redistributionof operations to second-tier hubs (see the follow-ing section).

Figure 17.–Possible Long-Term Impacts of PATCOStrike on ATC Workload Levels

1981 1984A = Built-up demand causes rapid recovery and

workload quickly matches projected levels.B = Steady recovery and projected rate of growth, but

workload matches projection later than anticipated.C = Strike stunts demand growth and ATC workload

never achieves projected level.

NOTE For Illustrate purposes only, and not based on speclflc FAA forecasts

SOURCE Off Ice of Technology Assessment

IMPLICATIONS FOR AIRPORT CONGESTION

Despite the uncertainties involved in forecast-ing precise rates of growth, there is a generalconsensus that air traffic and the demand forATC services will increase in the next 10 to 20years. There is also a consensus that much ofthis growth will come from the GA sector ratherthan the airlines, and within the GA sector frombusiness and commercial aircraft rather thanpersonal flying. There is far less agreement onhow this growth will be distributed through thesystem or how it will affect the problem of air-port congestion and delay.

FAA forecasts indicate that continued rapidgrowth of air traffic, if it occurs along existingpatterns at existing airports, will result in severeairside congestion at 46 air carrier airports by2000. FAA’s forecasts have consistently overesti-mated growth in the past, and a number of fac-tors may constrain growth in the future (seeabove). Nevertheless, airside capacity could be-

come an increasingly serious problem at more ofthe Nation’s airports by the end of the centuryunless there are improvements in airport capac-ity or traffic management (see ch. 6).

An alternative to this prospect, however, isthe redistribution of air carrier operations acrossmore of the top 50 airports, in combination withimproved facilities at additional GA reliever air-ports. This alternative is discussed below; spe-cific improvements in ATC technology and air-port management that would complement it areexamined in chapters 5 and 6. The economic andaviation growth rates on which the followingdiscussion is based are presented in table 5.

Continued Growth andAirport Saturation

The primary measure of aviation activity as itbears on airport and ATC decisions is “opera-

Ch. 4—Aviation Growth Scenarios • 59

Table 5.—Aviation Growth Assumptions for “Redistribution” Scenarios, Domestic Service, 48 States

Jets Propeller aircraft

1978:Revenue passenger miles . . . . . . . . . . . . . . . 200 billion 1.7 billionOperations at top 50 commercial

airports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 million 1.8 million

Low Average High Low Average Higheconomic economic economic economic economic economicgrowth growth growth growth growth growth

2000:Revenue passenger miles: average annual

growth rate ... ... ... ... ... ..percent...4.1 5.5 7.5 4.1 5.4 6.9Revenue passenger miles:

year 2000 . . . . . . . . . . . . . . . . . . . billions. . 450 600 900 - 4 - 5 - 7Operations: average annual growth

rate . . . . . . . . . . . . . . . . . . . . . . percent. . 1.6* 2.2* 3.0* 2.4 1.6* 2.4*Operations at top 50 commercial

airports ... ... ... ... ... ... .millions. . 10* 11 .2* 13’ 2.9 2.5* 2.9*

“Assuming effects of airport capacity constraints.

NOTE: Real GNP growth rates: Low 2.5Average 3.4High 4.3

tions’’—landings and takeoffs, or arrivals anddepartures (each flight generates two opera-tions). Figure 18 illustrates the 1978 mix of airactivity at the top 50 commercial airports,ranked by air carrier operations and aggregatedinto sets of 5 airports to simplify presentation.Most of the operations at these airports are gen-erated by scheduled passenger flights, but al-though there are few local operations at the top15 airports, GA traffic (predominantly cor-porate aircraft and air taxis) is seldom less than10 percent of operations.

Figure 18 also shows the estimated airside ca-pacity of these airports, expressed in terms of the“practical annual capacity” (PANCAP) that canbe handled safely, as estimated by FAA in 1978.Actual airside capacity is variable, however,changing with weather conditions or aircraftmix; the balance is a delicate one, and at busyhubs even a slight deterioration from optimumconditions can cause long lines of delayed air-craft. PANCAP—the level of operations atwhich 80 percent of aircraft encounter delays of4 minutes or longer— thus represents an approx-imate figure based on assumed average utiliza-tion of the existing number and configuration ofrunways, rather than an absolute or reliablemeasure of capacity.

Saturation—the level at which delay is chron-ic— may not occur at a given airport until oper-ations are as much as 100 percent abovePANCAP, so that small differences between ac-tual operations and PANCAP are not necessar-ily significant. Large differences, on the otherhand, indicate a rising probability of encounter-ing delays at the airport at least part of the time.The discrepancy at most of the top 10 airports infigure 18 represents a significant capacity short-age relative to demand (the desired level of oper-ations), and in most cases this situation has ex-isted since the late 1960’s. * It is assumed in thefollowing discussion that when operations aremore than 10 percent above PANCAP, the re-sult will be airport saturation and chronic delay.

Figures 19 through 21 show the PANCAP, the1978 level of operations, and the levels of opera-tions in 2000 projected under three aviationgrowth scenarios. These projections assume thattraffic growth will occur at the same rate acrossexisting airports, irrespective of capacity limita-

*The discrepancy between PANCAP and actual operations inthe sixth airport group (which includes Phoenix, Fort Lauderdale,Orlando, San Diego, and Portland) does not indicate a significantcapacity problem. These airports handle a large volume of GAtraffic that is discretionary as to time of day and weather condi-tions, both of which increase actual capacity over a PANCAPfigure.


Figure 18.–Activity at Top 50 Commercial Airports, 48 States, 1978

500

400

Averageannualoperations 300(thousands)perairport

200

100

Groups of 5 airports, ranked by air carrier operations

SOURCE: Office of Technology Assessment.

Figure 19.—Airport Airside Capacity Perspective—Low Economic Growth Scenario(Jets plus propeller service plus 10 percent for general aviation)

(1.3 percent average growth rate in operations)

A B

Growth unconstrained by capacity

2000

1 2 3 4 5 6 7 6 9 10Airport group

(5 airports per group)

800

600

400

Expanded hub structure(operating 10 percent over PANCAP)

1 2 3 4 5 6 7 8 9 10Airport group


Ch. 4—Aviation Growth Scenarios • 61

Figure 20.—Airport Airside Capacity Perspective —Average Economic Growth Scenario(Jets plus propeller service plus 10 percent for general aviation)

(2.3 percent average growth rate in operations)

800

600

400

200

AGrowth unconstrained by capacity

1 2 3 4 5 6 7 8 9 10Airport group

(5 airports per group)


B

600

t

400

200

1 2 3 4 5 6 7 8 9 10Airport group

Figure 21.—Airport Airside Capacity Perspective— High Economic Growth Scenario(Jets plus propeller service plus 10 percent for general aviation)

A

800 With growth constrained assumptions:2,36 percent average growth In aircarrier operations

600

400

200

1 2 3 4 5 6 7 6 9 10

Airport group(5 airports per group)

Airport group

SOURCE: Off Ice of Technology Assessment


tions. Under conditions of low economicgrowth, desired operations exceed PANCAPonly at the top 10 airports; at the top 5 airports,however, demand will be about 50 percentabove PANCAP (fig. 19A). Under conditions ofaverage economic growth, desired operationswould exceed PANCAP at the top 20 airports,and traffic at the 5 busiest hubs would be almost200 percent of PANCAP (fig. 20A). Under con-ditions of high economic growth, desired opera-tions would be higher than PANCAP at over 30airports, and the tops hubs would experience al-most 250 percent of PANCAP (fig. 21A). Toavoid these conditions, the carriers would prob-ably increase aircraft size and drop servicepoints, particularly in short-haul markets, inorder to reduce overall operations. This adjust-ment, also shown in figure 21A, could reduceoverall traffic levels by roughly 24 percent, butthere would still be serious congestion problemsat the top 10 or 15 hubs.

Redistribution of System Operations

In 1978 the level of scheduled commercial op-erations at the top 50 airports was about 52 per-cent of their combined PANCAP. However,these operations were heavily concentratedtoward the five largest airports (where trafficlevels exceeded PANCAP by 20 percent), whileconsiderable excess capacity existed at the other45 hubs. In addition, over half of the passengersarriving at the five largest hubs did so only to

change planes.

OTA examined the effect of redistributing theexpected increases in operations to these lesscrowded airports. In the following discussion itwill be assumed that 110 percent of PANCAP—i.e., saturation—represents a desirable level ofoperations (or an acceptable level of delay) atany given airport. The results, shown on theright side of figures 19 through 21, indicate thatthe combined existing capacity of the top 50 air-ports could accommodate substantial increasesin commercial operations if they were redis-tributed.

Low economic growth would result in 20 air-ports at 110 percent of PANCAP, instead of 5airports at 150 percent (fig. 19 B). Average eco-

nomic growth would result in 38 airports at 110percent of PANCAP, instead of 10 airports over150 percent and the top 5 at almost 200 percent(fig. 20B). High economic growth would resultin traffic levels of 113 percent of PANCAP at allof the top 50 airports even if redistributed, in-stead of almost 15 airports at 150 percent andthe top 5 airports at almost 250 percent; but ifairlines respond to capacity constraints by in-creasing aircraft size and dropping some servicepoints, as well as redistributing operations, theresult would be levels of 110 percent ofPANCAP at only 38 of the top so air carrier air-ports (fig. 21 B).

Such a redistribution would be accomplishedprimarily by “rehubbing” airline route struc-tures-that is, by moving the interline function(that of providing a transfer point) from con-gested airports to the “second tier” hubs whereexcess capacity still exists. There are indicationsthat such changes in the airline network arealready taking place. United Airlines, for in-stance, has been shifting some of its operationsfrom Chicago-O’Hare to St. Louis over the past5 years; in addition, Denver (the western hub)has been growing in importance relative toChicago in United’s overall system. Similarshifts by other carriers can be detected fromChicago to Kansas City, from Atlanta to Birm-ingham, from Dallas-Fort Worth to Houston,from Miami to Tampa, and from Memphis toNashville. FAA, for its part, has been tryingfor years (with only limited success) to shiftairline operations from Washington-National toDunes International.

Market forces will continue to promote thisredistribution, as will the traffic restrictions im-posed by FAA at the 22 largest hubs as a resultof the PATCO strike. Direct-service links al-ready exist between most of these new transferhubs, but the frequency and aircraft size of traf-fic between them would increase. Nevertheless,some hub airports will continue to experiencehigher than desirable levels of traffic and delaysunless further measures are employed, such aspeak-hour landing fees, access quotas, or slot-al-location schemes. Commuter airlines would behardest hit by these restrictions, and even withnew hubs available they would be hard pressed


to improve service at existing points or add newservice points to their networks. In addition, itwould eventually be necessary to shift most GAtraffic out of the top 20 or more airports (downto the supposedly “irreducible” 10 percent),which implies the need for improved facilities atreliever and other IFR-equipped airports if fu-ture GA growth is to be accommodated.

Expanded Capacity andImproved Management

The above scenarios indicate that attemptingto accommodate expected aviation growth with-in the existing airport capacity will have mixedeffects on the air service network. Although theadverse effects of growth, such as increasingdelays or reductions in service, might be toler-able, it would nevertheless seem both prudentand desirable to increase capacity, where feasi-ble, if this can be done at a reasonable cost andto the benefit of system efficiency. However, it isnot feasible to supply the amount of new capac-ity required to eliminate or even appreciablyreduce airside delay, particularly in major urbanareas. In the short and long term, the alleviationof delay will be best achieved through tighter

Figure 22.—Number of Commercial

control over the level and distribution of airportoperations, rather than the addition of newcapacity (see ch. 6).

However, both commuter access and overallcapacity constraints could be addressed by theconstruction of short, independent “stub” run-ways for turboprop aircraft where feasible, andespecially at the most congested airports. Thisalternative (discussed in detail in ch. 6) wouldincrease propeller capacity as an addition—rather than a detriment—to jet capacity, thereby

reducing the severity of hub saturation andallowing GA and commuter aircraft to competemore effectively with jets for airport access. Fig-ure 22 shows the effect of such runways in re-lieving saturation at commercial airports in2000: by adding about 25 percent to the effectivecapacity of an average hub, they would allow aconsiderably higher level of traffic growth or,alternatively, reduce the number of airports sat-urated by any given level of economic and traf-fic growth. However, the addition of stub run-ways would also result in more complex trafficpatterns, which might require new landing sys-tems and improved traffic management in ter-minal areas.

Airports Over Capacity–Year 2000,48 Contiguous States

(Jet plus propeller operations plus 10 percent allowance for general aviation)

Saturation at - 1.1 1.2 1.375 Times currentpracticalannualcapacity

Average annual growth rate of operations—props plus jets

SOURCE Off Ice of Technology Assessment

Chapter 5

TECHNOLOGY AND THE FUTUREEVOLUTION OF THE ATC SYSTEM


Air traffic control

. .

Contents

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Goals and Services of the ATC System. . . . . 68Major Components of the Existing

ATC System. . . . . . . . . . . . . . . . . . . . . . . 68Surveillance Radar . . . . . . . . . . . . . . . . . . . . 70Airborne Transponders. . . . . . . . . . . . . . . . 70Navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Communication. . + . . . . . . . . . . . . . . . . . . . 73

Future Requirements, Opportunities,and Constraints. . . . . . . . . . . . . . . . . . . . . 73

Future Requirements . . . . . . . . . . . . . . . . . . 73Technological Opportunities. . . . . . . . . . . . 74Constraints and Other Factors

Affecting Future Evolution . . . . . . . . . 76Continuity of Service. . . . . . . . . . . . . . . . 76Timing of Design Decisions and

System Implementation. . . . . . . . . . 76User Costs . . . . . . . . . . . . . . . . . . . . . . . . . 76Locus of Decisionmaking. . . . . . . . . . . . . 77Freedom of Airspace and Equipage. . . . 77International Requirements. . . . . . . . . . . 77Military Requirements . . . . . . . . . . . . . . . 78

Technical Options. . . . . . . . . . . . . . . . . . . . . . . 78En Route Computer Replacement. . . . . . . . 78

Total Replacement . . . . . . . . . . . . . . . . . . 79Hardware-First Replacement

(“Rehosting”). . . . . . . . . . . . . . . . . . . 81Software-First Replacement

(“Offloading”). . . . . . . . . . . . . . . . . . 81Modularity and Other Concerns. . . . . . . 82

Automated En Route Air TrafficControl . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Potential Benefits . . . . . . . . . . . . . . . . . . . 84Potential Implications and Issues . . . . . . 84

Data Link. . . . . . . . . . . . . . . . . . . . . . . . . . . . 85Potential Benefits . . . . . . . . . . . . . . . . . . . 85

Page

Mode S. . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Modes B and D......... . . . . . . . . . . . 87 VHF Data Link. ........ . . . . . . . . . . . 87 Potential Implications and Issues . . . . . . 87

Collision Avoidance. . . . . . . . . . . . . . . . . . . 88 Beacon Collision Avoidance System.. . 89Tri-Modal BCAS. . . . . . . . . . . . . . . . . . , 90Traffic Alert and Collision

Avoidance System . . . . . . . . . . . . . . 90Airborne Collision Avoidance System. . 92

Microwave Landing System . . . . . . . . . . . . 92Instrument Landing System. . . . . . . . . . . 92Microwave Landing System . . . . . . . . . . 94Potential Implications and Issues . . . . . . 95

Alternative ATC Processes . . . . . . . . . . . . . . . 96Role of the Human Operator. . . . . . . . . . . . 97Tactical v. Strategic Control . . . . . . . . . . . . 97Autonomy and Flexibility of

Operation . . . . . . . . . . . . . . . . . . . . . . . 97Ground v. Satellite Basing, . . . . . . . . . . . . . 97Levels of Service. . . . . . . . . . . . . . . . . . . . . . 98

LIST OF TABLES

Table No. Page6. Perform ATC Automation Processes. . . . 807. Summary of Functional Characteristics

of Alternative Collision AvoidanceSystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

LIST OF FIGURES

Figure No, Page23. National Airspace System. . . . . . . . . . . . . 6924. Major AERA Functions. . . . . . . . . . . . . . . 8325. Comparison of Microwave Landing

System and Instrument LandingSystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Chapter 5

TECHNOLOGY AND THE FUTUREEVOLUTION OF THE ATC SYSTEM

INTRODUCTION

The present air traffic control (ATC) systemhas evolved over several decades from the onethat was first put in place in the 1930’s. The op-erational characteristics and organization of theoriginal system were determined largely by thetechnologies then available—radio for naviga-tion and air/ground communication, and tele-phone and teletype networks for distribution ofinformation among ATC ground facilities. Newtechnologies—such as surveillance radar,Air Traffic Control Radar Beacon System(ATCRBS) transponders, microwave relays, andelectronic data processing—were added as de-mand increased and the state of the art pro-gressed after World War II, but they did notchange the essential characteristics of the earliergeneration of air traffic control—a ground-based, labor-intensive, and increasingly cen-tralized system.

Advanced data-processing and communica-tion technologies have been introduced to meetthe growing demand for ATC services* and toprovide the controller with the informationneeded to make the decisions required for thesafe and efficient movement of aircraft. How-ever, these technologies were applied largely toimprove the acquisition, integration, and dis-play of information, or to speed its dissemina-tion among ATC facilities. Recently, the auto-mated transmission of certain types of informa-tion to pilots has also been introduced, e.g.,weather and terminal area briefings. However,the making of ATC decisions and transmissionof ATC messages have remained essentially ahuman function.

As the air transportation network grows andevolves in response to economic conditions,

● These technologies have also found use in the cockpit whereRNAV and other systems have provided capabilities that have in-directly affected the ATC system.

market forces, and changing Government regu-lation, the requirements for the ATC system willbe affected in turn. In addition, new technologi-cal developments will make possible new func-tions and modes of operation that would havebeen impossible with older equipment and re-sources. The extent to which the system mustgrow depends primarily on the rate at which thelevel of air traffic and the demand for ATC serv-ices increase. There is considerable uncertainty

on this score. The direction in which the systemevolves will be influenced by what services areoffered, how they are delivered, and how theyare paid for. The answers to these questions,too, are subject to great uncertainty. Budgetaryconstraints and the continuing effects of the airtraffic controllers’ strike have introduced furthercomplications. In addition, the evolution of theATC system takes place slowly: some of themodernization programs now reaching fruitionwere first conceived a decade or more ago. Dur-ing this period new technologies have becomeavailable, and there has been continuing contro-versy regarding the technical choices that willdetermine the character of the future ATC sys-tem.

This chapter presents an overview of some ofthe technologies and technological issues thatare of concern in decisions that will soon bemade about the future development of the ATCsystem. It is not a detailed treatment of thetechnological and engineering complexities ofthe subject, nor does it attempt to resolve any ofthe related economic and funding controversies.Instead, this discussion is intended to providedecisionmakers and the public with useful infor-mation about the implications of some of the ad-vances in technology that have occurred or

which are on the horizon. This informationforms a background against which to assessFAA’s 1982 revision of the National AirspaceSystem (NAS) Plan.

67


GOALS AND SERVICES OF THE ATC SYSTEM

In order to accomplish the goals of safety, effi-ciency, and cost-effective operation, the presentATC system offers the following services to theaviation community:

●

●

●

●

●

separation assurance—tracking aircraft inflight, primarily with surveillance radars onthe ground and airborne transponders, inorder to ensure that adequate separation ismaintained and to detect and resolve con-flicts as they arise;navigation aids —maintaining a system ofdefined airways and aids to navigation andestablishing procedures for their use;weather and flight information— informingusers of the conditions that may be expectedalong the intended route so they may plan asafe and efficient flight;traffic management-processing and com-paring the flight plans, distributing flightplans to allow controllers to keep track ofintended routes and anticipate potentialconflicts, and ensuring the smooth and effi-cient flow of traffic in order to minimizecostly congestion and delays; andlanding services-operating airport controltowers; instrument landing systems, andother aids that facilitate the movement ofair traffic in the vicinity of airports and run-ways, particularly during peak periods orbad weather that might affect safety orcapacity.

These services together comprise an integratedprogram, no part of which can be fully effectivewithout the others. Flight plans must take intoaccount weather and traffic, for instance, andtraffic must be routed to destinations so that itarrives on time and can be handled at the airportwith a minimum of delay. Similarly, clearances

MAJOR COMPONENTS OF

The present ATC system can be divided intotwo major subsystems: en route and terminalarea (see fig. 23). The en route subsystem is pri-marily concerned with aircraft moving along the

have to be modified so that traffic can be routedaround severe weather or away from bottle-necks that develop in the system. In a practicalsense, the aircrew and ground controllers co-operate as a team using various human and elec-tronic resources to maintain safety and to movetraffic expeditiously. While the ultimate respon-sibility for safety of flight rests with the pilot, heremains dependent in many ways on data or de-cisions from the ground.


One of the Nation’s first air controllers—1929

THE EXISTING ATC SYSTEMairway network, generally cruising at higher al-titudes. To an increasing degree, it is also con-cerned with traffic flying point-to-point withoutfollowing the airway network. The terminal

Ch. 5—Technology and the Evolution of the ATC System ● 6 9


area subsystem handles aircraft flying at lowerspeeds and altitudes as they arrive at and departfrom airports, but it must also control IFR trafficthat is passing through a terminal area withoutlanding. * The major equipment componentsthat support these ATC facilities are surveillanceradar, airborne transponders, navigation aids,computers, and communication links.

Surveillance Radar

Two types of radar are used for the surveil-lance of aircraft. Primary surveillance radar(PSR) uses the return from the aircraft structureto determine range and bearing. Secondary sur-veillance radar (SSR), triggers a response fromaircraft equipped with an ATCRBS transponderand is able to obtain, in addition to range andbearing, the aircraft’s identity and altitude.**Because the transponder enhances the returnfrom primary radar, it improves the controller’sability to track individual aircraft. SSR is theprincipal aircraft surveillance tool of the ATCsystem; PSR is used as a backup for SSR and forlong-range weather data.

Airborne Transponders

The returns to surveillance radar vary consid-erably with range, aircraft structure, back-ground clutter, weather, and several other fac-tors. In addition, the present radar system doesnot permit aircraft altitude to be determinedfrom the ground on the basis of raw return fromprimary radar. This makes it difficult to trackspecific aircraft using a reflective return alone,although computer processing can be used toisolate a moving aircraft from background clut-ter. Transponders are radio transmitters de-signed to respond to ground interrogation with astrong signal that can easily be distinguishedfrom a purely reflective return. The groundequipment and airborne transponders constitutethe ATCRBS.

● In a terminal control area, all traffic is controlled by the ATCsystem.

**Altitude data is available only from aircraft equipped with atransponder having Mode C and an encoding altimeter. Onlyabout one-third of the transponder-equipped aircraft have altitudereporting capability.


Air control in the 1940’s using table top plots


Air control using a modern console

Current procedures require that all aircraftoperating in the busiest terminal control areas(TCA), or flying above 12,500 ft must beequipped with a transponder capable of report-ing both an aircraft identification code and alti-tude, Modes A and C, respectively. These de-vices respond to a Mode C inquiry from an

Ch. 5—Technology and the Evolution of the ATC System ● 71

ATCRBS interrogator by giving the altitudeof the aircraft, reported to the nearest 100 ft assensed by an onboard barometric altimeter.Transponders also have the ability to transmitone of 4,096 different identity codes in responseto a Mode A query from an ATCRBS interro-gator. Under Instrument Flight Rules (IFR) thecode to be used is specified for each aircraft bythe ground controller; for all VFR aircraftequipped with a transponder a common iden-tifier code (1200) is used. Some blocks ofnumbers within the 4,096 identity codes arereserved for classifying traffic such as coast-to-coast flights. Other codes have been set aside foremergency purposes—aircraft that have lostradio communication, aircraft in distress, or hi-jacked flights.

Navigation

Navigation aids are another important ele-ment of the ATC system. Although they are nottraffic control devices per se, they do have an in-fluence on the structure and the operation of thesystem. * As described in chapter 3, the primaryradio navigation aid is the very high frequencyomnidirectional range (VOR) system that oper-ates in the VHF band immediately below the fre-quencies used for voice communication. VORground stations provide coverage of nearly allthe continental United States and adjacent off-shore areas, and most aircraft that have commu-nication transceivers also are equipped to useVOR for navigation. VOR equipment enablesthe aircrew to determine the bearing to theground station. Distance measuring equipment(DME), colocated with VORS, emits signals thatallow the aircrew to determine the distance tothe station as well. A station where VOR andTACAN, the military navigation system that isfunctionally equivalent to VOR/DME but moreaccurate, are colocated is called a VORTAC.Other navigation systems that are available arelisted in chapter 3.

Many large commercial transports, militaryaircraft, and a growing number of corporate,

*For example, aircraft will follow radials to or from VOR sta-tions. This tends to add some order to the flow of traffic even if itis not operating within the ATC system.

general aviation (GA) aircraft are equipped withinertial navigation systems (INSs) that permitthem to navigate without primary reference toground-based radio transmitters. INS-equippedaircraft are not completely independent ofground aids since VOR/DME, LORAN-C, orOMEGA navigation signals are used for periodiccrosschecks of INS accuracy and realinement ofinertial platforms.

A growing number of commercial and GA air-craft are being equipped with navigational com-puters that enable them to operate off VOR-de-fined airways along direct origin-to-destinationroutes. This capability for area navigation(RNAV) can be achieved either with an INS orwith equipment that uses VOR/DME, OMEGAor other navigation aids as the primary refer-ence. The ability to fly RNAV makes it possibleto achieve considerable savings in time and fuelconsumption, and also allows aircraft to avoidthe congestion that sometimes occurs at VORairway intersections. FAA has begun publishing

RNAV routes for use by suitably equipped air-craft. At present, however, controllers grantdirect clearances only to the extent that they donot conflict with traffic along airways or affectadequate separation. While FAA is making aneffort to accommodate the increasing demandfor RNAV clearances, there are still cases inwhich the limitations imposed by the presentVOR airways system prevent users from realiz-ing the full benefit of installed RNAV equip-ment.

Computers

Computers are used extensively throughoutthe ATC system to process flight plans, to corre-late radar and transponder returns, to filter outextraneous signals that could obscure controlledaircraft, and to generate displays on the control-ler’s console. All control decisions, however, aremade by human operators. In the busiest ter-minal areas, an ARTS II or ARTS III computersystem combines SSR data and flight plan infor-mation to create a display on an analog terminal(see fig. 23). Displayed alongside the position in-dicator for each aircraft is a data block that in-cludes the transponder code of the aircraft, itsaltitude and groundspeed, and the aircraft regis-


tration number or flight designation to the ex-tent that they are available, For example, noneof these data will appear for an aircraft flyingVFR without an operating transponder unlessthey are entered manually.

The principal computer used at the 20 en routeATC centers is the IBM 9020, an assemblage ofIBM 360 components that have been modifiedfor ATC applications. The technology incorpo-rated in these machines is of 1962 vintage, andthere have been considerable advances in the de-sign and construction of computers since theywere first built and installed. The IBM 9020s aretied to either IBM or Raytheon digital displaysubsystems that present radar surveillance andclearance information in a brighter, sharper im-age than the analog displays used in the terminalcontrol facilities. In addition to driving the con-troller displays, the IBM 9020s also handle com-

munications with computers in other en routecenters and terminal area control facilities aswell as other tasks such as flight plan processing.

In case of an IBM 9020 failure, the controllershave a backup system, called Direct Access Ra-dar Channel (DARC), that digitizes the raw datafrom the secondary surveillance radar to create acomparatively clean image on the control con-soles. However, to use DARC, the controllersmust manually shift their display screens fromthe vertical to the horizontal position and makeplastic markers (“shrimp boats”) to identify thetargets on the screen, because the DARC systemcannot obtain the clearance data from which togenerate a display of the aircraft call sign or in-tended route. If the DARC system is inoperable,controllers have a second backup, a broad-bandsystem that displays radar data without com-puter enhancement and thus provides no data

Photo credit: Mitre Corp.

Computers for air traffic control system for aircraft en route

Ch. 5—Technology and the Evolution of the ATC System • 73

block for individual targets. FAA has indicatedthat it plans to remove the broad-band capabil-ity when sufficient operational experience withDARC has been established.

FAA is considering the option of installingcompatible computer and display systems in theen route and terminal area control facilities. Ifthis were done, much of the line of demarcationbetween these classes of facilities could be re-moved.

Communication

Communication is a key element in the pres-ent ATC system, and advances in communica-tion technology may open new options for con-figuring the system in the future. Historically,voice radio has been the primary and almost ex-clusive means of communicating between air-craft and the ground. Digital communication—the transmission of data in the form of machine-readable binary signals—has come into use forlinking ground stations (particularly for com-puter-to-computer interchanges), but it has notyet been applied for air-ground messages, exceptin the limited case of transmitting aircraft iden-tity and altitude by means of ATCRBS trans-ponders. In the future, it is expected that an air-ground digital data link will play an increasinglyimportant role as the automation of ATC func-tions requires more direct communication be-tween airborne and ground-based computers.

Another important advantage of the digitaldata link is that it permits messages to be trans-mitted selectively. The present voice-radiomethod is broadcast—i. e., available to any andall aircraft equipped with an appropriate receiv-er, regardless of the intended recipient. This

“party line” feature has certain advantages, sinceit permits pilots to develop a sense of what ishappening in the surrounding airspace. Never-theless, a “discrete address” technology that per-mits messages to be sent to a specific recipientcan be more effective than broadcast for proc-esses that require computer-to-computer com-munication. This is the underlying principle ofthe Mode S data link (formerly the Discrete Ad-dress Beacon System, or DABS), which is an im-portant building block in FAA’s plans for futuresystem development.

In the future, with the introduction of a digitaldata link capable of selective address, two dis-tinct modes of communication can be expected.Broadcast, the mode now used, will continue forvoice or digital transmissions of general interest,such as weather, airport status, and traffic ad-visories. Other transmissions, pertinent only tospecific aircraft, will be sent by a discrete-address digital data link that allows isolation ofspecific receiving stations. However to the ex-tent that communication relative to position andintent uses a discrete address data link ratherthan broadcast, the side benefits of the party linewould be diminished.

The application of a digital data link is notlimited to air-ground communication; it couldalso be used for exchange of messages betweenaircraft. For instance, most of the air-to-air com-munication in proposed collision avoidance sys-tems would be digitized; and by allowing air-borne computers to direct messages to specificaircraft, maneuvers intended to resolve conflictscould be coordinated between aircraft. Alterna-tive plans for the implementation of a digitaldata link are discussed later in this chapter.

FUTURE REQUIREMENTS, OPPORTUNITIES, AND CONSTRAINTS

Future Requirements ity of applying them to achieve greater effective-ness of the ATC system through higher levels of

The evolution of the ATC system will be in- automation. In many cases there are severalfluenced by changes in user demand, market ways of meeting specific needs, and the choice offorces, and regulatory policy, as well as the which path to take will reflect a combination ofavailability of new technologies and the possibil- technological, economic, and policy considera-


tions. In general, however, prospective changesin the system will be dictated by three relatedtechnical requirements:

●

●

●

replacement of obsolete equipment, whichwill become increasingly difficult to main-tain and repair, with more modern equip-ment that offers higher reliability and mightalso provide greater flexibility, higher capa-city, or lower costs;increase of system capacity in order to ac-commodate growth when and where it oc-curs, by improving the management of ex-isting resources where feasible and by add-ing new resources where necessary; andaddition of new capabilities in order to sup-port improvements in efficiency and pro-ductivity by automating more functionsand by introducing features that make itpossible to take advantage of improvementsin avionics and other newly available tech-nologies.

Advances in technology have increased thenumber of options that could meet these require-ments. Computers will probably assume roles ofincreasing importance, both in the air and on theground, because they present opportunities toincrease efficiency, productivity, or capacity byrelieving human participants in the system ofroutine tasks, by facilitating human decisions,and by improving the timeliness and quality ofinformation. As a result, the human operator’srole will become more that of a manager of sys-tem resources than that of a direct controller ofaircraft. Communications will also be a criticalelement, and digital communication betweenmachines (computers and various avionic de-vices) will be at least as important as voice com-munications between humans. Future systems,therefore, may have to provide for one or morehigh-speed data links of sufficient capacity tohandle the large volumes of data and messagesthat will be generated. Collision avoidance willreceive increasing attention as the volume oftraffic grows, and both navigation and landingaids may need to be upgraded in order to main-tain safety and improve the efficiency withwhich airways and airports are utilized. Specifictechnical options for each of these functions arediscussed in later sections of this chapter. The

more general opportunities created by advancedtechnology are discussed below.

Technological Opportunities

The development of microelectronics has beena primary source of expanded technological op-portunities for the ATC system. Data-processingcapabilities can now be tailored to meet virtuallyany computational requirement, hardware costshave fallen significantly, and reliability contin-ues to increase. The ATC system as presentlyconstituted is highly labor-intensive; and sincethe PATCO walkout, the system has been keptoperating with a greatly reduced work forceonly by administratively limiting traffic. Someobservers have suggested that the current situa-tion presents an opportunity to review the basicstructure of the system and to apply new tech-nology so as to make it less labor-intensive andless dependent on (or vulnerable to) the actionsof any specific group within the work force.

Computer software figures prominently in thepresent ATC system and will have an even moresignificant role in the future as the need for newcapabilities expands. Many systems related tothe safety of flight, both ground based and air-borne, will be “software driven, ” in that theprocessing of sensor data and the generation ofdisplays will be more dependent on computerprograms than is now the case. Processes run-ning on different computers will communicatedirectly with one another. There will thus be aneed for systems with the ability to identify er-rors and to take compensator action automat-ically. * Present ATC software uses a combina-tion of computer languages, but new high-levellanguages that are now available (like those usedfor military command and control) and thosethat will be developed in the future may make iteasier and cheaper to implement, modify, andmaintain ATC software.

Commitment to a highly automated mode ofoperation is not without risk. When there is acomputer failure in the present system, the con-trollers can revert to manual methods and keeptraffic flowing. However, experiments with

● Systems with this type of capability are within the state of theart and some are available “of f-the-shelf.”

Ch. 5—Technology and the Evolution of the ATC System . 75

more highly automated systems have shownthat traffic levels can reach a point that, al-though well within the capabilities of the auto-mated system, is beyond the point where theycan be handled manually. At these traffic levels,controllers experience considerable difficulty inreverting to manual operations during computeroutages. ’ This suggests that even though com-puter technology offers promise for the future,there may be a point of no return beyond whichthe commitment to automation is absolute—theonly backup system for a highly automatedATC system is another highly automatedsystem.

Decreasing size and costs of computers alsomean, however, that data-processing capabilitycan be located anywhere in a system, and thatredundancy can be provided where exceptional-ly high degrees of reliability are required. Micro-

1Leonard Tobias and Paul J. O’Brian, “Real-Time Manned Sim-ulation of Advanced Terminal Area Guidance Concepts for ShortHaul Operations, ” Ames Research Center, August 1977, NASA-TN-D-B499.

The experiments, conducted in 1971 at the Ames Research Cen-ter and jointly sponsored by FAA and NASA, were to deter-mine the comparative utility of 3-D and 4-D RNAV as aids to fly-ing landing approaches. Controllers and pilots were placed in asimulated high traffic environment and required to control trafficand fly approaches in STOL [short takeoff and landing] aircraftequipped with the two onboard navigation aids. Results showedthat the performance of both controllers and pilots improvedalthough the controllers were only secondary beneficiaries of theequipment installed in the aircraft.

Generally, when the effects of 4-D RNAV were compared withthose of 3-D RNAV, two types of effects were observed. The firstrelated to improvements in the effectiveness of both pilots andcontrollers. Pilots were able to fly a better track when assigned aroute and a time to arrive at a checkpoint. The range of deviationsof arrival time at a checkpoint from the time assigned droppedfrom about 4 minutes to about 30 seconds. Requirements for voicecommunication between controllers and aircraft under their con-trol were cut by more than half. Traffic flows were more orderly,and the number of aircraft in the system increased by about 25 per-cent.

A second major conclusion was that there maybe a point in thedevelopment of automated systems beyond which it is no longerpossible to return to a manual back-up. At higher levels of traffic,it was more difficult for controllers to make the adjustments re-quired to handle an emergency and restore traffic once the emer-gency had been resolved. Controllers expressed a definite need formore automated support for handling emergencies and restoringtraffic afterwards.

From this it seems that automated systems may have to be builtso that they are self-diagnosing, self-correcting and/or backed upwith other automated systems. Such back-up systems may not of-fer all of the features of the primary system but would be adequatefor an interim period while repairs to the primary system are un-derway.

processors have become integral elements of air-craft instrumentation, and modern aircraft canand do carry general-purpose computers thatcan be used for a variety of applications, such asflight management, processing digital communi-cations with the ground or other aircraft, up-dating the navigation system, developing alter-native flight plans, or driving multifunctioncockpit displays that replace several electrome-chanical instruments. The introduction of theseairborne capabilities means that ATC functionsneed no longer be wholly resident in ground-based computers. As a result, it might be possi-ble to improve system operation and safety byredistributing these functions among the variousparticipants in the ATC process. Many of thesefunctions will be critical to the safety of flightand, therefore, the computer based systems thatperform them will fall within the airworthinesscertification program of FAA.

An ATC system that places more informationand functions in the cockpit will also requirechanges in communication technology. As ATCautomation becomes more widespread and moreintegrated into the system, digital data commu-nication will come into greater use. Transmis-sions directed to a specific receiver—the princi-ple underlying the proposed Mode S data linkdescribed later—would facilitate communica-tion between ground-based and airborne com-puters. They would also allow a computer tocontinue with other functions once it determinesthat it is not the intended recipient, a feature thatincreases the effective capacity of a processor.The capacity required for data links betweenground facilities would also need to increase.While telephone and other ground links are usedat present, point-to-point satellite channelsmight provide an alternative in the future.

Satellites could also be used for aeronauticalnavigation and surveillance. Singly or in con-stellations, satellites with accurate sensors andcomputing capabilities can be used to determineaircraft position and relay the information toother aircraft and ground stations. Satellite-based collision avoidance systems have beensuggested. Large satellites that support a numberof functions are also being considered for civilaviation, notably in the Aerosat system of the


European Space Agency. The reliability andlongevity of satellites are high and likely to in-crease in the future. The space shuttle makes itpossible to recover, refurbish, and relaunchsatellites, or even make repairs while in orbit.However, the leadtimes for scheduling shuttlepayloads will preclude its use in respondingrapidly to unforeseen emergencies. In addition,frequencies and orbital slots are limited andATC applications must compete with other po-tential users of space technology, Both NASAand FAA have spent considerable amounts onR&D for ATC satellite applications, but no sig-nificant U.S. program is currently under way.Much of the required technology is available,but it has not yet reached the point of being acost-effective alternative to ground-based ATCfacilities and configurations. At some point inthe future, however, this may change and theoption of using satellites in ATC applicationsmay have to be reevaluated.

Constraints and Other FactorsAffecting Future Evolution

Continuity of Service

ATC is an ongoing activity that cannot be in-terrupted while a replacement system is put intoplace. Any changes in the system must thereforebe implemented gradually, and new and oldequipment will have to be operated in parallel toassure continuity of service throughout the tran-sition period. FAA can reduce the length of thistransition period by mandating equipage by cer-tain dates. If installation is voluntary, however,some users will hold off replacing existing equip-ment until it wears out, and some users mightnever make the change. At a minimum, paralleloperation will be needed for perhaps as long as adecade while users install new equipment. Insome cases, it could be in the best interest of allparties to establish a firm date on which existingservices will terminate and by which all userswill have to be equipped to use the new service.

Timing of Design Decisions

and System Implementat ion

Identifying future needs and installing the fa-cilities to meet those needs take a considerable

amount of time. In periods of rapid technologi-cal progress, new equipment or facilities maybecome obsolescent before the implementationphase is completed. Redesigning the system toincorporate newer technologies, however, maytake so long that a badly needed function re-mains unavailable, or that a deterioratingsystem is kept in place long after it has becomeinadequate. At some point, therefore, the deci-sion to go ahead with system enhancementsmust be made, despite the realization that the in-corporation of newer technologies will have tobe deferred until a later cycle of systemmodifications.

The design and development of some prospec-tive ATC systems and facilities began over 10years ago, and it will be late in the present dec-ade or early in the next before implementationcan be completed. A substantial portion of theneeded ground facilities would have to be in-stalled before users would begin to install the re-quired equipment on their aircraft, since theywould see little benefit in spending money forequipment before it is of practical value. Therate of installation of airborne componentswould also be limited by the rate at which theycan be produced, and the avionics industrywould be unlikely to commit to production untilit foresees a market of sufficient size to assureprofitability.

User Costs

FAA is responsible for the design, procure-ment, installation, operation, and maintenanceof equipment used in ATC installations and forestablishing standards for the equipment to becarried on aircraft. However, the responsibilityfor and costs of procuring, installing, operating,and maintaining the airborne equipment restswith the users. Any adverse impacts on aircraftperformance resulting from the installation ofairborne equipment also translates into in-creased user costs. Decisions about changes tothe ATC system must consider these user costsand the effect that required equipment mighthave on aircraft performance.

Large aircraft have the space to accommodatenew avionics, but in small GA aircraft or dense-ly packed tactical military aircraft space is at a

Ch. 5—Technology and the Evolution of the ATC System . 77

premium, and room for additional equipment tomeet the needs of the ATC system may be hardto find. Antenna location, in particular, ofteninvolves a tradeoff between aerodynamic andelectromagnetic characteristics. For instance, thesmall blade antenna used for a standardATCRBS transponder has little effect on aerody-namics, but the larger direction-finding antennasrequired for some collision avoidance systemsmay adversely affect aircraft performance oreven structural integrity when retrofitted intoexisting aircraft.

Not all new functions require the replacementof existing equipment. Some experts suggest thatthe capabilities in existing equipment are amplefor future needs and that new or upgradedequipment is not required. Some entrepreneurshave been successful in adapting existing equip-ment to new purposes without making any fun-damental changes. RNAV, as mentioned, usesVOR/DME signals and existing receivers to ob-tain the data required for navigation outside thedefined system of airways. Tri-Modal BCAS, acollision avoidance system, is designed to oper-ate with the installed ATCRBS transponders andinterrogators.

In addition, the ATC system serves a broadmix of users who operate aircraft having a widerange of performance characteristics and whouse the airspace for a variety of purposes. Overhalf of all air operations are not under the con-trol of FAA terminal and en route facilities, butthe ATC system must recognize the existence ofthese “off system” activities so that the availableairspace and airport facilities are used in a safe,efficient, and equitable manner. The heterogene-ity of the user mix complicates both the designand the implementation of new systems, and theGA community is particularly sensitive to theissues of user costs and mandatory equipage.

Locus of Decisionmaking

Decisionmaking in the ATC system is distrib-uted between ground controllers and aircrew.Ultimate responsibility resides with the pilot,but controller-supplied services are particularlyimportant in high-density traffic and at times ofpoor visibility. Some pilots feel that the amount

of ground control is becoming excessive and thatthey are burdened with the responsibility ofoperating the aircraft safely without havingavailable the information required to meet thatresponsibility. Technologies now available orunder development could make additional infor-mation available to both ground controllers andaircrew and might permit redistribution of thedecisionmaking function. These alternative con-cepts have not yet been validated and tested; butthey could lead to an ATC system that is less de-pendent on ground-based equipment and con-trol decisions.

Freedom of Airspace and Equipage

The passage of time has also brought increas-ing limitations on the amount of airspace avail-able for VFR operations. The GA community

(traditionally vocal in this matter) has beenjoined by the military services, who believe thataccess to suitable training areas is becoming ex-cessively restricted. The airlines, faced with highfuel prices and low profitability, have also ar-gued that they should be permitted to fly themost fuel-efficient routes possible betweenpoints served.

While FAA has not required all aircraft to beequipped to participate in the ATC system, ithas imposed limitations on the operations of air-craft lacking specific pieces of equipment. Whilethe requirements are still minimal, freedom ofairspace is already directly affected by theamount of avionics an operator is able and will-ing to install on an airplane. As more airspacebecomes congested, the areas in which unre-stricted VFR flight is permitted may have to bereduced, or some other method be found to as-sure separation and preserve safety of flight. Itmay not be possible in the future to permit thesome degree of flexibility and freedom of air-space use that has been accorded in the past to

those operating outside of positive control bythe ATC system.

International Requirements

The United States is party to a number of in-ternational agreements that affect the operationof the air transportation system. It is legally obli-


gated to provide ATC services that conform tointernational standards at gateway facilities un-less airspace users are notified that particular ex-ceptions are taken to the applicable agreements.Foreign-flag carriers enter U.S. airspace at gate-way facilities with the understanding that theywill receive full services if they are equipped inaccordance with international standards. U.S.aircraft similarly expect a full range of servicesfrom foreign controllers. There is no legalobligation to operate the domestic ATC systemin conformity with international standards,although many nations (including the UnitedStates) find it desirable to do so.

Two international bodies establish standardsthat affect aeronautical operations. The Interna-tional Civil Aviation Organization (ICAO) pro-mulgates standards that establish flight proce-dures and aircraft equipment specifications. Forexample, one ICAO standard governs the signalformat used by each mode of the ATCRBStransponder. Mode S, the signal format of theDABS data link, is currently being consideredfor establishment as an ICAO standard, withoutwhich it cannot be implemented for interna-tional operations.

The second organization, the InternationalTelecommunication Union (ITU), also estab-lishes conventions that affect aeronautical oper-ations, but the relationship is not as close as thatof ICAO. ITU assigns portions of the radio fre-quency spectrum to various applicationsthroughout the world. The spectrum is a finiteresource, and competitition among alternative

applications is intense. Aeronautical radio hasbeen assigned bands that are of sufficient capac-ity to meet present needs, but it may be difficultto obtain additional spectrum allocations fornew aeronautical applications in the future.However, it may be feasible to reduce the chan-nel spacing in bands that are currently allocatedand thus increase total effective capacity. Onearea where there is significant pressure is in theallocation of spectrum to satellite applications;and this may be a factor that could limit the de-velopment of ATC services that use satellites.2

Military Requirements

The ATC system will be constrained by na-tional security considerations. In time of war thesystem must meet the needs of the military with-out aiding an enemy in locating and hitting tar-gets in the United States. In addition, ATCequipment and facilities must not compromisethe operational integrity of military equipment.The military is a full participant in the ATC sys-tem, and FAA is charged by law with ensuringthat the system meets both civil and military re-quirements. Some arrangements for coordinat-ing the activities of FAA and the Department ofDefense (DOD) have been established, but thesehave not been completely formalized.

‘For further information on this subject see OTA’S assessment,Radio frequency Use and Management: impacts From the WorldAdministratiz]e Rudio C o n f e r e n c e o f 1979, OTA-CIT-163(Washington, D. C.: U.S. Government Printing Office, January1982).

TECHNICAL OPTIONS

En Route Computer Replacement

The computer now in use at en route ATCcenters is the IBM 9020, a designation given to aderivative of the IBM 360 line that has been spe-cially modified to perform ATC functions. Al-though the IBM 9020 was first commissioned byFAA in 1974, it incorporates a technology that isclose to 20 years old. It has less speed and capac-ity, is less reliable, requires more energy and

floor space, and is not as easy to maintain asmore modern computers that could be used insupport of the ATC system.

Growth in the demand for ATC services hasexceeded the data-processing capability of theIBM 9020. Some ARTCCs are already operatingat capacity, while others are expected to reachcapacity later in this decade. Alleviating capaci-ty problems by acquiring additional IBM 9020


computers is not a practical alternative, since theIBM 360 has been out of production for severalyears. Buying used IBM 360s and modifyingthem to make them IBM 9020s would be expen-sive in the short term and would provide, atbest, only a stopgap solution.

The reliability of the IBM 9020 hardware andsoftware has also been troublesome, giving riseto concern that the cost of repairing and main-taining the system will become excessive. Astime passes and existing stocks of spare parts areexhausted, maintenance of the computers couldbecome very expensive because spares wouldhave to be fabricated to order. Similarly, thetask of modifying and maintaining software tomeet evolving needs is likely to be increasinglydifficult to perform. In the future, it will be diffi-cult to recruit and retain programmers capableof maintaining the software because those whoare best able to do this job prefer to work onmore modern equipment. Further, there is ampledemand for their talents outside of FAA.

FAA is now in the process of planning the pro-curement of a replacement computer system thatwill overcome present operational problems andprovide additional capacity to meet the needs ofthe en route centers during the last decade of thiscentury and into the next. Plans are to use the in-creased capacity of the replacement computersto provide a variety of new and improved serv-ices, as well as to satisfy the requirements gen-erated by the anticipated increase in aviation ac-tivity. Table 6 indicates the range of services andactivities FAA expects to support with the re-placement computer system. These applicationsfall in three major areas: control of individualaircraft, conflict alert and resolution, and man-agement of traffic flow.

The basic technical issue is not whether the9020 system needs to be replaced—there is wideagreement that it does—but what replacementstrategy should be pursued.

There are many strategies for replacing theIBM 9020s, but all can be placed in one of threegroups:

• replace all hardware and software simulta-neously;

●

●

Thethat

place initial emphasisthe hardware; orplace initial emphasisthe software.

first strategy—total

on the replacement of

on the replacement of

replacement— impliesthe present system, with minor modifica-

tions needed to keep it operating, will be kept inplace until the replacement hardware and soft-ware are ready for commissioning. The lattertwo strategies are incremental approaches thatprovide for a transition to the new system incomparatively small steps over an extended peri-od. Some believe that either of these strategies, ifsuccessful, could provide relief from the mostpressing problems within a period of 3 to 5years, as opposed to the more than 8 years re-quired for the total simultaneous replacementoption.

The en route computer replacement strategy

has been reviewed as part of the FAA effort toproduce a revised NASP. Implicit in past FAAstatements is the presumption that the replace-ment computer, like the IBM 9020s, would haveto be uniquely designed for ATC applications.Critics of the full replacement strategy have putforth options that would effect the replacementof the computers incrementally.3 Generally,these plans envision using off-the-shelf equip-ment to replace the IBM 9020s rather than ob-taining a computer that has been designed ormodified specifically for ATC applications.

Total Replacement

The total replacement strategy has much torecommend it. First, FAA has learned from itsexperiences with the present system and, giventhe opportunity to make a fresh start, would bein a position to design a replacement that wouldcorrect present weaknesses. Second, advances inhardware, software, and communication tech-nologies have created new options that were notavailable when the present system was installed.A complete replacement of the present system

‘See, for example, FAA Air Traffic Control Computer Moderni-zation, Hearings before the Subcommittee on Transportation,Aviation, and Materials of the Committee on Science and Tech-nology, U.S. House of Representatives, June 16-18, 1981.


Table 6.—Perform ATC Automation Processes

Sustain ATC system operation . . . . . Assemble system information:● Acquire or negotiate decisions● Collect and analyze system status information

Calculate state of ATC system:● Calculate system load● Predict system state

Resolve management actions:● Resolve differences in system state and decisions. Translate resolutions into automation directives

Manage ATC automation processes performance:● Formulate required processes actions● Monitor processes status and performance● Monitor plan status and performance

Perform ATC planning processes . . . Assemble planning information:● Assemble trajectory information● Assemble flow information● Create multidimensional profile

Identify strategic planning problems:● Predict strategic delays● Predict long-term conflicts

Resolve strategic planning actions:• Absorb strategic delays● Resolve long-term conflicts

Issue strategic planning actions:● Formulate clearance plan

Perform ATC controlling processes . Assemble control information:● Assemble control information● Convert to appropriate reference● Apply control conditions

Identify control problems:● Predict short-term AC/AC conflicts● Predict environmental conflicts● Detect track/trajectory deviations

Select control actions:● Assess “accept/handoff” situations● Resolve tactical situations● Generate clearances

Control ATC system:● Perform aircraft accept/handoff● Deliver clearances● Deliver advisories

SOURCE: ATC Computer Replacement Program System Level Specification (Preliminary). En Route ATC Automation System.FAA-ER-130-003, May 1981 (draft).

offers the opportunity to explore all of these op-tions fully and to select the one that best suitsATC requirements in terms of both technicalcharacteristics and overall system productivity.

On the other hand, the total replacement op-tion would do little or nothing to relieve the defi-ciencies of the present system in the short term.If procurement were to start immediately, it isunlikely that the first replacement computerswould be in operation before the end of the dec-ade. In the interim, the IBM 9020s would have to

be kept in operation to meet the ongoing needsfor ATC services—a task that could become in-creasingly difficult and costly.

Critics of FAA have pointed out that the num-ber of interruptions to service experienced withthe present computers constitutes a threat to thesafety of flight. ’ A more recent study by the Na-

*Air Traffic Control Computer Failures, Committee on Govern-ment Operations, U.S. House of Representatives, House ReportNo. 97-137, June 11, 1981.


tional Transportation Safety Board5 indicates asignificant decrease in the number of computeroutages since the controller strike in the summerof 1981 due in part to the subsequent reductionin the level of traffic. Concern with the reliabil-ity of the ATC computers remains, however,and FAA has pointed out that some of the enroute centers were approaching capacity limitsat the time of the strike. This last considerationwould favor a conversion strategy that will havea positive short-term effect on en route trafficcapacity.

Hardware= First Replacement (“Rehosting”)

Either of the alternative strategies for the in-cremental replacement of the computers entails anumber of assumptions about the structure andoperational characteristics of the present system.For example, a proposal to move some of thefunctions from the IBM 9020s to an auxiliarycomputer assumes that it is possible to isolatethe software elements that perform those func-tions from the rest of the IBM 9020 software. Aproposal to move the existing software to a newprocessor assumes that interface problems aris-ing from differences in the internal timing of themachines can be overcome. Such assumptionsare critical both to the feasibility of incrementalreplacement strategies and to the schedule andbudget to carry them out.

The second option—incremental replacementwith initial emphasis on substituting new hard-ware—would “rehost” or move the existing soft-ware to a new processor capable of supportingthe IBM 360 instruction set. Several manufac-turers produce machines with this capability,but in every case some modification of the exist-ing software would be required. * At a mini-mum, some allowance would have to be madefor handling the instructions unique to the IBM9020. Real-time applications, such as the ATCsoftware, are characteristically sensitive to thetiming of internal machine operations, and this

‘Air Traffic Control System, Special Investigative Report,NTSB-SIR-81-7 (Washington, D. C.: National TransportationSafety Board, December 1981).

*The ability to modify software rests on an understanding of theexisting structure and the procedure it executes in performing re-quired functions.

●

too could cause severe problems in rehosting thesoftware. There could also be problems in meet-ing the requirements of the interface between themain processor and the IBM or Raytheon sys-tems that drive the displays used by the control-lers. However, there are probably technical so-lutions to these problems given enough time andresources to work them out.

Even though there may be problems with re-hosting the existing software in a new processor,there are several points that recommend thisstrategy. Some suggest that this approach couldbe implemented by 1985. Second, once the con-straint of machine capacity has been relieved, itwould be possible to begin restructuring the soft-ware to improve its maintainability and reliabil-ity. Finally, the replacement computer could beselected with a view toward providing enoughadditional capacity to support the new functionsand services planned by FAA as part of longerterm improvements of the ATC system.

The “hardware-first” approach does not reston the assumption that the processor to whichthe ATC software is moved would necessarily bethe long-term replacement for the IBM 9020. Itcould be viewed as an interim replacement thatwould serve while FAA proceeded with a pro-curement program for a totally new hardwareand software package, to be commissionedaround the turn of the century and intended toserve well beyond the year 2000. On the otherhand, the procurement of an interim computerreplacement would involve a sizable investmentthat might, for budgetary reasons, effectively

foreclose the option of initiating a second roundof computer replacement after the interim sys-tem was put in place.

Software-First Replacement (“offloading”)

The strategy emphasizing the replacement ofthe software first would involve separating indi-vidual functions of the existing software. This ofitself would be beneficial, since it would make iteasier to maintain the existing software and pro-vide an opportunity to increase overall operat-ing efficiency. Weaknesses in the software thatare known to have contributed to service inter- “ruptions could also be corrected during this ini-


tial reworking of the existing software. Once thisinitial phase had been completed, the softwarecould either be rehosted intact in a new com-puter, or some functions could be offloadedfrom the IBM 9020 to another processor. Theoffloading approach would free capacity on theIBM 9020, allowing it to absorb increases inworkload due to higher traffic levels.

In the short run, this strategy makes no provi-sion for adding the new functions envisioned byFAA. However, as various functions are movedfrom the IBM 9020s to other processors, therewould in effect be an incremental replacement ofthe present computer. This would offer consid-erable latitude in specifying the replacementprocessor. It could be a large main-frame proces-sor to which elements of the ATC system couldslowly migrate. Alternatively, the migrationcould be to several smaller processors, so thatthe system would finally evolve into a networkof distributed, modular processors. Comparedto the hardware-first strategy, this one offers theopportunity to migrate to a system that has beenselected specifically to meet the requirements ofthe ATC application. Since the software wouldbe designed first, and then a computer con-figuration suited to supporting it selected, itwould be less likely that a second conversionwould be required or that the resulting systemwould be less than optimal in terms of its abilityto meet the long-term needs of the ATC system.

A potential disadvantage of this strategy,however, it that it depends on being able to sep-arate specific functions in the existing software.There are indications that the subroutines withinthe present ATC programs are strongly interde-pendent, and that it might therefore be very dif-ficult to modularize the present software system.If this is true, then it might be necessary to essen-tially rebuild the existing software in order toimplement this strategy; and the cost of doingthis could be prohibitive relative to other avail-able options.

Modularity and Other Concerns

The total system replacement strategy advo-cated by FAA in the past recognizes the need toreplace the controller displays and other periph-

erals, as well as the 9020 mainframe. ETABS, theelectronic display of flight strip information,and other display features planned for the con-troller suite require replacing not only the maincomputer but the computers that generate dis-plays as well. In addition, FAA is contemplatingeventual replacement of the ARTS II and ARTS111 computers now used in the terminal areas.

The ATC functions performed by computersin the en route centers and those performed inthe terminal areas are similar. Therefore, onemight consider procuring a computer for the enroute centers that could also be used in the ter-minal areas. Most manufacturers produce linesof compatible machines with a considerablerange of capacity. Thus, the concept of using asmaller version of the en route computer in theterminal areas could be attractive. In fact, such astrategy could reduce the overall costs of soft-ware maintenance for the ATC system becausethere would be fewer software packages in use.

At some point, FAA will incur the cost of re-placing the IBM 9020s now installed in the enroute centers. Operational factors create consid-erable pressure to begin doing so in the nearterm. However, once the initial conversion hasbeen completed, future steps to upgrade or tomodify the system could be accomplished at aslower pace. Manufacturers of computers gener-ally design them so as to provide paths by whichusers can upgrade capabilities incrementallywithout large-scale rebuilding of software. Suchavenues would be available to FAA in the futureso long as off-the-shelf hardware was selected toreplace the IBM 9020s. If, on the other hand, aunique processor were to be selected, it is likelythat second conversion—of a magnitude similarto the one now being undertaken—would be re-quired at some point in the future to supportnew ATC services and capabilities.

Automated En Route Air Traffic Control

Another factor influencing the selection of theen route computer replacement is its compatibil-ity with the long-term evolution of the ATC sys-tem. The future requirements and operationalcharacteristics of the en route portion of the


ATC system are currently defined by FAA underthe concept of automated en route air trafficcontrol (AERA).

The essence of FAA’s AERA concept is toautomate the functions of maintaining aircraftseparation, metering traffic flow, deliveringclearances, and transmitting ATC messages.These functions would be assigned to comput-ers, thereby relieving the controller of manyroutine tasks. The controller’s role would thenbe primarily to handle exceptions and emergen-cies and oversee (manage) the operation of auto-mated ATC equipment. Operationally, AERAwould perform four principal functions: 1) auto-matically produce a clearance for each aircraftoperating in positive control airspace that wouldensure a conflict-free, fuel-efficient flight path;2) formulate messages to aircraft needed to exe-cute the planned flight profile and to assure sep-aration; 3) transmit those messages by data linkor VHF voice radio; 4) and monitor actual flightmovements relative to flight plans, revising

Figure 24.–Major

those plans and clearances as necessary to ensurecontinued freedom from conflicts. Major AERAfunctions are summarized in figure 24.

As currently envisioned, AERA would be acontinuation and extension of the presentground-based ATC system. It could be imple-mented incrementally over an extended periodautomating first those functions that are mostroutine and repetitive for the human controller.Instructions to ensure separation and coordinatetraffic flow would still come from ground facili-ties. However, these instructions would be for-mulated and issued by computers operating

under the supervision of human controllers, Fur-ther, the control instructions would be derivedfrom a more extensive data base (geographically

broader and covering a greater span of time)than the present system. In effect, the AERAsystem would operate strategically—planning

overall traffic flow as well as individual aircraftmovements so that conflicts do not arise—al-though some form of tactical control would also

AERA Functions

Adjacentfacilities

I ICurrent Coordination Alerts Feedback Alerts

trajectoriesI

toi control I

— S u r v e i l l a n c edata



be provided in order to resolve potential con-flicts before backup collision avoidance systemswould be activated. G

While AERA would entail extensive ground-based data-processing capability, detailed analy-sis of aircraft flight plans, and close surveillanceof actual flight paths, it would not necessarilylead to undue restrictions on aircraft move-ments. As envisioned, AERA could in fact re-duce or eliminate many of the procedural con-straints now imposed on the use of airspace. Itwould be a system of management by exception,in which controller intervention would be lim-ited to situations (or localities) where conflictscould not be reliably resolved by computer rou-tines. The controller would not have to visualizeor direct overall traffic patterns, as in the presentsystem, because the AERA concept envisionsautomated planning, monitoring, and meteringof traffic flow in a four-dimensional region madeup of several airspace sectors over an extendedperiod of time. ’

Potential Benefits

Initial estimates of the benefits of AERA indi-cate important savings in two areas: fuel savingsdue to more direct routings and reduced laborcosts. The fuel savings for domestic airlinescould be on the order of 3 percent; at presentfuel prices, this would amount to a $250 millionreduction in annual fuel costs.

The principal benefit to the Governmentwould come in the form of increased controllerproductivity and the attendant reduction in op-erating costs: the volume of airspace assigned toa control team could be greatly enlarged; itmight also be possible to reduce the size of thecontrol team by automating the routine tasks ofclearance coordination and flight data entry.Preliminary estimates are that controller produc-tivity could be doubled, i.e. that individual enroute controllers could handle perhaps twice asmany aircraft as with the present system. a This

‘R. A. Rucker, Automated En Route A TC (A ERA): Operationalconce~fs, MTR 79WO0167, The Mitre Corp., May 1979.

‘L. Goldmuntz, et al., The AERA Concept, Economic and Sci-ence Planning, Inc., for the Federal Aviation Administration,December 1980.

‘Personal communication, S. B. Poritzky, Director, FAA Officeof Systems Engineering Management, Dec. 21, 1981.

in itself would not necessarily increase the ca-pacity of the system, but it could significantlyreduce future operating costs. One recent esti-mate places these savings at $300 million annu-ally (1979 dollars), g but these preliminary figureswould need to be refined as the AERA programprogresses and a more precise picture of its oper-ational characteristics is obtained.

A third advantage of AERA—and a strongpart of the rationale for seeking a high level ofautomation—is that it would help reduce systemerrors. * In the present ATC system about 60percent of these errors are attributable to mis-takes on the part of controllers: improper coor-dination between controllers, inattention, for-getting, failure to communicate, poor judgment,and the like. 10 The underlying causes of many ofthese errors can be traced to the nature of ATCas a work activity—routine, repetitive tasks re-quiring vigilance and close attention to detail,and often conducted at a forced pace. Comput-ers are ideally suited to this kind of activity; andif the tasks to be automated are judiciously se-lected and the software carefully designed, anautomated system such as AERA could elimi-nate a major part of system errors, or at leastprovide a backstop to the shortcomings of hu-man operators. In this sense, AERA is expectedto be safer than the present system of traffic con-trol.

Potential Implications and Issues

It must be emphasized that AERA is still in theearly stage of engineering development. Exten-sive effort, over perhaps 5 to 10 years, will beneeded to bring AERA to a precise and detaileddefinition of requirements and equipment speci-fications. Installation, test, and full operationaldeployment will take an additional 5 to 8 years.

‘Goldmuntz, op. cit. This benefit is calculated by taking the$375 million annual expense (1979) to operate ARTCCS, increasingit by a factor of 1.6 to account for traffic growth by the time AERAwould become operational taking so percent of that as the benefitdue to AERA productivity improvements.

● By FAA definition, a “system error” occurs whenever the ac-tual horizontal or vertical separation between aircraft is less thanprescribed minima.

‘OGoldmuntz, op. cit.; and G. C. Kinney, M. J. Spahn, and R.A. Amato, The Human Element in ATC: Observations andAnalyses of the Performance of Controllers and Supervisors inP r o v i d i n g ATC Seruicest MTR-7655, The MITRE C o r p . ,December 1977.


Thus, AERA cannot be expected to replace thepresent generation of en route ATC until some-time near the end of the century. Similarly, thedevelopment costs and subsequent expendituresfor facilities and equipment (F&E) have not yetbeen estimated, except in the most generalterms. The latest available projections of R&Dexpenditures for en route control systems overthe coming 10 years, much of which would befor AERA, show a total outlay of $170 million(1980 dollars).11 As of the writing of this report,detailed estimates of the required F&E invest-ments and costs to users for avionics appropriateto AERA have not been published.

Three major implications of AERA are al-ready apparent, however. One is that AERAwould require computer capacity and softwarefar beyond what is now available in ATC appli-cations, although not beyond the present orforeseeable state of computer technology. Sec-ond, AERA will require a two-way data link ca-pable of rapid and high-volume exchange of in-formation between the air and the ground. FAAnow envisions that Mode S will provide thisdata link, and plans for AERA are predicated onthe availability and widespread use of Mode Sby the early 1990’s. (See the discussion of “datalink” in the following section.) Third, AERA im-plies equally extensive automation in terminalareas and in a central flow management facilitycapable of coordinating traffic throughout theATC system.

This last point is particularly important bothfor the immediate plans to replace en route com-puters and for the design of the entire ATC sys-tem over the long term. It implies a modularcomputer architecture, in which en route andterminal facilities utilize similar hardware andsoftware. This would make possible a flexiblesystem design, in which individual moduleswould be capable of mutual support and backupin the event of local equipment or software fail-ure. Human controllers would have difficultyoperating the ATC system manually in the eventof a failure of AERA if adequate automatedbackup were not provided.

‘‘National Aviation System Development and Capital Needs forthe Decade 1982-1991 (Washington, D. C.: Federal Aviation Ad-ministration, December 1980).

The development and implementation ofAERA is likely to raise several important issues.Some are technical and concern the reliabilityand safety of AERA, specifically its vulnerabil-ity to undetected software errors or hardwarefailures, and the adequacy of current hardwareand software design techniques. The degree ofautomation envisioned for AERA may also becontroversial, and this could give rise to issuespertaining to the division of tasks between hu-man operators and computers or the design ofthe man/machine interface. The design will haveto include features that keep the controller’s at-tention and insure that he has enough informa-tion to deal promptly with anomalous situationsas they arise. Acceptance of the system by bothcontrollers and airspace users may prove to betroublesome.

A third set of issues pertains to the costs andbenefits of AERA, especially the savings in oper-ational costs ascribed to AERA in comparisonwith the investments needed to implement thesystem. A corollary question will be the costsand benefits to various classes of airspace users,especially if AERA entails mandatory equippagewith data link or other avionics in order to par-ticipate in the automated ATC environment.Resolution of these issues, rather than the some-what narrower questions of technical feasibilityor system design, may prove to be critical to theacceptance and success of the AERA concept.

Data Link

Potential Benefits

Communication is central to the ATC proc-ess, and at present voice communication is theprimary medium even for messages that involvecomputers processes. For example, a controllerreads data from a computer-generated display,transmits it by voice radio to an aircraft, and thecrew then enters the data manually into an on-board computer. This process wastes crew andcontroller time and is prone to reading or trans-mission errors. As the ATC system changes toincorporate higher levels of automation, there-fore, great benefits could be gained from a digi-tal data link that permits direct communication


between automated components. Among thesepotential benefits are the following:

● Digital messages can include special codesto detect and correct transmission errors.

● Processes that are running on computerscan exchange data of little immediate inter-est to the human participants without hu-man involvement.

● Digital transmissions can be addressed to aspecific recipient such as an aircraft withoutdiverting the attention of others to whomthe information is not of concern.

● Digital messages can be transmitted, storedby the receiving terminal, and recalled ondemand by the recipient.

In the present ATC system, the ATCRBStransponder provides limited data communica-tion, Digital messages are sent by the transpon-der in reply to interrogations from the groundthat request aircraft identity (transponder code)or altitude. Some observers, as discussed later inthis section, argue that the inherent capability ofthe ATCRBS transponder is currently underuti-lized and that it is capable of meeting many ofthe future requirements for a digital data link.Others, including FAA and a significant segmentof the user community, question this conclusion.

While there is little dispute that a data link isneeded for the ATC system of the future, there isconsiderable discussion on how it would best beimplemented. * FAA has suggested the additionof a data link capability—Mode S—to the speci-fications for the standard ATCRBS transponder.Others have suggested alternatives, and one or-ganization, Aeronautical Radio, Inc. (ARINC),is now operating a nationwide data link that isused by the airlines for administrative communi-cation. These alternatives are described in thesections that follow.

● Data links are also used to connect computers at the variousATC facilities operated by FAA. They use leased commercial tele-communication facilities at the present time; but in the future, sat-ellites might be used to perform this function more efficiently. Forthis discussion, which will focus on data links for air-to-groundand air-to-air communication, the links between the ground-basedcomputers are not of direct interest.

Mode S

The operating characteristics of the ATCRBStransponder conform to a standard establishedby the International Civil Aviation Organiza-tion (ICAO). For civil aviation, four modes ofoperation are defined, of which only two are inactual use: Mode A for aircraft identity, andMode C for aircraft barometric altitude. Interro-gation messages are formatted so that the trans-ponder will recognize the mode of the query andreply appropriately. Since the transponder is al-ready the primary link between ATC computerson the ground and aircraft in flight, it is logicalto argue that the data link function be incorpo-rated in the transponder.

FAA has suggested adding a fifth mode, ModeS, to the specification for the ATCRBS trans-ponder. ** This mode would provide a general-purpose data link designed to operate in a man-ner compatible with the existing ATCRBSmodes. Mode S was on the agenda at the April1981 meeting of the ICAO Communications Di-vision, and position papers relating to it havebeen circulated among members. Great Britainand the Soviet Union have independently devel-oped data link specifications that are compatiblewith Mode S. As of now, however, no memberof ICAO has formally proposed detailed specifi-cations that could be adopted as a Mode S stand-ard.

Mode S permits a digital message to be ad-dressed to a specific recipient. Each aircraftwould have a permanently assigned code toidentify itself in all ATC-related communica-tions using the data link. When a Mode S inter-rogation or message is sent, replies from alltransponders operating in Modes A and C aresuppressed. Thus, during any transition period,interrogations cycles would have to be dividedbetween Mode S interrogations and those in theexisting Mode A and Mode C formats.

One of the applications of Mode S is for thesurveillance function of the ATC system. Whentwo aircraft are in proximity (i.e. in line or al-most in line and differing in range from the inter-

● *Until recently, Mode S was referred to by FAA as DABS(Discrete Address Beacon System).


rogating ground station by 1.5 miles or less),their replies to a Mode A or C interrogation willinterfere with one another, creating what iscalled “synchronous garble. ” The ability to ad-dress the interrogation to a specific aircraft isone method of resolving this difficulty. Othermethods, such as computer processing of returnsor the use of multiple sensors, can accomplishmuch the same thing.

A second anticipated benefit from Mode Swould be the ability to deliver control messages,such as clearances and en route weather infor-mation, to specific aircraft. The data needed togenerate onboard displays of traffic could alsobe transmitted using this technique. Further, aMode S data link could be useful in an exchangeof data between aircraft, allowing them to coor-dinate conflict-resolution maneuvers (i.e., as anelement in a collision avoidance system). Again,however, Mode S is not the only means bywhich these needs could be met.

Modes B and D

Most of the cost of implementing Mode Swould be borne by the users, although some ex-penditures by FAA for the modification of itscomputers and software would be required.Some observers, however, consider the expenserequired for the introduction of Mode S to beunwarranted. They argue that the capability ofthe present ATCRBS transponder is underuti-lized. Modes B and D, it is suggested, could beused for some data link purposes, since theyhave sufficient capacity to meet the needs of theATC system and would require no change in theexisting ICAO specification. In addition, themessage format for Modes B and D is shorterthan that suggested for Mode S, and thereforeless likely to result in the interference that mightoccur between Mode S transponders replying tosimultaneous interrogations from different sta-tions. However, in considering this alternative,one should also note that existing transpondersdo not include the components needed to proc-ess Mode B and D interrogations and wouldhave to be modified (at users’ expense) to do so.

VHF Data Link

A second alternative to Mode S is the use of apart of the VHF radiofrequency band assigned toaeronautical voice communication. ARINC, acorporation organized and owned by the airlinesto provide communication services, already op-erates a data link of this type, know as ARINCCommunication Addressing Reporting System(ACARS), which is being used by airlines for ad-ministrative messages. At present, small printersin the cockpit are used to record ACARS mes-sages. A future modification could be conver-sion of the onboard weather radar screen or oneof the multipurpose displays used by electronicinstrument systems found in some aircraft todouble as a display for ACARS messages.

Some critics suggest that ACARS would notmeet the requirements for an ATC data link,pointing out that the VHF voice band is alreadycrowded and that the one frequency used byACARS (although currently underutilized)would not have sufficient capacity to meet theneeds of the ATC system. This deficiency couldbe overcome by assigning multiple frequenciesand scanning them automatically to detect in-coming messages. There has also been a start(for reasons having little to do with data link) atreducing the current 50 kHz spacing in the VHFband to 25 kHz, effectively doubling the numberof channels available. Some of these new chan-nels could be allocated to the data link function.


A data link is a primary resource that can beapplied in a number of ways, and the benefitsobtainable will be a function of the purposes towhich it is applied. If the data link is to be usedprimarily for surveillance, then it would be ad-vantageous to integrate it with the radar beaconsystem. On the other hand, if it is used primarilyfor nonsurveillance purposes such as deliveringclearances, reporting weather conditions, orsending and receiving advisories, the need to as-

sociate it closely with the radar beacon system isless compelling. The balance in traffic betweenthe uplink and downlink is also significant. If the


great majority of the message traffic is “up”-from ground to air— the ground station couldassume responsibility for allocating time amongusers. If there is a substantial flow of informa-tion in the opposite direction—air to ground,with a large part of it initiated by aircraft—thetask of coordinating the activities of the userswould become much more difficult. The lattersituation would be complicated further by theintroduction of substantial amounts of air-to-airtraffic, as in the Traffic Alert and CollisionAvoidance System (TCAS) concept (describedlater).

In considering the candidate forms of datalink, another important consideration to keep inmind is that the data link is not an isolated sub-system of ATC, nor does it provide any uniqueservice. Some form of data link is indispensableto the future scheme of operation and servicesenvisioned by FAA, such as AERA and the col-lateral improvements of terminal area controland central flow management. The level of auto-mation and the degree of strategic and tacticalcontrol that AERA would bring about requires ahigh-speed and high-volume flow of informa-tion, decisions, and replies between the air andthe ground. Thus, even though FAA is com-mitted to Mode S, it is important that all ques-tions about data link be promptly resolved andthat the necessary ground facilities and aircraftavionics be put in place so as to keep pace withthe parallel computer replacement program.Both of these resources will have to be availablewithin a decade if longer range improvementsare to be accomplished in the 1990’s.

It is also important to recognize that the datalink decision is not one where the United Statescan act with complete independence. ATC re-quirements and development programs of othernations must also be considered, and the di-rection chosen by FAA must be coordinatedthrough ICAO to ensure compatibility of signalformat, modes of operation, equipment charac-teristics, and the like. On balance, a data linksystem that is compatible with the needs of otherICAO member nations is preferable to one thatis unique to the United States.

Another important aspect of the data link de-cision concerns the avionics equipment that air-space users will have to install in order to takeadvantage of the services that data link offers.The data link is more than just a special kind ofhigh-speed receiver-transmitter: to make any

meaningful use of this capability, aircraft willalso have to be equipped with processors to en-code and decode messages, and with some kindof input-output device (displays and controls)that presents information to the aircrew and al-lows them to interact with the onboard proces-sors and ground stations. Such equipment iscostly to acquire (about $10,000 for a commer-cial aircraft, but somewhat less for GA) andwould require special maintenance. For com-mercial and corporate operators the expenses ofacquisition and maintenance could be absorbedwithout great difficulty, and the costs wouldprobably be offset by operating benefits such asfuel savings, avoidance of delay, and greaterflexibility of flight planning. For smaller GA op-erators, on the other hand, the cost-benefitequation may not be as favorable, and they mayconsequently conclude that the expense is notjustified by the improved services or operationalsavings made available to them.

The matter could become particularly acutefor GA if equipage with data link avionics wereto be made mandatory for access to airspace orfor receipt of essential ATC services. FAA cur-rently envisions a tiered program of services inwhich users receive progressively more extensiveservice in relation to the sophistication of theavionics carried on the aircraft. The concern ofGA is that the areas in which they will be al-lowed to operate with only minimal equipment(that is, without a two-way data link) will be-come so restricted that small GA aircraft will beeffectively excluded from the Nation’s airspace.The extent to which these concerns are war-ranted will depend heavily on the type of datalink that is selected and how it is to be incorpo-rated in various classes of aircraft.

Collision Avoidance

A primary function of the ATC system is pro-viding separation assurance. Ground-based sur-


veillance equipment and computer software in-clude features that will alert the controller to sit-uations where separation standards have beenviolated or are about to be violated. Neverthe-less, a small number of midair collisions andnear misses continues to occur, most of them in-volving aircraft not under positive control. Atthe present level of traffic, the probability of col-lision is very low, but as traffic density in-creases, so does the threat of collision. The fewaccidents suffered by commercial carriers haveheightened public awareness of the conse-quences of a midair collision involving large pas-senger aircraft. This common concern has led tosignificant public and private efforts to developcollision avoidance systems that would give theaircrew direct warning of the threat of collision.

A collision avoidance system is conceived as alast-resort measure to protect against collisions;it would come into play only after all othermeans to ensure separation have failed. A colli-sion avoidance system is not intended to be theprimary method of ensuring the separation ofaircraft. But the extra margin of safety providedby a collision avoidance system could lead tochanges in ATC procedures for separation assur-ance. For example, a reliable collision avoidancesystem could justify a reduction in separationstandards, thus effectively increasing the capaci-ty of the airway and airport system. This sectiondiscusses some of the alternative collision avoid-ance systems that have been proposed over theyears in order to give the reader an awareness oftheir relative merits and implications.

In general, two major classes of collisionavoidance systems have been proposed: thosethat depend on ground facilities; and those thatrequire only airborne equipment. Ground-basedcollision avoidance systems characteristically re-quire the expenditure of Government funds forfacilities and equipment, while airborne systemsdo not. Some of the so-called airborne systems,however, are in fact passive users of ATC equip-ment—that is, they “eavesdrop” on replies toATCRBS interrogations from ground surveil-lance stations in order to obtain the data neededto locate nearby aircraft. Some systems wouldbe effective only when a large portion of the air-craft in the fleet are equipped, while others

would provide some protection regardless of thenumber of users who install the equipment.

Beacon Collision Avoidance System

The Beacon Collision Avoidance System(BCAS) is one that had been under developmentby FAA for some time and was nearing the pointof implementation when FAA made the deci-sion, in the summer of 1981, to adopt anothersystem that is a derivation of BCAS (see below).The initial version of BCAS, known as ActiveBCAS, would have been implemented first; andFull BCAS, a more complex version designed tooperate in congested airspace, would have fol-lowed several years later.

In operation, Active BCAS on board aircraftwould emit interrogation pulses to whichATCRBS and Mode S transponders on the otheraircraft would reply in the same manner as theywould reply to an interrogation from a groundstation. The BCAS concept offered immediateprotection against aircraft equipped with ModeC ATCRBS transponders and altitude encodersand promised more efficient performance andbroader protection against aircraft equippedwith Mode S Transponders. The BCAS systemused the elapsed time between interrogation andreply to determine the range to other aircraft,and by calculating the rate of closure it deter-mined the potential for collision. If a collisionthreat were detected, an indicator would advisethe pilot whether to climb or descend to resolvethe conflict. The DABS data link was to be usedto coordinate the maneuvers of two BCAS-equipped aircraft. Active BCAS did not, how-ever, provide the pilot with the relative bearingof the intruder aircraft. * Full BCAS, in additionto originating interrogations, also gathered databy listening to replies to interrogations from theground and correlated these replies to determinebearing as well as range.

There was little question that BCAS would beeffective in low-density airspace, but there wasconsiderable concern that the system would be-come saturated in areas of high-traffic densitywhere a collision avoidance system is most

● A proposed follow-on version of Active BCAS would haveprovided direction-finding capability.


needed. For this reason, FAA planned to installground equipment (an RBX transmitter) to sup-press BCAS and prevent system saturation inareas of high-traffic density where it planned torely instead on a ground-based system called theAutomatic Traffic Advisory and ResolutionService (ATARS) to resolve conflicts. ATARSwould use ATCRBS and Mode S interrogationsand replies to gather traffic data and conveytraffic information to suitably equipped aircraftby means of the Mode S data link; ATARS wasdesigned to provide a turning manuever as wellas the climb or descend maneuver of BCAS.

While ATARS would overcome the majorweakness of BCAS, however, it would also re-quire considerable expenditure for both groundand airborne equipment. Both BCAS andATARS planned to use the Mode S transponderas a key element, and both therefore werecaught in the debate that surrounded the Mode Sdata link concept. Some critics have claimedthat Full BCAS would be required to support acockpit display of traffic information (CDTI),since the simpler Active BCAS provided no in-truder bearing and thus could not provide theaircrew with a picture of surrounding trafficanalogous to that available to ground control-lers. In many cases it was difficult to separate thearguments for and against DABS from thosepertaining to a collision avoidance system.

Tri-Modal BCAS

Tri-Modal BCAS was one proposed alterna-tive to the BCAS program. It was similar toBCAS in concept but based on the existingATCRBS transponder rather than the new ModeS capability, and it would operate in three differ-ent modes. In areas of high traffic density, Tri-Modal BCAS would operate passively, generat-ing all of the required information by analyzingstandard ATCRBS transponder replies to inter-rogations from ground surveillance stations. Inareas without coverage by ground radar, itwould operate like Active BCAS. Where cover-age was provided by only one ground radar sta-tion, it would operate in a semiactive mode togenerate its own interrogations while also listen-ing to replies to interrogations from the groundstation. The logic used by Tri-Modal BCAS

would enable it to determine both range andbearing in airspace adequately covered byground interrogators and, thus, to generate thedata needed to support a CDTI.

Advocates of Tri-Modal BCAS cited the fol-lowing advantages of this system:

●

●

●

●

It does not require the Mode S transponderand provides full protection from all air-craft equipped only with a standardATCRBS transponder.In airspace where the geometry of the distri-bution of ground-based interrogators is ap-propriate, it provides bearing without re-quiring the directional antenna that isneeded for TCAS (discussed next) and FullBCAS.It requires no change to the ground facilitiesexcept for the activation of the north pulseon the secondary surveillance radars nowinstalled.It can operate independently of all groundfacilities in the same manner as activeBCAS.

NASA, with the sponsorship of FAA, success-fully tested Tri-Modal BCAS, but its report indi-cated that the tests were not exhaustive becausea working model that included all of the featuresof the system was not available. However, thedevelopers of the system have continued theirwork since the NASA tests and claim that theirsystem is ready for certification and operationaluse.

Traffic Alert and CollisionAvoidance System

After supporting the development of BCASfor several years, FAA announced in the sum-mer of 1981 its decision to adopt an enhancedair-to-air version of BCAS, the Traffic Alert andCollision Avoidance System (TCAS). The ab-ruptness of this change has led to controversy inthe aviation community, and various observershave questioned both the suitability of TCASand its superiority to alternative systems.

TCAS is a direct derivative of BCAS and isdesigned to meet the following criteria:


● It does not require ground-based equip-ment.

● It is compatible with the present ATC sys-tem and a logical extension of it.

● It is more suitable for use in high-densitytraffic than BCAS.

● It offers a range of capabilities suitable tothe needs of various classes of airspaceusers.

To meet the last criterion, two versions ofTCAS have been specified; both include theMode S data link as an integral component.

TCAS I is designed for use by general aviationand the basic system is estimated by FAA to costin the range of $2,500 to $3,500 per aircraft.TCAS I would indicate to the pilot the presenceof a transponder-equipped aircraft without pro-viding either range or bearing information; itwould be the responsibility of the pilot to locatethe intruder by visual means and to take the ap-propriate action. An upgraded version of thisbasic system would provide the pilot with in-truder range and bearing and with informationdescribing the maneuver that a TCAS II-equipped aircraft intended to execute. TCAS Iestimates range by the strength of the signal re-ceived from another aircraft, at best an impre-cise measure, and in high-density airspace theproximity-warning indicator tends to be trig-gered repeatedly, thus minimizing its value as awarning device (if the false alarm rate is high, pi-lots might tend to ignore the warning). The addi-tion of an altitude stratified in TCAS I, however,appears effective in minimizing high alarm rates.

TCAS II is a more sophisticated version de-signed for use by air carriers and larger corpo-rate GA aircraft. FAA estimates that the neces-sary avionics will cost on the order of $45,000 to$50,000 per aircraft, slightly more than the pro-jected cost of an Active BCAS unit. TCAS II op-erates in the same way as Active BCAS, but withtwo major enhancements:

●

●

A directional send-receive antenna that willprovide both range and bearing withoutcreating the interference in areas of hightraffic density expected with Active BCAS.The ability to transmit to TCAS I and otherTCAS II aircraft information regarding its

relative location and the intended maneuverto resolve a conflict.

Initially, the TCAS 11 antenna will provide bear-ing information accurate to within 300, suffi-cient to provide the pilot with an “o’clock” indi-cation of relative bearing and activate a climb ordescend indicator. In later versions, FAA plansto specify an antenna with much higher angularresolution (1° to 2o), which would permit thesystem to generate a command for a horizontalas well as a vertical maneuver. The improvedversion would also support a CDTI.

FAA has issued a contract for the develop-ment of the high resolution antenna to determineif or when an antenna with this degree of resolu-tion, yet suitable for installation on commercialaircraft, can be designed and tested. One earlyversion of the sector scan TCAS II antenna wasapproximately 18 inches in diameter and extendslightly above the fuselage contour. Mountingsuch an antenna might require significant modi-fications of aircraft structure even on a large air-craft; the problem would be more severe in thecase of small GA or tactical military aircraft.Further, if a large antenna were to result fromthe development efforts, it could have detrimen-tal effects on aerodynamics, aircraft perform-ance, and fuel consumption.

The adoption of TCAS means that the DABStransponder remains a key element in FAAplans. However, the fact that TCAS is ground-independent and capable of operating in air-space with high-traffic density puts in questionthe need for ATARS, one of the key applicationsheretofore envisioned for DABS. There arestrong indications that FAA will drop ATARSfrom its plans and that, as a result, the level ofexpenditures on ground equipment will be sig-nificantly less than they would have been hadthe ATARS program been implemented.

FAA has also made a point of leaving the wayopen for entrepreneurial innovation in the devel-opment of TCAS, Thus, it is conceivable thatFAA might certify other collision avoidance sys-tems if their capabilities were demonstrated andif they would not interfere with TCAS or otherelements of the ATC system.


For instance, TCAS and Tri-Modal BCAScould operate in the same environment becauseboth depend primarily on responses from air-borne equipment and neither requires the instal-lation of equipment on the ground. However, asnoted above, the TCAS concept contains a pro-vision for coordinating conflict-resolutionmaneuvers of TCAS-equipped aircraft. Suchcoordination would not be possible between anaircraft equipped with TCAS and one equippedwith Tri-Modal BCAS as these systems arepresently designed. On the other hand, theTCAS concept does not assume that it willalways be possible to coordinate the maneuversof aircraft in a conflict situation. Therefore, theinability to coordinate the maneuvers of TCASand Tri-Modal BCAS aircraft does not presentan insurmountable barrier to operation of thetwo systems in the same environment.

Airborne Collision Avoidance System

All of the alternative collision avoidance sys-tems that have been discussed to this point arecapable of providing users some level of protec-tion from aircraft that are not similarlyequipped. The Airborne Collision AvoidanceSystem (ACAS), which was developed and dem-onstrated in the 1970’s, was not based on theATCRBS transponder and could have beenmade available for about $1,500 per aircraft(1977 dollars), considerably less than the alter-natives being considered at that time. A majordrawback of this system, however, was that itwould not be effective unless a substantial por-tion of the aircraft operating in a given area wereACAS-equipped.

Conceptually, the operation of the ACAS sys-tem was simple. It generated interrogations towhich all aircraft within a specified altitudeband would respond. Range was determinedfrom the delay between interrogation and reply,and when an aircraft was detected at closerange, subsequent interrogations narrowed thealtitude band from which a reply was requestedin order to determine whether a detected aircraftpresented a threat of collision.

ACAS is no longer being actively consideredas an alternative collision avoidance system, but

it is presented here to illustrate another group ofalternatives that have been explored in the past.

Microwave Landing System

Instrument Landing System

Providing precise and reliable guidance forapproach and landing in conditions of reducedvisibility is a prime consideration for safety offlight, but it also has important implications forthe efficient use of terminal area airspace andairport runways. Generally, the highest runwayutilization rates are achieved under VFR. Whenrestricted visibility or weather conditions dictateincreased separation and the use of instrumentapproaches, one consequence is a reduction inthe number of aircraft that can be landed in agiven space of time.

In part, this reduction in airport capacity utili-zation is a result of the guidance system in use.The present Instrument Landing System (ILS),which has been the standard U.S. system since1941, provides guidance along a straight path ata fixed slope of 3° or less extending 5 to 7 milesfrom the runway threshold. All aircraft ap-proaching the airport must merge to follow thispath in single file, spaced at intervals dictated byseparation minima and the need to avoid wakevortex. Aircraft flying at different speeds alongthis single fixed path complicate the controllerstask in achieving a uniform rate of traffic flowand diminish the capability to use the full capac-ity of the runway served by ILS.

The runway utilization rate under IFR couldcome closer to that attainable under VFR if air-craft could be permitted to follow multiple ap-proach paths, descend at different flight angles,fly at different approach speeds, or aim at differ-ent touchdown points on the runway—none ofwhich can be done with ILS. If these variationswere possible, as they are under VFR, the IFRcapacity of the airport would be increased to alimit determined almost solely by the rate atwhich successive aircraft could touch down, de-celerate, and clear the runway. *

● Wake vortex, for example, would remain a constraint on ca-pacity even if MLS with curved and variable glide slope ap-proaches were installed.



Microwave Landing System (see fig. 25). This capability is useful in avoiding

A precision approach and landing system thatovercomes these inherent disadvantages of ILS isthe Microwave Landing System (MLS). BecauseMLS uses a scanning beam, rather than a fixedbeam like ILS, it allows aircraft to fly any ofseveral approach angles (including two-stepglide slopes) and, in the lateral plane, to ap-proach along complex paths that intersect thealinement of the runway at any selected point

noise-sensitive areas on approach paths and re-ducing the impact of the wake vortex problem.

MLS offers other important advantages incomparison with ILS. The reliability of the MLSsignal is not influenced by ground-plane effects(snow buildup, soil moisture, tidal effects, etc.);this permits MLS to be installed at sites whereILS will not function properly. Fixed or movingobstacles in the approach zone do not interfere

Figure 25.—Comparison of Microwave Landing System and Instrument Landing System

Middlemarker

SOURCE: Federal Aviation Administratlon.


with MLS signals to the same degree as with ILS.In addition, MLS also provides precisionguidance for departures and missed approaches,a feature of particular importance when trafficpatterns of closely located airports are in con-flict. MLS operates in a frequency band that pro-vides 200 transmission channels; ILS has usedonly 20 of the 40 channels theoretically availableto it, and these are very near saturation in largehubs such as New York and Los Angeles. Final-ly, ILS does not meet the joint civil/militaryoperational requirement for precision approach,since it does not afford the tactical flexibilityneeded by military aircraft. MLS does.

For these reasons, FAA has designated MLS asthe precision approach guidance system to re-place ILS. The MLS transition plan, publishedby FAA in 1981,12 calls for 1,425 installations tobe carried out in three phases over the next 20years. In the first phase, between 10 and 25 sys-tems will be installed over a period of 2 years atselected airports in order to develop a base of ex-perience and reach an operational confirmationof the benefits that MLS can provide. The sec-ond phase will see the installation of 900 addi-tional MLS units at a rate of 100 to 150 per yearover a period of 6 to 9 years, with priority givento large and medium hub airports. The thirdphase involves installation of an additional 300to 500 units to meet the growth in demand antic-ipated by the end of this century. FAA estimatesthe cost of purchasing and installing 1,425 MLSground units to be $1.332 billion (1981 dollars);the cost to users to equip their aircraft with MLSis estimated to be an additional $895 million,yielding a total cost of roughly $2.2 billion.l 3

In selecting the transition plan, FAA workedin consultation with various user groups underthe auspices of Radio Technical Commission forAeronautics, and considered 10 deploymentstrategies—9 submitted by FAA and 1 developedby RTCA Special Committee 125. These strat-egies differed in terms of the order and rate ofdeployment at various sites, the length of the pe-riod of duplicative operation with ILS, and as-

‘ ‘ M i c r o u u m w LandirIg Syst~m Trnmsition Plan, APO-8 I-I(Washington, D. C.: Federal Aviation Administration, May 1981).

“Ibid.

sumed rates of user equipage. Each strategy wasanalyzed to estimate costs, benefits, and opera-tional effects. All strategies yielded favorable netbenefits in the range of $2.4 billion to $2.7billion. The costs of the 10 strategies varied nar-rowly ($1.20 billion to $1.35 billion for groundunits), as did the benefits ($3.65 billion to $4.05billion). These results led FAA to conclude that“there is no clear-cut economic rationale forchoosing among the MLS implementation strat-egies” and that “the choice should be based uponoperational considerations or on the special op-portunities for improved precision guidanceservice created by the installation of MLS equip-ment .“l 4 The strategy selected by FAA reflectsthese considerations.


There are two factors that may complicate theMLS transition plan, both of them involving thereplacement of the existing ILS. As of March1981 there were 653 ILS units in commission at458 airports, and an additional 155 units were invarious stages of procurement or installation.Thus, the MLS transition plan has to take intoaccount how these ILS sites, many of them re-cently commissioned and with many years ofservice life remaining, are to be phased out. ILSand MLS can be colocated and operated simulta-neously without signal interference or procedur-al difficulty, but the length of the period of jointoperation and the timing of ILS decommission atspecific sites could create difficulties for someclasses of airport users. FAA transition plan stip-ulates that no ILS will be removed until all of thenetwork’s Ill-equipped airports have operation-al MLS and at least 60 percent of the equippedaircraft routinely using the ILS/MLS runway areMLS-equipped. When this occurs however, 40percent of the regular users of a given airportcould lose the precision-landing service, eventhough they continue to operate with function-ing ILS equipment.

The second complication is that, by ICAOagreement, the United States is committed to re-tain ILS service at international gateway airportsthrough 1995. There are 75 such airports at pres-

“Microuun~e La)ldit~g Systetn Transition Pla/1, op. cit


ent, and generally they are among the busiestU.S. airports. The retention of ILS service atthese sites may cause some users to delay pur-chasing MLS equipment, since the installed ILSequipment will still be usable for another 10years or more.

Despite the overall favorable benefit-costratio of MLS indicated by FAA analysis, the spe-cific benefits and costs to various classes ofairspace users remains a subject of controversy.FAA’s analysis showed high positive net benefitsto air carriers and commuters largely due to thevalue attributed to passenger time saved. Forgeneral aviation as a whole, the costs exceededthe benefits for all 10 deployment strategies,although some classes of GA (notably corporateGA operating multiengined piston and jet air-craft) were shown to derive substantial benefitsfrom MLS. Thus, there is likely to be continuedresistance to MLS from some GA operators,probably in the form of opposition to decom-missioning ILS at specific sites and reluctance topurchase MLS equipment (at a cost of $5,000 ormore) so long as ILS is available.

It is also likely that specific details of the MLStransition plan will continue to arouse debate.Comment received by the FAA during thecourse of preparing the plan indicates that thereare several sensitive points. One potential issueis the priority given to installation of MLS at dif-ferent types of airports. For example, commuterairlines favor early deployment at small commu-nity airports, while the Airline Pilots Associa-

tion seeks to have MLS first installed at hub air-ports on runways not now Ill-equipped. Otheruser groups, for example the Air TransportAssociation, recommend an installation strategythat would create a network connecting majorairports (including many now equipped withILS), in order to encourage users who fly theseroutes frequently to install MLS equipment ontheir aircraft. Another, slightly different, recom-mendation would involve establishment of amajor-city network but with priority also givento installation at sites where it is not possible tolocate an ILS and at small community airportsthat have commercial service but not an ILS.

AS a final point, the MLS transition plan pro-posed by FAA may encounter administrativeand budgetary difficulties. The plan, particular-ly Phase II, is highly ambitious in that it calls forinstallation of 900 units at a rate of 100 to 150per year. It may be technically and administra-tively difficult to sustain such a pace, and itmight be even more difficult to justify the re-quired annual outlay of funds in a time ofbudget austerity. Implementation of Phase IIwould entail annual expenditures of $125 millionon a 6-year schedule, or $85 million on a 9-yearschedule. Stretching out Phase II, in order tohold it within some imposed budgetary limit, isan alternative that may have to be adopted,even though it might increase overall programcosts and defer realization of the full benefits ofMLS.

ALTERNATIVE ATC PROCESSESFAA is nearing the end of research and devel-

opment of several major components of theATC system and is about to begin operationaldeployment of these new technologies. Most ofthe system improvements planned by FAAwould continue the present trend toward aground-based, centralized control system withincreasingly more extensive requirements foravionics and more restricted forms of operation.These plans would also entail a major commit-ment of funds by the Federal Government andthe aviation community. It is important that the

Congress be satisfied, not only as to the sound-ness and appropriateness of these prospectivesystem changes, but also as to whether FAA’splans take into account the new alternatives thatare being made available by emerging technol-ogies.

There are five aspects of the future ATCsystem on which new technologies might havean especially important influence in creatingnew options:

● the role of the human operator;


●

●

●

●

tactical v. strategic control;autonomy and flexibility of control;ground v. satellite basing; andlevels of service.

Role of the Human Operator

The AERA concept implies that computerswill assume many of the controller’s routinedecisionmaking tasks and, by means of digitaldata link, many of the communications tasks aswell. The immediate consequences would bethat fewer human operators would be needed tohandle a given volume of traffic and that thehuman role would evolve toward that of amanager of automated resources.

However, there would also be important con-sequences for the pilot. The increased level ofautomation on the ground would bring a corre-sponding increase in opportunities to employautomation in the cockpit. Aircrew dependencyon airborne data processors and displays wouldincrease as more information would be trans-mitted digitally and the relative importance ofthe voice channel waned.

Another consequence of automation is thatthe burden of responsibility for operational reli-ability would shift. Safety would be assuredmore and more through the design process andless through the compensatory actions of the hu-man operator.

Tactical v. Strategic Control

A system supported by powerful data proces-sors can collect, analyze, and distribute informa-tion on a much wider scale than the present ATCsystem. This makes it possible to plan and coor-dinate the movement of traffic over a broaderarea and a longer span of time. The basic modeof control could therefore become more strategicand anticipatory— relying more on preventionof conflict through planning, and less on tacticalor reactive response to actual or imminent viola-tions of separation minima.

For the ground controller, whether human orcomputer, the principal task would be monitor-ing aircraft movements to ascertain conform-ance with a flight plan that, through planning,

had been determined to be conflict-free. For thepilot (aided by a flight management computerand onboard ATC systems), the principal taskwould be to fly from origin to destination with-out deviating from that flight plan unless unfore-seen circumstances (such as weather or devia-tions of other aircraft) forced rerouting. Tacticalcontrol measures would still be available, butthey would be called into play only whenstrategic measures proved inadequate to fore-stall conflict.

Autonomy and Flexibility of Operation

IFR control is now centralized on the groundbecause only the ground controller has the infor-mation needed to assure separation and an or-derly flow of traffic. However, improvements incommunication and processing technologieshave made it possible to redistribute informationamong the various participants in the ATC sys-tem.

Given greater access to information, aircrewcould become more active participants in theATC process. As the quality and timeliness ofthe information improves, interaction withground controllers could become infrequent.However, there is a logical limit to their inde-pendence from ground control, because overallstrategic control of the flow of traffic will remaina ground-based function.

Ground v. Satellite Basing

Navigation and surveillance functions in thepresent system are ground-based, as are the fa-cilities for relay of air/ground radio transmis-sions. The development of space technology

makes it possible to consider satellites as alterna-tives for all three purposes. Satellites could beused in either an active or a passive mode. In thepassive mode, they could serve as relay stationsfor communication between the air and theground or between ground sites where presentmethods are limited to line of sight. Satellite-mounted transponders could also provide posi-tion reference for airborne navigation systems.In an active mode, data processing capabilitiescould be installed in satellites to track aircraftand report their location to ground-based con-


trol facilities, eithersurveillance radar.

replacing or supplementing mediate levels of ATC services between thesetwo extremes. The level of service could vary ac-cording to 1) the density of traffic; 2) the mix of

Levels of Service aircraft; 3) the avionics carried by those aircraft;4) flight conditions;

Under the present ATC system there are onlyand 5) the ground-based

capability for separation assurance and traffictwo forms of operation—controlled (corre- management. The result could be a more variedspending roughly to IFR) and uncontrolled (cor- range of services, more closely tailored to theresponding roughly to VFR). In the future, im- needs and capabilities of the airspace users, thanprovements in ground-based and airborne tech- is now the case.nologies could make it possible to provide inter-

Chapter 6

AIRPORT CAPACITYALTERNATIVES


Dunes International I

Contents

PageIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . 101Airside Components. . . . . . . . . . . . . . . . . . . . 102Limitations on Airside Capacity. . . . . . . . . . 103

Aircraft Performance Characteristics. . . . 103Wake Vortex... . . . . . . . . . . . . . . . . . . . . . 103Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103Airfield/Airspace Configuration. . . . . . . 105Aircraft Noise . . . . . . . . . . . . . . . . . . . . . . . 106ATC Equipment and Procedures.. . . . . . 107Demand Considerations. . . . . . . . . . . . . . 107

Delay and Delay Reduction. . . . . . . . . . . . . . 107Demand-Related Alternatives . . . . . . . . . . . . 109

Peak-Hour Pricing. ......... . . . . . . . . . 109Quotas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110Balanced Use of Metropolitan Area

Airports . . . . . . . . . . . . . . . . . . . . . . . . 110Restructuring Airline Service Patterns. . . 111Reliever Airports. . . . . . . . . . . . . . . . . . . . . 112

Airport Development Alternatives. . . . . . . . 113Expanding Existing Airports. ....... . . 113Development of Secondary Runway

Operations. . . . . . . . . . . . . . . . . . . . . . 113Building New Airports. . . . . . . . . . . . . . . . 114

ATC Improvement Alternatives. . . . . . . . . . 116Airfield/Airspace Configuration

Management . . . . . . . . . . . . . . . . . . . . 116Wake Vortex Prediction. . . . . . . . . . . . . . . 116Microwave Landing System . . . . . . . . . . . 117Reducing Separation or Spacing

Minimums . . . . . . . . . . . . . . . . . . . . . . 117

Page

Automated Metering and Spacing . . . . . . 118Cockpit Engineering. . . . . . . . . . . . . . . . . . 118

Summary of Alternatives. . . . . . . . . . . . . . . . 119Future Research Needs. . . . . . . . . . . . . . . . . . 121

Wake Vortex Avoidance. . . . . . . . . . . . . . 121Wake Vortex Alleviation. . . . . ........ 121Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Airport Design . . . . . . . . . . . . . . . . . . . . . . 121Ground Access. . . . . . . . . . . . . . . . . . . . . . 122

LIST OF TABLES

Table No. Page8. ’’Top’’ U.S. Airports, by Enplaned

Passengers, by Air Carrier Operations,and by Reported Delays.. . . . . . . . . . . . 101

9. Arrival and Departure Separations. . . . 10410. Operational Characteristics of Airports

With Potential Benefits From aSeparate General Aviation Runway. . . 115

11. Summary of Alternatives.. . . . . . . . . . . 119

LIST OF FIGURES

Figure No. Page26. Airport Hourly Capacity Varies

Strongly With Weather. . . . . . . . . . . . . . 10427. Runway Configuration. . . . . . . . . . . . . . 10528. Typical Distributions of Delay. . . . . . . . 108

Chapter 6

AIRPORT CAPACITY ALTERNATIVES

INTRODUCTION

The ability of airports to accommodate trafficcan be expressed in terms of “airside” or “land-side” capacity. “Airside” capacity is defined hereas the number of air operations—landings andtakeoffs—that the airport and the supporting airtraffic control (ATC) system can accommodatein a unit of time, such as an hour. The capacityof an airport is not a single number, but willvary with the number of runways in use, the vis-ual or electronic landing aids available, the typesof aircraft being accommodated, the distancebetween aircraft in the approach pattern, andthe noise abatement procedures in effect. Thetime each aircraft occupies the runway and thefacilities for handling aircraft on the ground, ontaxiways, or at gates also affect airside capacity.All of these factors will vary depending on theweather.

“Landside” considerations, such as the sizeand number of lounges or the adequacy of bag-gage-handling equipment, affect the number ofpassengers an airport terminal can accommo-date. Ground access, including the adequacy oftransit connections, roadways, and parkingareas for passengers’ cars, is an important part of

an airport’s landside capacity, and in some caseshas become a limiting factor on an airport’s abil-ity to handle passengers. Recent discussionabout putting a quota on operations at Los An-geles International Airport, for example, is re-lated to growing ground access problems, notlack of airside capacity.

This chapter discusses alternatives to increaseairport airside capacity. Landside problems willonly be treated here as they affect airside capa-city.

When the traffic demand for an airport ap-proaches or exceeds its capability, the result isdelay. Delay has been a major problem at theNation’s busiest airports, resulting in millions ofdollars of increased operating costs for air carri-ers and wasted time for travelers. Although sev-eral different methods of measuring delay exist(as will be discussed later) it is generally agreedthat the six airports most affected by delay in1980 were: O’Hare (Chicago), Stapleton (Den-ver), La Guardia and JFK (New York), Harts-field (Atlanta), and Logan (Boston). As shownin table 8, most of the airports which report

Table 8.— “Top” U.S. Airports, by Enplaned Passengers, by Air Carrier Operations,and by Reported Delays

Passenger Air carrier Delays overenplanements operations 30 minutes

1.2.3.4.5.6.7.8.9.

10.11.12.13.14.15.

Chicago O’HareAtlanta HartsfieldLos Angeles InternationalNew York J.F. KennedySan Francisco InternationalDallas-Ft. WorthDenver StapletonNew York La GuardiaMiami InternationalBoston LoganHonolulu InternationalWashington NationalDetroit MetroHouston IntercontinentalSt. Louis Lambert

Chicago O’HareAtlanta HartsfieldLos Angeles InternationalDallas-Ft. WorthDenver StapletonMiami InternationalSan Francisco InternationalNew York La GuardiaNew York J.F. KennedyBoston LoganWashington NationalSt. Louis LambertDetroit MetroHouston IntercontinentalHonolulu

Chicago O’HareDenver StapletonNew York La GuardiaNew York KennedyAtlanta HartsfieldBoston LoganLos Angeles InternationalSt. Louis LambertSan Francisco InternationalDallas-Ft. WorthPhiladelphia InternationalNewarkWashington NationalMiami International

SOURCE: Federal Av/at/on Adm/n/strat/on, Term/na/ Area Forecasts, Fisca/ Years 1981-92, Washington, D.G. 1981 p 13; in-terview, FAA, Air Traff/c and A/rways Fac//dies, Aug. 20, 1981.

101


serious delay problems rank among the top 15airports in terms of both enplaned passengersand air carrier operations.

This chapter first describes the airside compo-nents in the operation of a typical airport. Itthen reviews those major factors which influenceor limit airside capacity. Next the chapter dis-cusses the problem of delay—how it comesabout and the methods for measuring it and esti-

mating its costs. The next sections outline somealternative methods for reducing delay or in-creasing the airside capacity. These includechanging the pattern of traffic demand, expand-ing the runway system, or modifying the termi-nal area air traffic control procedures and equip-ment. Finally, some suggestions for future re-search are made.

AIRSIDE COMPONENTS

The airside capacity of an airport is governedby factors related to its runway system and theairspace above and around the airport, as wellas the terminal area ATC and navigation equip-ment and procedures.

The number of runways, their layout, length,and strength will in large measure determine thekinds of aircraft that can use the airport andhow many aircraft can be accommodated in anygiven time period. The layout depends on anumber of factors including the local terrain andpredominant direction of the wind. Federal Avi-ation Administration (FAA) safety regulationsdictate how close the runways may be to one an-other and to buildings, trees, or other obstruc-tions.

In order to land on a runway, aircraft ap-proach the runway in single file, with a safe dis-tance between them. Air traffic may enter theairspace around the airport (“terminal area”)from many directions at a number of differentpoints (“entry fixes”), and in many metropolitanareas the aircraft may be destined for one of sev-eral different airports. Thus, the task of deliver-ing aircraft one by one to a particular runway ata particular airport must begin many miles fromthe airport itself, and controllers must orches-trate the orderly merging and diverging of manydifferent traffic streams until each aircraftreaches the final approach to its destination run-way. By the same token, departing aircraft mustbe safely routed from the airport to the “depar-ture fix” where they leave the terminal area andjoin the en route ATC system.

Controllers use bothseparation to maintain

vertical and horizontalsafe distances between

aircraft, a task that is complicated by their dif-ferent performance characteristics. Jets flying ata very slow (for a jet) 160 knots will neverthelessovertake and pass slower aircraft. The controllermay assign different altitudes so that this cantake place safely, or he may vector the faster air-craft along a longer path so that it will safelyovertake and pass around the one ahead.

In good visibility conditions, tower control-lers may clear aircraft, once they are in sight ofthe airport, to make a visual landing undertower control. The pilot assumes responsibilityfor separating himself from other aircraft, withthe controller standing by to warn pilots to “goaround” in case of a potential conflict. Duringtimes of poor visibility the ATC team retains re-sponsibility for separating the aircraft on finalapproach. In this case the Instrument Flight Rule(IFR) radar minimum separation is observed, so

Photo credit Neal Callahan

The variety of airspace system users .

Ch. 6—Airport Capacity Alternatives ● 103

that distances between aircraft are greater thanin good weather. Under IFR conditions, pilotsare much more dependent on landing aids suchas the Instrument Landing System (ILS) to guidethem to the runway.

An aircraft is considered to be on the runwayfrom the moment it flies over the runway thresh-old until it turns off onto a taxiway. Angled“high-speed” turnoffs can allow aircraft to leavethe runway at higher speeds than perpendicular

convenient to most of the aircraft using a run-way is important for getting maximum capacityfrom the runway system.

Departures from the airport may take placeon a separate runway or may be “interleaved”between arrivals on the same runway. Aircraftpreparing to depart can wait beside the runwayon holding aprons until the runway is clear; thenthey can then taxi onto the runway and take offfairly quickly—the time spent on the runway for

ones. Placing the turnoffs where they will be departure is on the order

LIMITATIONS ON AIRSIDE CAPACITY

Among the major factors influencing airportcapacity are: aircraft performance characteris-tics, wake vortex turbulence, weather, airfieldand airspace configuration, aircraft noise, ATCequipment and procedures, and demand con-siderations.

Aircraft Performance Characteristics

Characteristics of the aircraft—their size, aer-odynamics, propulsion and braking perform-ance, and avionics—will affect the capacity ofthe runways they use. Pilot training, experience,and skill will also influence performance, andthe capacity of a runway can vary greatly withthe types of aircraft using it. Runway capacity isusually highest if the “traffic mix” is uniformlysmall, slow, propeller-driven aircraft. The nexthighest capacity would come with a uniform mixof large jets. Where the traffic mix is highly di-verse—with jet and propeller aircraft of widelyvarying sizes and speeds—it is usually difficultto maintain optimum spacing and optimum run-way usage, and runway capacity is reduced. Thedirection of traffic also affects runway systemcapacity. When arrivals predominate, capacityis lower then when departures predominate.

Wake Vortex

Related to aircraft performance characteristicsis the problem of wake vortexes. Aircraft pass-ing through the air generate coherent energeticair movements in their wakes, and under quies-

cent weather conditions

of 30 seconds.

the wake vortex canpersist for 2 minutes or even longer after an air-craft has passed. The strength of the vortex in-creases with the weight of the aircraft generatingit. As the use of wide-bodied jets (e.g., B-747and DC-10) became more common in the early1970’s, it became apparent that wake vortexesbehind these heavy aircraft were strong enoughto endanger the following aircraft, especially if itwas smaller. Until the potential danger of wakevortex to transport sized aircraft was demon-strated (e.g., the 1972 crash of a DC-9 landing inthe wake of a DC-10) standard separations of 3nautical miles (nmi) were required under IFRconditions. In order to prevent accidents causedby wake vortexes, FAA increased the separa-tions for smaller aircraft behind larger ones dur-ing weather conditions when persistent vortexesmay be a danger. These minimums are shown onthe right side of table 9.

Weather

Heavy fog, snow, strong winds, or icy run-way surfaces reduce an airport’s ability toaccommodate aircraft and may even close anairport completely. For a given set of weatherconditions, several of the different runway con-figurations available at an airport may be suit-able but only one will have the maximum value.Using these maximum values, and plotting themwith the percentage of the year during which dif-ferent weather conditions are likely to prevail, a


Table 9.—Arrival and Departure Separations

Minimum ArrivalVisual Flight Rules*

Leads L H

s 1.9 1.9 1.9

L 2.7 1.9 1.9

H 4.5 3.6 2.7

Separations— Nautical MilesInstrument Flight Rules

Leads L H

s 3 3 3

L 4 3 3

H 6 5 4

Minimum DepartureVisual Flight Rules*

Leads L H

s 35 45 50

L 50 60 60

H 120 120 90

Separations— SecondsInstrument Flight Rules

Leads L H

s 60 60 60L 60 60 60

H 120 120 90

“VFR separations are not operational minima but rather reflect what field data show under saturated condition. Adapted fromParameters of future ATC Systems Re/atirrgr to A/rport Capacify/De/ay (Washington, D. C.: Federal Aviation Administration,June 1978), PP. 3.3, 3.5.

“capacity coverage curve” for any given airportcan be constructed.

An example of a capacity coverage curve isshown in figure 26. The highest hourly capacityof Boston Logan Airport is 126 operations perhour in Visual Flight Rule (VFR) weather. Thiscombination of highest capacity runway use andgood weather is available 40 percent of the year.Strong winds create crosswind componentswhich close some of the runways of that con-figuration, and hourly capacities continue todecrease as marginal weather and finally badweather cause restrictions in safely operating therunway system. There is a small percentage (2percent) of the year when poor visibility, ceil-ings, and snow completely close the airport.Notice that there is a wide variation in the hour-ly capacity from 126 operations per hour downto 55 operations per hour before the airportcloses. This is typical of many major airportswhere several runway combinations exist. Thiswide variation in hourly capacity prevents theestablishment of a single capacity value for theairport; instead, it will be variable depending onweather conditions.

It is difficult to foresee any capital investmentin runways or technological improvements toATC facilities which can completely eliminate

Figure 26.—Airport Hourly Capacity Varies StronglyWith Weather

(There is a 3 to 1 or 2 to 1 ratio between good weather/badweather capacities)

Capacity Coverage Curve—Boston Logan Airport

20t

0 10 20 30 40 50 60 70 80 90 100

Average percent of time

SOURCE: Robert W. Simpson, “Airside Capacity and Delay at Major U.S. Air-ports,” draft report prepared for the Office of Technology Assess-ment, U.S. Congress, Washington, D. C., October 1980.

this degradation of capacity with weather condi-tions. New runways can raise the overall level ofthe capacity coverage curve, but they do not

Ch. 6—Airport Capacity Alternatives . 105


Snow control at a terminal

prevent its degradation with weather. Some ofthe ATC improvements discussed later in thischapter attempt to improve overall capacity byreducing the gap between IFR and VFR perform-ances.

Airfield/Airspace Configuration

The capacity of an airport depends to a largeextent on the number of runways available andtheir interactions. For example, for a given traf-fic mix a particular runway can handle 65 opera-tions an hour in VFR conditions and 55 in IFRweather. The VFR capacity of two parallel run-ways, 2,500 ft apart, might then be 125 opera-tions per hour— twice the capacity of a singlerunway. Yet the IFR capacity of this two-run-way system would be more like 65 operations

per hour, because under IFR conditions runwaysless than 4,300 ft apart are considered “depend-ent” for purposes of landings—that is, an opera-tion on one prevents a simultaneous operationon the other. Similar safety restrictions applywhere runways converge or intersect with oneanother. Thus, not only is the capacity of eachrunway reduced during bad weather, but the ca-pacity of the airport is further reduced becausenot all runways may be fully used.

In the illustration in figure 27, the three run-ways could be used in several different ways,four of which are shown. Each of these combina-tions may have a different operating capacity,and each might be suitable for a different set ofwind, visibility, and traffic conditions. A largeairport like O’Hare might have 40 or 50 possiblecombinations of runway uses. The limitationimposed by the available runway system variesamong the top air carrier airports. ChicagoO’Hare has seven runways, Kennedy has five,and La Guardia has only two (La Guardia’sadditional short 2,000-ft runway can be usedonly for departures during good weather condi-tions). Yet the capacity relationship is not linear:La Guardia manages to handle 40 percent ofO’Hare’s total aircraft movements with less than30 percent of its runways. An adequate taxi-way/gate configuration is also needed in orderto support optimum runway usage. For in-stance, the La Guardia Airport capacity task

Figure 27.—Runway Configuration

SOURCE: Federal Awatlon Admlnlstration, Techniques for Determinmg Awport Alrside Capacity and Lle/ay, FAA-RD-74-124, June 1976,


force found that additional taxiways in one areawere critical to minimizing delays. This is be-cause space at gates was limited, and the addi-tional taxiways could be used to hold and se-quence departing aircraft during periods of con-gestion.

Aircraft Noise

Aircraft noise, especially the noise of jet air-craft, has made airports unpopular with theirneighbors. The greatest noise impact is usuallyin the areas just beyond the ends of the runways,where arriving and departing aircraft fly at lowaltitudes. If a high-noise area is occupied by afactory or a highway cloverleaf there maybe lit-tle difficulty, but such land uses as residences,hospitals, and schools are not compatible withthe amount of noise generated by an airport. Insome areas, ineffective or nonexistent zoningand land use controls over the years have al-lowed these incompatible land uses to occupyhigh noise impact areas near many airports. Thecourts have generally found that the airport op-erator is responsible for injury due to reducedproperty value, and owners of nearby prop-erty have been able to collect damages in somecases. In Los Angeles, the courts have recentlyawarded nuisance damages as well. In someareas, including Atlanta, St. Louis, and Los An-geles, airport operators have been required topurchase noise-impacted property and either useit as a buffer zone or resell it for a more compat-ible use.

One method for reducing noise is to introducequieter aircraft or, as many air carriers havebegun doing, to re-equip old aircraft withquieter engines. FAA has set standards for newaircraft that are much quieter than in the past,but noisy aircraft will remain in the fleet formany years. The increasing sensitivity of thepublic to noise may have offset much of the re-cent improvement.

FAA, at the request of individual airport oper-ators, has also developed operational proce-dures that reduce noise impact. For example, useof certain runways may be preferred, or pilotsmay be required to make approaches over lesssensitive areas, weather permitting. However,


Air use and land use

FAA has established very few mandatory noise-abatement procedures. Over the past few yearssome operators have conducted airport noisecompatibility and land use studies for use as abasis for their own noise planning. The newFederal Aviation Regulation, Part 150, requiredunder the Aviation Safety and Noise AbatementAct of 1979 (Public Law 96-193), providesoperators with guidelines for voluntary noise-abatement standards and establishes a standard-ized method for measuring noise exposure.

Many of these noise-control procedures havea negative effect on capacity, and airports withboth capacity and noise problems have foundthat the available solutions to one problem oftenaggravate the other. The highest capacity run-way configuration, for instance, may be onewhich requires an unacceptable number offlights over a residential area. Enforcing noise-abatement procedures may also cause an unac-ceptable level of delay at peak hours. Thus, air-ports must balance tradeoffs between usable ca-pacity and environmental concerns.

The FAA Administrator recently reempha-sized that the responsibility for establishingproper land-use controls around airports restswith local government. He also predicted that


more communities will be establishing localnoise limits by ordinance or statute. f

A local government, whether or not it is theowner of the airport, can exercise some controlover noise, but must do so in a manner that isnondiscriminatory and does not place an undueburden on interstate commerce. For example, acity may select a reasonable noise exposure limitand exclude or fine aircraft exceeding that limit.However, the total ban on jet aircraft in SantaMonica, Calif., was overturned by the courts asunduly discriminatory against one class of air-craft (some new jets are quieter than propeller-driven aircraft).

ATC Equipment and Procedures

Improvements in aircraft surveillance, naviga-tion, and communication equipment over thepast decade have greatly increased the ability ofpilots and controllers to maintain high capacityduring all weather conditions (see ch. 5). How-ever, there are still ATC-related limits on airportcapacity. Clearances used in the en route air-ways and the terminal airspace are frequentlycircuitous, routing aircraft through intermediate“fixes” or control points rather than allowingthem to travel directly from origin to destina-tion. While this places aircraft in an orderly pat-tern so that controllers can better handle them, italso reduces capacity and consumes time andfuel.

“’Helms Places Airport Noise Problems on Operators, Commu-nities, ” Alliatiou Daily, Sept. 29, 1981, p. 154.

The limitations in the accuracy of surveillanceequipment also can influence how airports areconstructed and how they may be used. For ex-ample, the spacing requirement between inde-pendent IFR runways was developed based onthe limitations of surveillance, navigation, andcommunications equipment. Improvements inequipment and procedures have allowed thisminimum to be reduced over the years.

Constraints on capacity can arise whenairspace near one airport must be reserved toprotect operations at another airport. This is anespecially pressing problem in some busy areas.There is such an airspace conflict between LaGuardia and Kennedy in certain weather condi-tions, for example.

Demand Considerations

The daily pattern of demand is characteristicof the airport and the travel markets it serves.Air travelers prefer to travel at certain times ofthe day—midmorning and late afternoon, forexample—and air carriers wish to accommodatethem. Heavy scheduling at peak hours makes iteasier for passengers to transfer to other planesor other airlines, yet (as will be discussed short-ly) peaks in demand can be major causes of de-lay. Even at airports with a high percentage ofscheduled traffic it is not possible to predict theactual number of aircraft which will appear at aparticular hour of a given day, as nonscheduledtraffic volume can vary substantially. At quotaairports, the quota is set at a value between theVFR and IFR capacity, resulting in a built-indelay situation whenever weather conditions de-teriorate.

DELAY AND DELAY REDUCTION

Airport delays received a great deal of public- peaks, there may be delays even when the num-ity during the late 1960’s and they continue to be ber of aircraft using the airport is less than thea major waste of time, money, and fuel. Delay capacity for that peak time period. Somecan be expected whenever instantaneous traffic amount of delay arises every time two aircraftdemand approaches or exceeds the airport’s are scheduled to use a runway at the same time.capacity. When traffic occurs in bunches or The probability of simultaneous arrivals in-


creases rapidly with traffic density, so that aver-age delay per aircraft increases exponentially

well before traffic levels reach capacity levels.

A typical variation of delay with operationrates is shown in figure 28. When the traffic levelis above capacity, the accumulation of aircraftawaiting service is directly proportional to theexcess of traffic over capacity. For example, ifthe capacity of a runway system is 60 operationsper hour and traffic rates are averaging 70 opera-tions per hour, then every hour will add an aver-age of 10 aircraft to the queues for service, and10 minutes to the delay for any subsequent ar-rival or departure. Even if the traffic level dropsto 40 operations per hour, delays will persist forsome period since the queues will be depleted ata rate of only 20 aircraft per hour.

The principal delay-reporting systems of FAAcurrently measure only the occurrence of largedelays. The National Airspace CommunicationsSystem (NASCOM) delay reports record in-stances of delays of 30 minutes or more at 46participating airports. The Performance Measur-ing System (PMS) records delays of 15 minutesor more at 15 major airports. The PMS also at-tempts to estimate “average delay per aircraftdelayed.” Both NASCOM and PMS rely on con-troller’s manual recording of instances andcauses of delays during periods when he is al-ready busy. Weather is listed as the primarycause for these delays, ranging from 76 percent

Figure 28.—Typical Distributions of Delay

50 percent 100 percent

Traffic rate capacity


of the 30-minute delays in 1976 to 84 percent in1979 in the NASCOM system. The total numberof delays reported also increased, from approxi-mately 36,200 in 1976 to approximately 61,600in 1979. It must be emphasized that whileweather may indeed be the primary cause, theability of the system to anticipate, adjust to, andrecover from weather-related problems is de-pendent on a number of the other determinantsof airside capacity.

Another major delay-reporting system issponsored by FAA and three airlines—Eastern,United, and American—which have been pool-ing their operational flight-time data since 1976.This Standard Air Carrier Delay ReportingSystem (SACDRS) covers 36 airports and meas-ures taxi times, gate holds, and flight timesagainst standard values in an attempt to deter-mine delay. Unfortunately, an error in thismethod causes an overestimation of delay: forexample, the standard times used for taxi in andout are based on the average over all runways ata given airport, but at some airports there iswide variation in taxi times for different run-ways and terminals; some percentage of theselonger taxi times are always counted as delayunder the SACDRS. FAA recognizes the defi-ciency in this system, but no correction has yetbeen devised. Estimates of the annual cost ofdelay based on SACDRS have ranged as high as237 million gallons of fuel and $273 million ofadditional operating costs to the three airlinesinvolved, although these costs too are overesti-mated. 2 The PMS and NASCOM systems, onthe other hand, because they only count long de-lays, probably underestimate delay. The truevalue of delay lies somewhere in between andhas not been determined with accuracy. Thus,estimates of the cost of delay based on any ofthese reporting systems have to be viewed withsome caution. However, all observers agree thatdelay is a serious and expensive problem at someairports, especially in light of the high cost offuel in recent years.

One method of dealing with delay is to con-strain traffic to manageable levels. This is the

‘Virginia C. Lopez (cd.), Airport and Airway Congestion, A Se-n“ous Threat to Safety and the Growth of Air Transportation(Washington, D. C.: Aerospace Research Center, July 1980).


origin of the quota systems which have been im-posed at a few major airports. Each carrier hasrepresentatives on the “scheduling committees”to negotiate the carrier’s share of allowed peakhour operations. FAA, through its flow controlcenter, also works to ameliorate the costs ofdelays by forewarning air carriers when delayconditions develop at major airports. For ex-ample, when weather deteriorates and capacitygoes down in Chicago, FAA may advise aircraftscheduled into Chicago to delay their arrival

there by waiting on the ground at other cities.Waiting on the ground is much less wasteful offuel than waiting in holding patterns in the air.

Although the lengthy delays of the late 1960’sare no longer typical, delay remains a majorproblem at many airports. Further, the numberof operations will increase as air traffic grows,and additional airports may experience thisproblem. Some possible approaches to dealingwith delay are discussed below.

DEMAND= RELATED ALTERNATIVES

Delay problems tend to be concentrated at theNation’s major airports, and even at these loca-tions the problem is most acute during certainhours of the day (usually midmorning and lateafternoon). If operations could be shifted fromthese peak hours to less busy times, delay couldbe reduced and the overall capacity of the air-port better utilized. Variable user fees or quotasduring peak hours are tools which have beensuggested, and tried at some locations, to reducepeak demand and increase operations in non-peak hours. All these mechanisms,duce the ease of transferring fromanother at hub airports, makingachieve ideal airline economics.

however, re-one flight toit harder to

Peak-Hour Pricing

Most airports now charge a landing fee basedon the weight of the aircraft. This fee schedule isdesigned to recover construction and operatingcosts of the airport, not to ration capacity. How-ever, when the use of an airport is nearing ca-pacity it could be more economically efficient tobase landing fees on the marginal costs imposedby each additional aircraft served. This meansthat the user should pay not only for use of theairport, but for the delay caused other users whowant to use it at the same time. This methodallows users who value access to the airport atpeak times to pay for their preference; thosewho do not wish to pay the higher fee would usethe airport at other times, or perhaps useanother airport.

In general, peak-hour pricing would have lit-tle effect on air carrier operations unless theprice changes are very large. Airlines scheduleflights when they think passengers will want tofly, and they would probably be willing to ab-sorb moderate increases in user fees in order touse the airport at those times. Even a landing feeof several hundred dollars would be small com-pared to the total operating costs of a largejetliner, and such an expense could be passed onto the passenger by a relatively small increase infares. Commuter air carriers, with their smallernumber of passengers, would be unable to paylanding fees quite as high as the larger carriers.

General aviation (GA) users on the otherhand, especially student and personal flyers, aremore sensitive to increases in landing fees. ThePort Authority of New York and New Jersey’s1968 decision to increase minimum landing feesfrom $5 to $25 during peak hours brought aboutan immediate decline of about 30 percent in GAoperations during peak hours at its three air car-rier airports (JFK, La Guardia, and Newark) anda noticeable decline in aircraft delays of 30 min-utes or more.3 In 1979, a $50 surcharge added topeak-hour landing fees at Kennedy and La Guar-dia resulted in a further decrease in GA traffic atthose airports.4 The remaining GA users were

3Airport Quotas and Peak Hour Pricing: Theory and Practice(Washington, D. C.: Federal Aviation Administration, 1976), pp.54-60.

“Port Authority of New York and New Jersey, Aviation Depart-ment, interview, Oct. 23, 1981.


primarily high-performance turboprop aircraftused for corporate travel; corporations, like theairlines, may be willing to absorb a fairly largeincrease in fees in order to use specific airportsduring peak hours.

One problem with a peak-hour pricing systemis that it is difficult in practice to determineprecisely what the marginal cost of an airportoperation is; several years of trial and errorwould be necessary to settle on a pricing schemewhich both controlled delay and allowed the air-port to cover its costs. However, if the same feewere charged to air carrier, commuter, and GAaircraft, peak-hour pricing might be stronglyresisted. Proportionately different fees for dif-ferent categories of users might therefore benecessary.

QuotasAn alternative method for managing demand

is to set a quota on the number of operationswhich can take place during a peak hour. Thequota can be placed on total operations, or acertain number of operations can be allocated todifferent classes of users. The quota levels areusually set between the IFR and VFR capacity ofthe airport; thus, in VFR conditions, additionalaircraft could easily be accommodated. Whencapacity is reduced, users without reservationshave to use the airport at another time or useanother airport.

Although reservations (slots) for GA or evenair taxis might be allocated on a first-come, first-served basis, slots for scheduled carriers presenta more complex problem. At major airportswhere quotas have been in effect for some time(O’Hare, JFK, La Guardia, and Washington Na-tional) representatives of the air carriers are al-lowed (with antitrust immunity) to meet asscheduling committees to negotiate how manyslots will be allocated to each carrier. Althoughnew entrants are able to participate in these ne-gotiations, quota systems do tend to favor thestatus quo. Since the air traffic controllers strikein August, 22 airports have been brought undera quota system designed principally to easepeaks of demand on the en route ATC system.The methods for assigning slots to new entrants

or allowing existing carriers to exchange slots arestill under development.

One objection to quota systems is that the al-locations are made without any price signals toshow that the capacity is being used efficiently.Thus, although the quota may provide somestop-gap congestion relief, it does not provideany long-run guide for allocating resources asthe system grows or changes. It has been sug-gested that this problem could be overcome byauctioning the reservation slots among the carri-ers or by combining the quota system with somesort of peak-hour pricing scheme.

Balanced Use of MetropolitanArea Airports

Many major metropolitan areas are served bytwo or more large airports. Where one or moreof these airports is underutilized, possibilities ex-ist for increasing airside capacity through a morebalanced use of the region’s airports. Examplesinclude: Newark Airport, which is underutilizedcompared to Kennedy and La Guardia; OaklandAirport, which could relieve San Francisco;Midway Airport, which is practically emptywhile Chicago-O’Hare has delay problems; andBaltimore-Washington and Dunes Airports,which might relieve Washington National. Theproblem of balancing use of metropolitan air-ports presents a chicken/egg dilemma: airlineswon’t serve the underutilized airport becausethere are so few passengers, and passengersdon’t go there because there is so little service. Itis difficult to foresee when congestion in itselfwill become great enough to cause redistribu-tion, or to what extent the process can or shouldbe managed by local or even Federal authorities.In some cases, better transportation between air-ports might make it easier to transfer betweenflights and to attract passengers to underutilizedairports.

The Washington, D. C., area is illustrative ofthe problems of imbalance airport use. Wash-ington National Airport, operating since themid-1940’s, is convenient to the downtown area.National has three runways (all under 7,000 ft)and does not accept wide-body jets. Both its air-


side and landside capacity are severely limitedand a quota system and airline scheduling com-mittee are used to ration peak-hour operations.Expansion is difficult due to surrounding devel-opment and the Potomac River. Complaintsabout the airport’s noise have led to a 10 p.m.curfew among other noise abatement policies.From time to time some groups even call for theairport to be closed.

Many of these problems could be alleviated ifsome operations were transferred to Dunes In-ternational Airport, 26 miles from Washington.Dunes, opened in 1962, has two 11,500-ft run-ways, one 10,000-ft runway and capacity tospare. FAA (which operates both airports) hasrepeatedly attempted to induce carriers to useDunes more; for example, only Dunes canreceive international and long-range domesticflights. Despite the constraints of the quotasystem, the curfew, and the restrictions on wide-body and long-range flights, however, Nationalhandled nearly 4 times the operations and 4½times the passengers that Dunes did in 1980. Fur-ther, National generated a net profit of $10 mil-lion that year, while Dunes incurred a net loss of$3 million. ’ The principal problem is ground ac-cess; it is more convenient to fly from Nationalthan from Dunes.

Some new airlines beginning service since de-regulation have sometimes deliberately chosento operate out of underutilized airports to avoidcongestion and delay. One example is MidwayAirlines, which uses the nearly abandoned Mid-way Airport for its Chicago service. Midway’sproblem is also related to ground access: con-gested highways make trips to the airport longeven though Midway is closer to downtownChicago than O’Hare. Another example is Peo-ple Express, which serves the New York areafrom Newark. The Port Authority of New Yorkand New Jersey has been offering incentives topassengers as well as airlines to increase the useof Newark Airport: improved ground access bytrain and express bus allows New York City pas-sengers to get to Newark without paying high in-terstate taxi fares, and new airlines are offered

51nterviews, FAA, Metropolitan Washington Airports, July 6,1981.

more and better space for future growth atNewark. In addition to People Express, NewYork Air has located part of its operation atNewark. Now that permission has been gainedto use Newark as a international airport, severalestablished airlines are also bidding to offertransatlantic service from there.

Restructuring Airline Service Patterns

When delay becomes intolerable at busy hubairports, users themselves may voluntarily movetheir operations to another facility. This move-ment might be to an underutilized airport near-by (e.g., Newark), but it could also be to a medi-um or small hub located at some distance fromthe congested hub. This is especially likely fortransfer traffic. (See ch. 4 for a discussion of thegrowth and capacity impacts of this redistribu-tion scenario. )

Many major airports currently serve as hubsfor a large amount of transfer traffic. Three-fourths of the arriving passengers at Atlanta andabout one-half the passengers at O’Hare, Dallas-Fort Worth and Denver pass through these air-ports only to change planes for somewhere else.Carriers choose to establish their hubs at thesebusy airports so that passengers can choose frommany transfer flights. However, when the trans-fer airport becomes too congested the disadvan-tages of delay may begin to outweigh the advan-tages of convenience, for airlines as well aspassengers. Hence carriers may decide to locatetheir new transfer operations, and even movetheir existing hubbing activities, to other citiesthat have more room for growth.

Redistribution of operations appears to be oc-curring under the new routing freedom availableunder the Airline Deregulation Act of 1978. Car-riers are finding it easier to change their routesand establish new “second-tier” hubs at less con-gested airports. Between 1978 and 1980 the num-ber of large hubs (handling more than 1 percentof total U.S. passenger traffic) fell from 26 to 24,while the number of medium hubs (handling0,25 to 0.99 percent) increased from 33 to 36—a

market shift reflecting the distribution of opera-tions over more airports. This trend may accel-erate as regional carriers modify their patterns of


service, and even the busiest airports such asAtlanta and O’Hare, may see actual declines inboth enplaned passengers and operations in thenext 10 years. A similar decline in operations oc-curred at Kennedy Airport when internationalflights were allowed to enter the United States atother gateway cities.

Reliever AirportsIn metropolitan areas where there is conges-

tion at the main airport and excess capacity atsurrounding airports, diversion of GA trafficwould be effective in improving the use of air-side capacity in the whole region. It would allowa higher level of service for both air carrier andgeneral aviation, and in most metropolitan areasthere are smaller airports which might potential-ly attract some GA traffic away from the mainairport. For example, FAA lists 27 airports in theChicago area, 51 around Los Angeles, and 52 inthe Dallas-Fort Worth metropolitan area. How-ever, most of these airports are quite small, andonly a few have runways long enough to accom-modate business jets or instrument landingequipment for bad-weather operations.

The FAA’s National Airport Systems Plan(NASP) designates 155 airports as “satellites” or“relievers” to major airports, and NASP pro-vides for separate Airport Development AidProgram (ADAP) funding to be set aside for re-lievers. Publicly owned reliever airports may useADAP funds for construction, installation ofsafety equipment, and other eligible expendi-tures. The 25 or so privately owned reliever air-ports, although they presumably provide thesame benefit in terms of diverting traffic fromcongested air carrier airports, are not eligible foraid. Local and State governments may, how-ever, use ADAP funds to help purchase private-ly owned reliever airports, and at least fivereliever airports have changed from private topublic ownership since 1973. One privatelyowned reliever, Chicagoland (a reliever forO’Hare) closed in 1978. Although the FAA re-liever program was initiated largely to segregatetraining activities from major commercial air-ports in the interests of safety, it also providesadditional airport capacity for a certain type oftraffic—namely, personal GA aircraft with ori-

gins or destinations in the local region; businessand commercial GA (i.e., corporate aircraft andair taxis) delivering or picking up airline passen-gers will probably continue to use the majorcommercial airport.

The process of diverting the personal GA traf-fic has already occurred at the Nation’s largestmajor commercial airports. The fraction of GAactivity at Atlanta, O’Hare, Kennedy, Los An-geles International, etc., is very small (about 10percent) because these regions have good alter-nate secondary airports with high levels of traf-fic. In fact, some of the large relievers such asVan Nuys and Long Beach, Calif., Opa Locka,Fla., and Teterboro, N. Y., are among the busiestairports in the country in terms of annual opera-tions. This trend toward establishing a system ofreliever airports is underway and has been en-dorsed by many user groups and observers,most recently the President’s Task Force on Air-craft Crew Complement. e

To be of maximum benefit the reliever airportshould be located so that approach and airspaceconflicts between the reliever and the commer-cial airport do not place capacity limits on both.In the New York area, for example, instrumentoperations at Linden and Teterboro reliever air-ports must alternate with operations at the New-ark Airport. In addition, the noise consequencesof increasing operations at the reliever airportmust be considered. Most reliever airports have,or will soon have, IFR landing aids and runwaysystems capable of handling sophisticated GAaircraft. To be most attractive to users, airportsshould also have commercial services for aircraftservicing, repair and maintenance, groundtransportation, and flight crew amenities. Withsufficient amenities, such an airport might evenattract some commuter airline service, althoughtransfers and interlining would be difficult un-less the airport is served by several carriers orhas excellent ground access to a major hub. Insome cases, however, the provision of better fa-cilities may not be sufficient to divert additionalGA traffic away from major hub airports. In-creased landing fees at the major airport can

‘Report of the President’s Task Force on Aircraft Creu) Comple-ment (Washington, D. C.: July 2, 1981).


provide additional incentives for this shift, looked upon as a complement to the Federal pro-and such pricing policies—the domain of local gram of investment in satellite airports.government and airport authorities—could be

AIRPORT DEVELOPMENT ALTERNATIVES

Expanding Existing Airports

Because runway availability is the major con-straint on airside capacity, one way to increasecapacity is to add more runways. A new longrunway, properly equipped for independent IFRoperations can increase an airport’s capacity by20 to 50 percent depending on the original run-way configuration.

Adding another runway, however, requires alarge amount of land. One 11,000-ft runway forlarge jet operations with its basic safety areascovers 130 acres, and when other necessary“clear zones” are considered, an area three tofour times that size would be directly affected.Further, the additional operations enabled bythe new runway would probably require land-side additions such as new gates, terminal space,and parking for more passengers. Few airportshave the necessary land for this kind of expan-sion, which could add approximately 10 percentto their present area, and for some airports likeWashington National and La Guardia, the pros-pect is especially bleak. Even for larger airports,obtaining proper spacing from other runwayswould be extremely difficult.

A 1977 report by the Department of Trans-portation (DOT) studied the possibility of majorexpansion at 24 airports to meet projected needsfor 1985-2000. Expansion was found to be “feasi-ble” in only four of these cases, and none ofthese four airports (Detroit, Houston, Minne-apolis, and Pittsburgh) are among those whichare experiencing the greatest capacity problems.In 9 other cities the DOT study found expansion“feasible within major constraints,” and in 11cases it was considered “not feasible.” Botheconomic and environmental reasons were citedfor preventing the land acquisition. ’ Airport de-

‘Establishment of New Major Public Airports in the UnitedStates (Washington, D. C.: Federal Aviation Administration,August 1977), p. 6-5.

signers foresaw the need for growth and mostmajor airports were built where land was plenti-ful, but sites that were on the edge of town in1925 or 1948 are now in the middle of urban de-velopment. In some cases the airport itself at-tracted businesses; in other cases developmentsimply resulted from good highways, suburbani-zation, and all the other forces which havecaused urban areas to expand over the years.Developed land tends to be expensive to buy: arecent study of the cost to acquire and clear landaround some major air carrier airports estimatedthese costs at between $100,000 and $200,000per acre.8 Noise is among the largest environ-mental obstacles to airport expansion. Chicago-O’Hare has sufficient land for an additional run-way, but the runway has not been built in partbecause it would cause unacceptable noise expo-sure in nearby neighborhoods. JFK Airport inNew York is surrounded by intensive develop-ment on one side and a National Park and Wild-life Sanctuary on the other, making expansionunlikely. Dallas-Fort Worth, on the other hand,is planning an additional major new runwaythat is expected to ease some of the capacity lim-itations imposed by noise abatement proceduresand airspace conflicts with nearby Love Field.

Development of SecondaryRunway Operations

At some airports where major expansion isunlikely it may still be possible to add one shortrunway for smaller, slow-moving commuter andGA aircraft. This could improve airport capaci-ty by diverting traffic from the longer runwaysand may also provide a partial solution to thewake vortex problem (previously discussed).Many airports routinely use short runways, orsections of long runways, for small aircraft dur-

t‘Louis H. Mayo, Jr., “Noise Compatible Land Uses in Airport

Environments, ” Environmental Comment, March 1979, p. 9.


ing good weather, but because of inadequatelanding aids or spacing these runways cannot beused during bad weather; all-weather operationswould require additional navigational and ap-proach guidance equipment.

One study found that the use of short IFR run-ways for small aircraft was feasible at 11 of 30major airports. Of these 11, suitable runways al-ready existed at 3 airports, existing runwayscould be extended for use at 2 others, and at 6airports space was available for short runwaysto be constructed. The study estimated that thevalue of reduced delays brought about by theaddition of such runways might be $450 millionto $810 million in current dollars between 1980and 1990 at the airports shown in table 10. Thebenefits would be unevenly distributed: Chi-cago, Atlanta, Philadelphia, and Denver wouldreceive 80 to 85 percent of the estimated savings;among the users, 86 to 89 percent of the savingsfrom reduced delays would accrue to the air car-riers. 9

A detailed study of the airfield and airspace ateach airport would be needed to see if the shortrunway could really be constructed. Such stud-ies done at Denver revealed two possible loca-tions for a short GA runway. Construction ofeither one could lead to a 35 to 70 percent in-crease in hourly operations, depending onweather conditions. Total cost was estimated atabout $10.8 million. 0

Building New Airports

Another way to increase airport capacity is tobuild a completely new airport to replace or sup-plement the existing one, an alternative that isespecially attractive where landside facilities(terminals, baggage equipment, parking) arealso outmoded or inadequate. A new site wouldprovide the opportunity to design and build run-ways, terminals, and parking space to meet fu-

‘John D. Gardner, Feasibility of a Separate Short Runway forCommuter and GeneraZ Aviation Traffic at Denz~er, prepared forthe Federal Aviation Administration by The Mitre Corporation,McLean, Va., May 1980, pp. 1-1.

‘“John D. Gardner, Extensions to the Feasibility Study of a Sep-arate Short Runway for Commuter and General Auiation Trafficat Denuer, prepared for the Federal Aviation Administration byThe Mitre Corp., McLean, Va., September 1980, pp. 4-3 and 7-1.

ture needs, rather than making do with what hasevolved over time. Sufficient land could be pur-chased to allow for future growth and properland-use controls could be applied so that noisecompatibility problems do not arise again. Insome recent airport relocations, however, thisdid not work as well as hoped. For example, atboth Dallas-Fort Worth Regional Airport andKansas City International Airport, built in themid-1970’s, encroachment by other land uses isagain leading to complaints about airport noise.On the other hand, Montreal’s new Mirabel Air-port seems to have little problem with noiseincompatibility; the airport itself covers 17,000acres, and is surrounded by an additional 21,000acres controlled by a specially created municipalauthority. However, its distance from the citymakes access a problem.

Building a new airport also provides an op-portunity to add a large amount of new airsidecapacity to a region. The opening of Kansas CityInternational, for example, more than doubledthe available capacity in that hub from the esti-mated 195,000 operations at the old municipalairport to about 445,000 with the new airport.Love Field in Dallas handled 410,000 operationsin 1972; in 1977, after air carrier operations weretransferred to Dallas-Fort Worth Regional Air-port, Love Field still had 310,000 operations(mostly GA), while the new airport had 385,000.

A 1977 investigation by DOT found that any-where from 2 to 19 new airports might be neededin the United States by the year 2000, dependingon the growth rate assumed. When the studyexamined the feasibility of new airport construc-tion for 10 hub areas, it found it to be “feasible”in four instances, “doubtful” in four, and “notfeasible” in two. The reasons for the “doubtful”and “not feasible” findings are related primarilyto site location, land acquisition, funding prob-lems, and the difficulty of providing adequateground access to a remote location. The FAA’s1980 NASP foresees the possibility of a new air-port opening at Palmdale, Calif. (near Los Ange-les), within the next 10 years; some initial workon new airports at Atlanta and San Diego mightalso be expected within the next decade. ’ 2

‘ ‘Establishment of New Major Public Airports, op. cit., p. 7-16.“National Airport System Plan, Revised Statistics 1980-1989

(Washington, D. C., Federal Aviation Administration, 1980) p. vi.

Ch. 6—Airport Capacity Alternatives • 115

Table 10.—Operational Characteristics of Airports With Potential Benefits From aSeparate General Aviation Runway

Parallel Parallel Nonparallelindependent dependent dependent

Modification operations operations operations

New runway . . . . . . . . . . . Chicagoc, Atlantac, Philadelphia,Dallas-Ft. Worthc, Denver Pittsburgh ab

Existing runway Portland,or taxiway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detroit a,b St. Louis

Extension of New York (JFK)a,Existing runway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Indianapolis

aThe ~enera] aviation runway is Independent of 1 of 2 air Carrier runways for departures.bGeneral aviation runway handles departures OnlYc_friple parallel runways.

SOURCE: J. D. Gardner, “Feasibility of a Separate Short Runway For Commuter and General Awatlon Traffic at Denver, ”prepared for the Federal Aviation Administration by Mltre Corp., McLean, Va., May 1980.

Building a new airport is a huge undertaking.A new air carrier airport can represent an invest-ment of $5 billion, to be shared among the air-port sponsor (and local taxpayers), airport con-cessionaires, the airlines (through their landingfees), and the Federal Government. Even a mod-est-sized GA airport would cost several hundredmillion dollars. The length of time required forplanning and construction of a large airport—up to 10 years—can also add substantially tocosts. Political and institutional factors can alsopose substantial difficulties. Building an airportrequires agreement from existing air carriers tomove to the new facilities, but while a new air-port can reduce delays it will also increase airlinecosts, and they must be convinced that the bene-fits will outweigh the costs. Further, approvaland support of a number of State, county, andmunicipal governments, not to mention high-way districts, zoning commissions, and variouscitizens’ interest groups, must also be secured.

In some cases the divergent interests of dif-ferent governments and constituencies can snarlthe process. In St. Louis, for example, a site for anew airport was selected across the MississippiRiver in Illinois. The Illinois State governmentwas a major supporter of the project, as were theSt. Louis city government and FAA. The oppo-nents included citizens groups of the countywhere the new airport would be located (whoobjected on environmental grounds), the Stateof Missouri (which did not want the airportmoved out of the State), and groups in St. Louis

(which did not want the city to give up the close-in Lambert Airport). The project was debatedfor several years, but it was shelved after achange in the St. Louis city government.

Photo credit: Federal Aviation Adrninistration

The design of a modern airport: Dallas-Fort Worth


ATC IMPROVEMENT ALTERNATIVES

As mentioned earlier, existing ATC proce-dures and equipment can represent constraintson the airside capacity. Improvements in theseareas can increase the number of aircraft opera-tions.

Airfield/Airspace ConfigurationManagement

The ATC team at an airport decides how therunway and ATC equipment should be usedbased on wind, visibility, traffic mix, ratio of ar-rivals to departures, noise-abatement proce-dures, and the status of the airport (which runways or landing aids are under repair, etc.). In alarge air carrier airport like O’Hare, there maybe 40 or 50 ways in which the runways can beused, so deciding which one offers maximum ca-pacity for any particular set of conditions is acomplex task. The problem is compounded bythe interdependence of runway use and the con-figuration of the surrounding airspace. For ex-ample, changing which runway is used for land-ings may change the route that approaching air-craft must take through the terminal airspace,which may in turn affect or be affected by activ-ity at other airports in the vicinity.

One FAA analysis of capacity and delay prob-lems in Chicago suggested that proper manage-ment of airfield and airspace could have a largepayoff:

Optimized management of the air traffic con-trol system . . . could achieve now, at mini-mum investment cost, savings comparable tothose that will be achieved much later at muchhigher cost when third generation ATC hard-ware is deployed. This highlights the impor-tance of FAA management exploration of op-portunities for improved system efficiency byplacing emphasis on optimization of operationsat least equal to that given development of ATChardware. 13

After study of the runway system of O’Hare air-port, the task force found that a computerized

“Delay Task Force Study, Volume 1: Executive Summary,O’Hare International Airport (Chicago: Federal Aviation Admin-istration Great Lakes Region: July, 1976), p. 4.

airspace/airfield management system could beused to assist the controller team in selecting thehighest capacity and most energy-efficient run-way use for each set of circumstances.

Such a system could have several levels ofcomplexity. In its basic form it would aid in se-lecting the preferred runway configuration for agiven set of conditions; this basic system isunder development by FAA. The intermediateform would update this assessment as changes inweather or traffic conditions arise, and thenselect the most efficient means of making thetransition from the one configuration to an-other. (This is important because the transitionperiod is often a time when airspace and airportcapcity are wasted. ) The advanced versionwould have the ability to make longer term stra-tegic decisions. The 1978 Chicago task force sug-gested that savings of $11 million to $16 millionannually in reduced delay costs might be ex-pected from the basic system alone.14

Wake Vortex Prediction

Alleviation of the wake vortex problem offersthe possibility of a substantial potential payoffin increased capacity without large capital ex-penditures for new runways. Research over thepast decade has shown some possible ways ofdoing this. For example, it has been found thatcertain wind conditions can quickly dissipate avortex or remove it from the path of oncomingtraffic. If wind conditions can be accuratelymonitored and quickly analyzed, then the likeli-hood of wake vortex danger can be known on aminute-by-minute basis.

FAA has been testing such a system at O’HareAirport since 1977. Wind sensors are located on50-ft towers near the runway ends. A computeranalyzes wind conditions and when persistentvortexes are unlikely it gives the controller teama “green light” to permit reduced separations onfinal approach. To have maximum effect (e.g.,to allow all separations to be reduced to 3 nmi),an advisory system would have to be able to————

“Ibid.


predict the likelihood of wake vortexes atgreater distances and higher altitudes than theChicago system now does. However even thisprototype system has been credited with allow-ing reduced average separations, and thus moreoperations per hour, at O’Hare. There are nocurrent FAA plans for implementing full scalewake vortex advisory systems at other airports.

Microwave Landing System (MLS)

As discussed in chapter 5, MLS allows air-space to be used more efficiently than the cur-rent ILS, since aircraft would be able to ap-proach the airport on curved paths, as they dounder visual conditions, and turn onto theirfinal approach much closer to the runway.”Variable MLS glide slope angles could also pro-vide a partial solution to the long separationsrequired to avoid wake vortex; with MLS, thetrailing aircraft could avoid the vortex by ap-proaching the runway at a steeper angle than thelead aircraft.

Models suggest that where the traffic mix con-tains a variety of fast and slow aircraft, the useof variable glide slopes could allow some capaci-ty improvements—perhaps around 10 to 15 per-cent. However, where aircraft have similar per-formance characteristics, MLS landing proce-dures would offer about the same capacity ascurrent ILS procedures. MLS would also allowthe restructuring of airspace at some airports, sothat small aircraft can approach the airport in aseparate arrival stream from jets and make useof a separate short runway. The Dash-7 aircraftin Ransome Airlines’ Washington-Philadelphiaservice use MLS equipment to land on short run-ways.

MLS equipment has been developed, tested,and accepted for international use. Field evalua-tion is taking place at such airports as Washing-ton National, and FAA has published a plan forfull-scale implementation beginning in themid-1980’s and to continuing into the next cen-

“An Analysis of the Requirements For and the Benefits andCosts of the National Microwave Landing System, Volume 1(Washington, D. C.: Federal Aviation Administration, June 1980),p. 2-3.

tury. One reason for this delayed schedule maybe the international agreement to maintain ILSuntil 1995; and another reason is the reluctanceof users, principally the airlines, to install MLSavionics in aircraft already equipped with ILSavionics.

Reducing Separation orSpacing Minimums

Several studies have suggested that wherewake vortex is not a problem (for example,where aircraft have similar performance charac-teristics) it maybe possible to reduce separationsfrom 3 nmi to as little as 2.5 or 2 nmi. Theamount of time each aircraft spends on the run-way is another constraint in reducing separa-tions, and depends on such factors as the num-ber and spacing of the exits, visibility, runwaysurface conditions, and the performance charac-teristics of the aircraft. In general, small, lightaircraft spend less time on the runway thanlarge, heavy ones. According to surveys, mostairports have an average runway occupancytime of between 41 and 63 seconds for landing,although these figures do not include the raresnowy or icy days when separations might haveto be extended to allow time for aircraft to brakesafely and exit the runway. Where the averagerunway occupancy time is so seconds or less, ithas been suggested that the minimum separationcould safely be reduced to 2.5 nmi instead of 3nmi. Greater reductions might be possiblethrough automated metering and spacing.

Another way of increasing airfield capacity isto reduce the required spacing between run-ways. For example, runways must be 4,300 ftapart for simultaneous IFR operations to takeplace. Reduction of this minimum to 3,500 or3,000 ft would enable some airports to make useof more of their runways during IFR conditions.Minimum spacing standards have been reducedbefore (e.g., from 5,000 to 4,300 ft for independ-ent parallel IFR runways in the early 1960’s) as aresult of improvements in surveillance equip-ment and procedures.

“William J. Swedish, Evaluation of the Potential for ReducedLongitudinal Sparing on Final Approach, prepared for the FederalAviation Administration by The Mitre Corp., McLean, Va., p.4-1.


FAA is also investigating the possibility of al-lowing instrument approaches to triple parallelrunways during poor visibility. Currently tripleparallels can be used only during good visibility.One of these three runways might be a shortrunway for commuter or GA aircraft. Efficientuse of triple parallels would require redesign ofthe airspace and approach patterns, a higherdegree of coordination between approach con-trollers than is currently the case, and possiblemodifications to the ILS. MLS, with its greaterflexibility and navigational precision, might beuseful in bringing this procedure into practicaluse. Use of triple parallels could make it possibleto make use of more existing runways duringpoor weather, as at O’Hare, or even to allowconstruction of new runways which are infeasi-ble under current procedures. Capacity im-provements would depend on traffic mix and onwhether the runways had sufficient spacing toallow independent operations. Models indicatethat triple parallel runway systems might handleup to 50 percent more IFR operations than dou-ble parallels with traffic mixes typical of today’smajor airports.l 7

A number of airports have been identifiedwhich might benefit from either reduced spacingstandards or from use of triple parallel ap-preaches.18 However, site-specific analyses ofthe airfield and airspace of each candidate air-port are needed to measure the capacity benefits,costs, and safety effects of these proposedchanges.

Automated Metering and Spacing

The controller’s ability to meter aircraft—todeliver them to a specific point at a specifictime—is based on aircraft speed and position asshown on the radar screen and the controller’s

‘ ‘T. N. Shimi, W. J. Swedish, and L. C. Newman, Requirementsfor lnstrurnent Approaches to Triple Parallel Runways, preparedfor the Federal Aviation Administration by The Mitre Corp.,McLean, Va., 1981, p, E-7.

‘“L. C. Newman, T. N. Shimi, and W. J. Swedish, Sur-ocy of 101U.S. Airports for New Multipfe Approach Concepts, prepared forthe Federal Aviation Administration by The Mitre Corp., McLean,Va., 1981, p. xxiv, 5-4, 6-2; and A. L, Haines and W. J. Swedish,Requirements for Independent and Dependent Parallel instrumentApproaches at Reduced Runway Spacing, prepared for the FederalAviation Administration by The Mitre Corp., McLean, Va., 1981,passim.

instructions to change speed or direction inorder to arrive at the runway threshold at theproper time. Using this manual system the con-troller’s training and experience allow him todeliver aircraft to the runway threshold with anerror (standard deviation) of about 18 seconds.l9

It has been suggested that an automated sys-tem could provide more accurate metering andspacing. In such a system, the ATC computercould analyze radar and transponder data di-rectly and compute future aircraft location withgreat accuracy, then generate commands de-signed to deliver each aircraft at a specific timeand thereby optimize the use of the runway’scapacity. It has been suggested that an auto-mated system could reduce the delivery error toabout 11 seconds.20 The automated concept hasbeen under development at FAA for about 10years but has not yet been approved for imple-mentation. FAA states that the computerizedmethods developed so far are not as reliable as ahuman controller. In addition, FAA believesautomated terminal metering and spacing willnot be of much value unless it can be tied in withen route metering and other aspects of ATCautomation now under development (see ch. 5).

Cockpit Engineering

Advances in technology are in fact changingthe basic character of the cockpit. Electrome-chanical instruments are being replaced withelectronic displays that present full-color imageswith a very high degree of resolution. Comput-ers are also expanding the range of functionsthat can be performed by aircrew. Advancednavigation aids such as area naviation (RNAV)make it possible to navigate from point to pointwithout following established airways. The FAAhas suggested the use of a data link to improvethe quality of the information available in thecockpit. A cockpit display of traffic information(CDTI), currently under investigation at the Na-

“New Engineering and Development initiatives—Polic y andTechnology Choices, coordinated by Economic and Science Plan-ning, Inc. (Washington, D. C.: Federal Aviation Administration,March 1979) p. 107.

20 Parameters of Future A TC Systems Relating to Airport Capac-ity/Delay (Washington, D. C.: Federal Aviation Administration,June 1978).


tional Aeronautics and Space Administration airport and airway facilities are used. Therecould show pilots the locations of nearby air- have been suggestions that the distribution ofcraft, thus reducing their dependence on ground the decisionmaking function in the ATC systemsurveillance. Both RNAV and CDTI offer pilots must or should be changed to take advantage ofsignificant independence from controllers, and the capabilities these technological advancesthis could increase the effectiveness with which have made possible (see ch. 5).

SUMMARY OF ALTERNATIVES

The alternatives discussed above all make use the other hand, rely on the application and en-of some combination of economic, regulatory, forcement of regulatory measures to deal withor technological tools to reduce delay or increase the delay problem. Automated metering andairside capacity. For example, peak-hour pricing spacing is a technological tool, but its use will re-is an economic alternative—allowing the market quire changes in existing rules and standards.to allocate scarce airport capacity. Quotas, on Table 11 summarizes the alternatives discussed

Table 11 .—Summary of Alternatives

EconomicAlternative incentives Regulation Technology Comments

Change of airlineservice patterns

Reliever airports

●

●

●

Demand-relatedPeak hour pricing ● Could be implemented by local airport authority.

Devising and managing the pricing scheme maybe complex, but it could provide a substantiallong-term payoff in reduced delay.

Quotas Could be implemented by local authority or FAA.Would provide some short-term relief for con-gestion and delay problems but is an inefficientlong-term solution. FAA has already imposedquotas at 4 airports since 1969.

Balanced use of ● Could be implemented by local authority whichmetropolitan airports might use economic incentives, improved ac-

cess, and better facilities to encourage use ofunderutilized airports; or could use regulation toimpose it.

Airlines may voluntarily shift some of theirhubbing activities to less congested airports tosave delay. (This trend seems to already beunderway.) The FAA might also be able toachieve this redistribution by regulation. Thiswould make better use of airport capacity na-tionwide, but might do little to reduce delays atcongested airports.

FAA has already designated reliever airports.Many are well used by GA traffic. Localauthorities encourage this trend with pricingstrategies, better facilities, or regulations requir-ing use of relievers by certain classes of users.Relievers have been and will continue to be suc-cessful in providing capacity for GA operationsaway from congested commercial airports.

Airport developmentAirport expansion ● Responsibility of local authorities, possibly with

Federal aid. Could greatly increase capacity,but is unlikely in many locations because of sur-rounding development or environmental prob-lems.


Table Il.–Summary of Alternatives (Continued)

EconomicAlternative incentives Regulation Technology Comments

Addition of short ●

runway

New airport construction

ATC alternativesAirfield/space

management

Wake vortexprediction

Microwave landingsystem

Reduced separationor spacing standards

Automated meteringand spacing

Cockpit engineering

●

●

●

●

●

●

●

●

●

●

●

●

Possible in several airports to provide a separatetraffic stream for GA and commuter aircraft.Increases capacity for both small and large air-craft. Responsibility of local authority withpossible Federal aid. Cost estimate for Denverwas $10 million to $11 million.

Responsibility of local authorities with Federalassistance. Could have a major impact on localairside capacity, but is unlikely in many areasdue to expense, lack of close in suitable land.Good high-speed ground access might makemore distant airports likely in long range.

Allows modest capacity gains by making betteruse of the runways available. Computerizedsystem has been tested in Chicago. Similarsystem could be developed and implemented inother areas by local authorities and FAA.

FAA would be responsible for installing vortexdetection or advisory equipment. FAA hastested one wake vortex advisory system whichprovides some capacity benefits, but is still inthe experimental stage.

Benefits are more efficient use of airspace andavailability of variable glide slopes which,among other things, can allow aircraft to avoidwake vortexes. Fairly substantial increases incapacity available where traffic mix is diverse.The technology now exists and FAA will prob-ably install ground equipment in the 1985-2000period. FAA’s installation costs are estimated tobe $300,000 to $500,000 per airport. Users costsfor avionics will range from $1,500 to $30,000per aircraft.

Responsibility of FAA. Reduction of thesestandards could offer large capacity increases,but FAA’s first priority is safety of the system.Reduction of standards is unlikely without sometechnological change—elimination of wakevortex problem or improved navigation orsurveillance.

increased accuracy of metering could optimizerunway use, offering modest capacity increases.FAA has not yet developed a program which itfeels ready to implement. FAA wants to in-tegrate terminal automated metering and spac-ing with the automated en route system, im-plementation might not be possible until afterthe replacement of the en route computersystem.

RNAV technology is already available. Users mustbuy the avionics, FAA is responsible fordeveloping RNAV procedures which mightreduce delays somewhat. Cockpit displays oftraffic information are being developed andtested by the FAA but will not be available inthe near future.



above and indicates generally what types oftools—economic, regulatory, or technological—would be required to implement them. The com-ments in table 11 touch on several points—whocan implement the change, whether it wouldmake a large or small change in capacity, andhow likely it is to take place in the short or longterm.

In general, the demand-related alternatives donot increase capacity; rather, they reduce delayby molding traffic activity to fit existing capaci-ty. Modest capacity gains are available throughATC improvements that increase the efficiencywith which airfield and airspace are used, espe-cially under IFR conditions, but the benefitavailable to each airport is heavily dependent on

local conditions of runway configuration andtraffic mix. The addition of new runways isclearly effective in increasing capacity, but thisoption is available to only a limited number ofairports. In a few cases, short runways could beconstructed to increase capacity by separatingjet and propellor traffic. New airport construc-tion also offers large capacity gains, but theywould likely be further from cities and thereforeface the problem of ground access. Reliever orsatellite airports to move GA out of air carrierairports are necessary unless the growth of bothuser groups is to be severely limited, but relieverairports will also be constrained by land prices,noise impacts, and community acceptance.

FUTURE RESEARCH NEEDS

Several areas offer possibly fertile ground forfuture research on means to increase airport air-side capacity.

Wake Vortex Avoidance

The FAA’s wake vortex advisory system hasbeen discussed, but more research is needed todevelop operational versions of this systemwhich can predict vortex problems at greaterdistances from the runway ends—say, back tothe ILS middle marker or outer marker. FAA hasalso studied the use of acoustical radar and lasersto detect actual vortexes. Although some prog-ress has been make in understanding the natureof vortexes, these techniques are far from opera-tional. However, with further research this lineof inquiry may be the basis for a ground-basedor airborne wake vortex detection system.

Wake Vortex Alleviation

Also important is the possibility of modifyingor minimizing vortexes at the source. NASA re-search has shown that certain combinations offlaps, spoilers, or protrusions on the wings ofaircraft can cause the wake vortex to be unstableand therefore to dissipate more quickly. Trailingaircraft can then follow closer in safety. These

methods, however, also tend to increase thenoise level and decrease the energy efficiency ofthe aircraft. More work needs to be done to de-velop a system which minimizes the vortex withan acceptable price in terms of noise and fuel.

Noise

Many current noise abatement procedures re-quire a tradeoff in terms of reduced airspace andairport capacity. As long as aircraft remainnoisy, however, there is little alternative to rout-ing them away from noise-sensitive areas. Somenew and re-engined jet aircraft are much lessnoisy than their predecessors, but it has beensuggested that technology may have gone as faras it can, and that administrative solutions arethe only alternative. In any case, a great deal offurther research is needed to develop creative so-lutions to the noise problem.

Airport Design

The scarcity of suitable land for expanding ex-isting airports or building new ones means thatnew research is needed on basic concepts of howan airport and its access system should be de-signed. For example, it may be possible to re-design the runway-taxiway system in a manner


that is less profligate of land. Research is neededinto the safety and capacity questions raised bythis type of design. In some locations where littleland is available for a new airport, it may bepossible to locate an airport on a nearby lake orbay. Such an airport would be expensive tobuild, even when the necessary technology hasbeen developed, but in some cases it might bethe most cost-effective alternative.

Ground Access

Airport access is a major area of concern.Research is needed not only to alleviate the ac-cess problems plaguing some of today’s majorairports, but also on cost-effective means to getpassengers out to new airports which may haveto be constructed at distances of 30 to 50 milesfrom the city center.

Chapter 7

POLICY IMPLICATIONS

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Air System Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Discussion . . . . . . . . . . . . ......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airport Capacity Alternatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ATC System Improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Computer Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Automated En Route Air Traffic Control (AERA). . . . . . . . . . . . . . . . . . . . . . . . . .Mode S Data Link. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Collision Avoidance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Microwave Landing System (MLS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Funding and Cost Allocation Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

General Fund.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Trust Fund. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Operating Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pending Legislation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .System Modernization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Airport Development Aid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Trust Fund Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .User Taxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURE

Figure No.

29. Airport and AirwaysTrust Fund Expenditures, 1971-80. . . . . . . . . . . . . . . . . . . .

Page125125125126127127128129129130130131132133133133134136136138138138139140140

Page135

Chapter 7

POLICY IMPLICATIONS

INTRODUCTION

The letter from the House Committee on Ap-propriations requesting this assessment indicatedthe

●

●

●

●

following areas of concern:

scenarios of future air transportationgrowth;alternative ways to increase airport and ter-minal capacity;proposed modifications of air traffic control(ATC) system technology; andalternatives to the present ATC process.

OTA’s analysis of these subjects is presented inchapters 4, 5, and 6; this chapter summarizes themajor points emerging from those analyses andexamines their implications in terms of congres-sional interests. The intent is to highlight those

aspects of air system evolution that may be ofparticular concern to the Congress in evaluating

the Federal Aviation Administration’s (FAA)1982 National Airspace System (NAS) Plan .

The following discussion is organized underthree major headings. Under each heading is abrief statement of findings followed by a discus-sion of specific problems and implications. Afourth section deals briefly with the related ques-tions of funding and cost allocation, which mustalso be addressed in the years ahead. The finalsection reviews recent congressional reports onthese subjects and identifies the relevant legisla-tion now pending before Congress.

AIR SYSTEM GROWTH

Findings

Chapter 4 compares recent FAA AviationForecasts and those of several other sources. Thefollowing major points emerge from that com-parison:

● FAA projections of future demand haveconsistently been too high in the past, inpart because of the way they are made: theyassume that past trends will continue, thatthere will be no constraints on continuedrapid growth, and that proposed ATC im-provements will in fact be made when andwhere needed to accommodate that growth.However, other sources (including RollsRoyce and the Air Transport Association)feel that the airline industry is already ap-proaching its mature size; this could lead toa leveling off or even a decline in air carrieroperations. There is also considerable un-certainty about a number of other factorsthat might affect future aviation activity,such as changes in U.S. economic or regula-tory policy, the long-term impacts of airline

deregulation and the PATCO strike, andthe ability of airlines to finance new equip-ment, Given these uncertainties and thequestionable economic assumptions under-lying the 1981 baseline projection on whichthe 1982 NAS Plan will be based, Congressmay wish to reexamine the deploymentschedule proposed by FAA for major ATCsystem improvements.There will be some growth in the system,but the rate of growth will be slower thanwas experienced in the past and may beslower than has been anticipated even in re-cent forecasts. The various scenarios sug-gest that a 2- or 3-percent annual growthrate for total operations at FAA-toweredairports would be a reasonable expectation,although the rate might be as low as —1percent or as high as +5 percent, dependingon a variety of economic, regulatory, andoperational factors that cannot be reliablypredicted. En route and flight service work-loads are likely to increase as fast or fasterthan tower operations.

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● There is disagreement about the exact distri-bution of this future growth among usergroups, but the forecasts generally agreethat general aviation (GA), and especiallyair taxis and corporate aircraft equipped forIFR operations, will be the fastest growingcategory. GA may account for as much as75 percent of the increase in tower work-load, particularly if FAA (as planned) in-creases the number of towered GA and re-liever airports. Commuter operations willincrease moderately, on the other hand,and air carrier operations (not passengertraffic) may actually decline at some hubs.

• The relatively rapid growth of GA demand,combined with the slower growth of com-muter and air carrier operations, couldhave several effects on the U.S. airport andATC system:—Unconstrained growth of operations at

major hubs would lead to saturation at 15to 20 airports by 2000, compared with 5to 10 airports today. Growth rates above4 percent annually, which are possiblebut unlikely, might result in saturation at

—all 50 of the top air carrier airports by theend of the century.

—In the absence of capacity improvementsat saturated hubs, increasing congestionand delay will probably result in furtherredistribution of air carrier operations(especially transfer functions) away fromsaturated major hubs to “second tier”hubs where surplus capacity still exists.

–Similarly, GA traffic is likely to beshifted out of more and more air carrierhubs to reliever and other GA airports.This will create a demand for improvedfacilities at those airports.

—As a result, the principal opportunitiesfor capacity expansion will come not atthe major hub airports but rather at thesecond-tier hubs and at GA and relieverairports, as well as at the air route trafficcontrol centers and flight service stations.If these increases in ATC system capacityare to be provided without greatly in-creasing FAA’s operation and mainte-nance (O&M) expenditures, expanded

use of automated andwill be required.

Discussion

remote facilities

Forecasts of aviation activity are subject tothree principal kinds of uncertainties, all ofwhich affect the accuracy and usefulness of theresulting projections of airport and ATC systemdemands:

●

●

●

There is no common purpose or focus—airline forecasts concentrate primarily onmeasures of carrier profitability, aerospaceforecasts on potential aircraft markets, andFAA forecasts on ATC workloads.All of the projections nevertheless employ asimilar methodology and rely on similardemographic and economic expectations.Specifically, the forecasts assume a continu-ation of the past relationship between grossnational product growth and increased de-mand for air travel. As a result, common-mode failure is possible—the forecastscould all be wrong for the same reason.All of the forecasts are subject to factorswhose future influence can only be guessedat, including the price and availability offuel, the effects of airline deregulation, theresulting changes in industry structure, thelong-term impacts of the air controllersstrike, the uncertain availability of financ-ing for reequipping airline fleets, and futurechanges in Federal aviation policy or costallocation.

As a result, there is general agreement on thelikelihood of future growth, but little certaintyabout its magnitude, and still less about themore important questions of when and wheregrowth will occur or what its impact will be onthe Nation’s airport and ATC system.

Continued growth along historic patternswould exacerbate congestion and delay at hubsthat are already saturated and would probablyspread these problems to additional airports.This would present two possible courses of re-sponse:

. accommodate the growth wherever it oc-curs (as FAA has done in the past) by at-

Ch. 7—Policy /replications . 127

●

tempting to expand the capacity of affectedhubs; orchannel the growth, either actively or pas-sively, so that it can be accommodated atother hubs.

Neither of these courses will be applicable in allsituations, and in most cases the solution will in-volve some combination of the two; finding theproper balance will require a case-by-case analy-sis of their relative costs and benefits.

Adding new capacity at congested hubs—inthe form of new runways or entirely new air-ports—could be extremely expensive in relationto the number of additional operations that canbe accommodated. There are, however, a num-ber of traffic management techniques that couldincrease the efficiency with which existing ca-pacity is utilized at airports and terminal areasthat are already saturated.

There are clear indications that market forceshave already begun to alter the historical pat-terns of demand distribution. Some airlines,faced with high delay costs and strike-related re-strictions at congested hubs, are finding it attrac-tive to move some of their “hubbing” or transferoperations to well-equipped second-tier hubswhere available capacity exists and delay costscan be avoided. Local service airlines, with thenew route and entry freedom of deregulation,are beginning to increase the number of direct-service flights, and, consequently, to decreasethe number of transfer operations. New entrantsand low-cost carriers, unencumbered by largeinvestments in facilities at congested hubs, are

basing their operations at second-tier hubs. Al-though this trend may involve a small decreasein the operational efficiency of system users, itwould greatly increase the efficiency with whichthe airport and ATC system’s aggregate capacityis utilized.

Growing congestion could have serious impli-cations for commuter and GA users, who wouldbeat a considerable disadvantage in any compe-tition for access to congested hubs. Neither usergroup is likely to be completely priced or regu-lated out of major hubs, but growing congestionmay nevertheless prove to be a significant con-straint on their future growth. Additional GAoperations might be accommodated at relieverand other GA airports; this would make morecapacity available at existing hubs, but it couldalso lead to additional FAA investments andoperating costs for new towers at lightly usedGA airports. (FAA plans have called for asmany as 50 new towers by 1993, but its experi-ence in closing over 60 low-volume towers sincethe PATCO walkout justifies a review of theseplans. ) Commuter carriers, on the other hand,will continue to require access to hub airports,since most of their passengers transfer to otherflights, Rehubbing by major airlines will notchange this requirement and might even createadditional complications in commuter routesand operations, although it might also createnew market opportunities for commuter air-lines. In addition, commuter and GA users willgenerate most of the new demand for en routeand flight services.

AIRPORT CAPACITY ALTERNATIVESFindings

The committee asked OTA to examine the“relative merits of alternative ways of increasingairport and terminal capacity to meet future de-mands and reduce safety hazards. ” The toolsthat can be used to increase capacity or reducedelay are examined in chapter 6, where the ma-jor findings and implications are:

● Changes in ATC equipment or procedurescan produce small increases in airside capa-

city by helping aircraft use available air-space and runways more efficiently. How-ever, large capacity improvements, such aswould result from greatly reducing thedistance between aircraft on landing andtakeoff, must await technological break-throughs like improved prediction of wakevortices.

. Where ATC improvements are made, they


would not necessarily eliminate the prob-lem of delay: latent demand at a popularairport could quickly consume new capaci-ty, and the length of delay would remainthe same.

. Major increases in the physical capacity of ahub would require building new runways orentire new airports. Such major improve-ments are unlikely to be made in the nearfuture because of the unavailability or highprice of land, costs of construction, andnoise and other environmental constraints.

• If growth continues, however, some newmajor airports may have to be built. Sincethey are likely to be some distance from thecenter city, the success of these airports willdepend upon suitable high-speed ground ac-cess. (Dunes International Airport demon-strates the need for such access. )

● Congestion at large hub airports may in-duce use of a variety of techniques to maxi-mize effective capacity, including hourlyquotas and peak-hour pricing. GA users arelikely to be the major losers in competitionfor slots at congested airports, althoughthese restrictions might also constrain thegrowth of commuter carrier operations.

● If air carriers continue to redistribute theirtransfer operations to second-tier hubs,some added investment will be required atthese airports.

● In the near term, two forms of capacity ex-pansion can be helpful: 1) construction atcongested airports of separate, short run-ways, equipped for instrument operations,for use by small aircraft; and 2) construc-tion or improvement of reliever airports toaccommodate GA traffic diverted fromcongested commercial airports.

Discussion

Some improvements can be expected fromchanges in ATC equipment or procedures incongested terminal areas; but the net effect ondelay would be quite small. For instance, com-puterized airfield/airspace management mightallow better utilization of existing physical capa-city, so that actual operations would approachthe theoretical maximum for each combination

of weather and traffic conditions. The Micro-wave Landing System (MLS) might also allow asmall increase in the number of InstrumentFlight Rules (IFR) operations under certain con-ditions of traffic mix. In general, mostly becauseof the separation required by the danger of wakevortex, there will be no significant ATC-relatedincrease in the number of aircraft operationsthat can be handled by a given runway, airport,or terminal area.

Past Federal, State, and local airport policyhas been to provide new capacity where demandseemed to warrant it, if at all possible. Most oftoday’s congested airports have gone throughperiods of major expansion, only to become sat-urated by subsequent growth. As urban trans-portation planners have discovered, additionalcapacity is not always the solution to the prob-lem of delay. Building a new lane does not ap-preciably ease traffic jams on a busy freeway,for instance, because new traffic is attracted bythe improved link and delays quickly reach theprevious level. The same principle applies tomany hub airports: the busier an airport is, themore demand there is for access to it, simply be-cause it is busy and thus offers a wide choice ofconnections and services. Adding new capacitymay merely tap this latent demand—the airportcan accommodate those it couldn’t handle be-fore, but the new traffic quickly saturates the ad-ditional capacity and delay soon rises to previ-ous levels. This doesn’t mean that expansion isfutile, but it should be evaluated in terms of itsbenefits and the available alternatives.

If expansion proves impractical, the 15 to 20airports that will become saturated by the end ofthe century will probably have to make wideruse of demand-managing alternatives—peak-hour pricing, quotas, or access restrictions—todeal with the problems of congestion and delay.These tools do not increase peak capacity; theyshift traffic to a time or place where it can be bet-ter handled, thus increasing effective capacity.Pricing schemes to ration scarce landing slotsplace the greatest burden on operators of smallaircraft, since they have a smaller base of pas-sengers over which to spread cost. Administra-tive quotas may also tend to favor larger air-craft, which serve more passengers and generate

Ch. 7—Policy Implications . 129

higher landing fees. In either case, commutersand GA users will have the greatest difficulty incompeting for slots at crowded airports. Not allGA activity could be displaced, since some GAflights must use the main airport to deliverpassengers connecting with commercial flights.Even at the busiest airports GA operations cur-rently tend to average about 10 percent of totaloperations.

The separation of fast and slow (or jet andprop) traffic is one ATC procedure that couldbenefit both types of traffic. Most GA and com-muter aircraft can use shorter runways thanthose required for large jet liners, and at somebusy commercial airports the construction ofshort runways equipped for instrument opera-tions could allow continued accommodation ofcommuter and GA aircraft, and at the sametime, could also allow some secondary increasein jet aircraft operations. These separate, shortrunways would be especially important for com-muter carriersable to land at

whose business depends on beingmajor airports, and in many cases

they would add more capacity relative to costthan a new mixed-traffic runway.

Another means of separating traffic that willbecome increasingly important is the diversionof some GA traffic from commercial airports toreliever airports. This technique has some draw-backs. For example, users may resist going to a“second best” airport which may not offer thesame services or ground access as the commer-cial airport. On the other hand, a properlyequipped GA reliever can often provide betterservice to nonscheduled private traffic than themain airport could. Constructing, improving, orupgrading these airports would be largely the re-sponsibility of local authorities, but Federal as-sistance (in the form of the Airport DevelopmentAid Program (ADAP) or other grants) is cur-rently available for the 155 reliever airports in-cluded in the NAS Plan. The level of funding forrelievers in the recent past has been a little under25 percent of all grants for GA airports, or 4 to 6percent of all airport grants.

ATC SYSTEM IMPROVEMENTS

Future improvements in the ATC svstem willbe directed toward three general objectives:

●

●

●

replacing obsolete equipment with im-proved technology that is more effectiveand reliable and less costly to operate andmaintain;expanding system capacity to accommodateexpected growth; andadding new capabilities to increase the pro-ductivity of the system and the efficiency ofits users.

Two improvements are basic to this process:1) achieving higher levels of automation on theground, and 2) taking advantage of the capabil-ities of flight-management avionics that are ap-pearing in the user fleets. In the 1980’s, the majoreffort will be devoted to replacing the computersin the en route centers, modernizing the flightservice stations, and beginning the deployment

of the MLS, the Discrete Address Beacon System(DABS, now Mode S), and the Traffic Alert andCollision Avoidance System (TCAS). For the1990’s, the FAA’s plans included further imple-mentation of the Mode S data link and MLS andthe start of a long-range program of automationin en route and terminal area ATC centers. TheFAA plans are undergoing a major review,however, and there are indications that theFAA’s 1982 NAS Plan will include changes inboth technology and timing.

In general, OTA finds that the ATC systemimprovements previously proposed by FAA inthe areas studied are technologically feasible. Infour of the five major areas addressed by OTA,however, detailed cost and benefit informationis not yet available. This information will beneeded on all major programs before final judg-ment can be made on FAA proposals. The spe-cific findings and potential issues in the five pro-gram areas studied by OTA are set forth underseparate headings below.

.


Computer Replacement

The computers used in en route ATC centerswill need to be replaced within the next 10 yearsbecause the present IBM 9020 computers do nothave the computing speed or storage capacityneeded to accommodate the expected growth inair traffic at the most heavily used en routecenters. These computers also lack the capacityto support more automated modes of operationthat FAA estimates will be needed to assurefuture system safety or to increase ATC systemproductivity. There is also concern that the costof repairing and maintaining the present com-puters will become excessive, largely because theIBM 360 series computers used in the 9020 are nolonger in production and replacement partswould ultimately have to be specially made.

An important issue in the computer replace-ment program is the procurement strategy to befollowed. The program previously recom-mended by FAA was a total replacement strat-egy which would require about 10 years to com-plete and would entail specially designed ATChardware and software to meet near-term needsand serve as the foundation for more advancedautomation in the 1990’s and beyond. Theschedule called for the first operational contractto be let in 1988, with installation of productionsystems starting in the 1990’s. The costs of thisprogram were at one time estimated at nearly$1.7 billion (1980 dollars), over the 1982 to 1991period.

Alternatives to this total replacement strategyinclude incremental approaches which couldprovide relief to computer capacity problems ina shorter time—perhaps 3 to 4 years as com-pared to 10 years for total replacement. For ex-ample, a “software first” approach would focuson rewriting ATC software to reflect modernmodular programing techniques. Then softwarefor particular ATC functions could be graduallytransferred to new computers which would atfirst supplement and finally replace the 9020s. A“hardware first” strategy would involve trans-ferring (rehosting) the existing software packageto a new computer. Later this software could bemodified along more modern lines or totally re-placed to support new functions and services.

There are technical difficulties to be overcomein each of these incremental strategies, but theyhave the advantages of allowing the replacementprocess to begin quickly. The use of off-the-shelfhardware would appear to offer some cost sav-ings over specially designed equipment. Furtherit would ensure that compatible hardware isavailable to upgrade or expand the system at afuture date.

Automated En Route AirTraffic Control (AERA)

Part of the rationale for en route computer re-placement is to satisfy the long-term evolution-ary requirements that are now defined in a gen-eral way under the concept of AERA. The es-sence of this concept is to transfer from control-lers to computers some routine activities, such asseparating and metering aircraft or formulatingand delivering clearances. Relieved of these rou-tine tasks, the controller’s role would be primar-ily to handle exceptions and emergencies and tooversee (manage) the operation of automatedATC equipment. Automation could achieve sev-eral benefits: increasing controller productivityand reducing FAA personnel costs; reducinguser costs by permitting wider use of fuel-effi-cient flight profiles; accommodating more oper-ations; and reducing system errors.

The AERA concept requires a great amount ofground-based data processing to perform exten-sive and detailed management of aircraft flightpaths. It could also reduce many of the pro-cedural constraints now imposed on the use ofairspace. In effect, it would be a system ofmanagement by exception: intervention by acontroller would be limited to circumstances orlocalities where conflicts could not be reliablyresolved by computer algorithms.

The major advantage claimed for AERA,aside from more comprehensive management oftraffic, would be a substantial increase in con-troller productivity. It is contemplated thatAERA control sectors would be staffed by oneor perhaps two (rather than the present three)controllers and that the volume of airspace con-trolled would be several times the size of presenten route sectors. A substantially greater number

Ch. 7—Policy Implications ● 131

of aircraft could thus be handled by a controllerteam. On the other hand, this load would almostcertainly be heavier than human operators couldhandle in the event of computer failure. As aresult, the AERA concept includes provisions forautomated backup for automated functions, aswell as a computer design that will allow thesystem to “coast” safely while backup pro-cedures are being initiated.

It must be emphasized that at present AERA isonly in an early stage of development. Extensiveefforts over perhaps 5 to 10 years will be neededto bring AERA to a precise and detailed defini-tion of requirements and equipment specifica-tions.

Three major features of AERA are already ap-parent. First, AERA would require computercapacity and software substantially beyond thatnow available in ATC applications, althoughnot beyond the present or readily foreseeablestate of technology. Second, AERA will requirea two-way data link capable of rapid and exten-sive exchange of information between the airand the ground. FAA now envisions that ModeS will provide this data link, but other possibili-ties could be considered. Third, AERA implies alike degree of automation in the terminal areasand in a central flow management facilitycapable of coordinating traffic throughout theATC system. This last point is particularly im-portant for both short-term computer replace-ment and long-term system design, since it im-plies the advisability of procuring a computerhaving a modular architecture. This wouldmake it possible for en route and terminal facil-ities to utilize similar hardware and software; itwould also encourage a flexible system design,in which individual modules would be capableof mutual support and backup in the event ofpartial equipment failure.

Close scrutiny by Congress will be needed asFAA’s plans mature. One major issue is likely tobe the acceptability to the users and controllersof an ATC system automated to the degree envi-sioned in the AERA concept, especially its safetyand operational reliability. A second major issuewill be evaluation of the savings in operationand maintenance ascribed to AERA, compared

to the needed investments in facilities and equip-ment to implement the system. A corollary issuewill be the costs and benefits to various classesof airspace users. The information to supportjudgments on these matters is not now available,and OTA can reach no conclusion beyond thegeneral observation that resolving these issues islikely to be far more important than seeking an-swers to the rather narrow question of technicalfeasibility.

Mode S Data Link

Another key element in the FAA’s overall planfor improving the ATC system is the Mode Sdata link, an improvement to the secondary sur-veillance radar that allows properly equippedaircraft to be interrogated selectively by groundstations. Mode S provides greater surveillanceaccuracy than the present Air Traffic ControlRadar Beacon System (ATCRBS) equipment andavoids the problem of “synchronous garble” thatoccurs when more than one aircraft respond si-multaneously to interrogation. The discreteaddress capability also provides a two-wayground-to-air data link to transmit clearances,weather information, traffic advisories, controlinstructions, and flight data automatically in adigital format without using VHF voice chan-nels. The Mode S data link feature provides thebasis for automation of ATC functions andother system improvements in the years beyond1990.

Mode S has been under development by FAAfor nearly 10 years at an estimated cost to dateof $58 million. The first prototype unit was de-livered for test and evaluation in 1978, and acontract for initial production will be awardedin 1982. FAA has not yet issued a formal imple-mentation plan, but the preliminary plan callsfor a multiyear procurement and deploymentstarting in 1986, at 197 sites—97 in terminalareas and 36 in the en route system, plus 60 forlow-altitude coverage and 4 at support facilities.

Deployment at these 197 sites would not con-stitute full implementation of Mode S. Addi-tional installations, which would not be com-pleted until early in the next century, might be


needed at another 100 sites to provide coveragedown to 6,000 ft for the continental UnitedStates and perhaps portions of Hawaii andAlaska.

An issue that will need to be addressed duringexamination of the plans for Mode S has to dowith the extent to which a Mode S transponderwould be required before permitting an aircraftto enter airspace or receive services (e.g. accessto and operation in a terminal control area[TCA]). Mode S and ATCRBS Mode C are com-patible, so that in the short run either systemwould qualify users to operate in TCA. GA op-erators, however, have expressed concern thatthe Mode S format would eventually supplantATCRBS Modes A and C and that they wouldbe required to reequip their aircraft with Mode Stransponders. This concern would be reduced byassurances that ATCRBS could be utilized for anextended period following the initial implemen-tation of Mode S.

Collision Avoidance

The primary function of air traffic control isto assure the safe separation of aircraft. In thepresent system, this is accomplished by control-lers on the ground using surveillance radar andcomputer aids; when conflict is detected, thecontrollers use voice radio to advise pilots oftraffic or instruct them to perform appropriateavoidance maneuvers. At present, the pilot hasno instrument or display in the cockpit to iden-tify potential threats or to indicate a maneuverthat would resolve a conflict.

For many years, FAA (in cooperation with theaviation community) has investigated a numberof collision avoidance systems that would pro-vide a backup (rather than a substitute) for thecurrent ATC procedures and ground-based sep-aration assurance service. During the summer of1981, FAA selected a system known as TCAS.FAA plans for TCAS to be operational by theend of 1984, a goal that is considered by some tobe optimistic. FAA has justified the choice ofTCAS on the following grounds:

. it does not require ground-based equip-ment;

●

●

●

it is compatible with the present ATC sys-tem and is a logical extension of it;it offers a range of capabilities suitable tothe needs of the various classes of airspaceusers; andit is more suitable for use in high-densitytraffic than the Beacon Collision AvoidanceSystem (BCAS), the system that was fa-vored by the FAA prior to the TCAS deci-sion.

TCAS provides the user with protection fromother aircraft regardless of whether they areequipped with TCAS or the standard ATCRBStransponder. In the active mode, TCAS interro-gates other aircraft to determine whether theyare threats. TCAS also identifies potentialthreats from ATCRBS-equipped aircraft bymonitoring their replies to interrogations fromthe ground. A central feature of TCAS is the useof the Mode S transponder for the communica-tion of data between aircraft. TCAS 1, the sys-tem intended for use by general aviation, pro-vides general Mode S capability and would cost$2,500 to $3,500 per aircraft. TCAS II, the ver-sion intended for use by commercial aircraft,would cost between $40,000 and $50,000 per set,plus the cost of antennas and installation. Somebelieve these estimates to be low. TCAS requiresessentially no expenditures by FAA, except fordevelopment and certification costs; but since itwill require Mode S for identification and datalink, aircraft equipped with TCAS will be pre-pared to take advantage of any new services re-quiring data link that may be offered by FAA.

Although FAA has decided that it will certifyTCAS as the collision avoidance system to beused in the United States, not all features of thesystem have been developed and demonstrated.The TCAS II direction-finding antenna is of crit-ical importance: there is some question regard-ing the aerodynamic effects of the antenna onaircraft performance, particularly the perform-ance of tactical military aircraft. TCAS I, on theother hand, has been demonstrated; but it is notclear how useful this more basic form will besince it only indicates the proximity of anotheraircraft without providing either bearing orrange.

Ch. 7—Policy Implications . 133

Prior to selecting TCAS, FAA was pursuingdevelopment of active BCAS. Because there wasconcern that omnidirectional BCAS might inter-fere with the surveillance system in congestedareas by saturating ATCRBS transponders, FAAwas also planning to base conflict resolution inareas of high traffic density on DABS/Auto-matic Traffic Advisory and Resolution Service(ATARS), a ground-based system that would re-quire expenditures of $518 million to equip ter-minal and en route facilities. The decision toadopt TCAS has led FAA to reevaluate the needfor DABS/ATARS.

Microwave Landing System

Another important component in the FAA’sdevelopment plans is MLS, a precision landingaid designed as a replacement for the InstrumentLanding System (ILS) that has been in use sincethe early 1940’s. MLS is less sensitive to interfer-ence and distortions than ILS and will work atsites where it is difficult or impossible to installILS. It is also anticipated that MLS equipmentwill be more reliable than ILS. The chief opera-tional advantage of MLS is that it permits vari-able glide slopes, curved and segmented ap-proaches, and precision missed approaches,where ILS does not. This would allow traffic tobe routed around noise-sensitive areas andwould also allow greater flexibility in handlingtraffic in crowded TCAS. MLS can operate on200 channels (compared to 20 for ILS) making itpossible to provide precision landing aid in areaswhere closely spaced airports limit the availabil-ity of ILS channels.

FAA has announced plans to implement MLSin three phases over the coming 11 to 16 years,

with 1,200 to 1,400 systems eventually installed.In the first phase, between 10 and 25 systemswill be installed at selected airports in order todevelop a base of experience and reach an empi-rical determination of the benefits that can berealized. The second phase would be the installa-tion of 900 MLS units at the rate of 100 to 150per year for a period of 6 to 9 years, with prior-ity given to large and medium hub airports andthose where ILS siting problems exist. The thirdphase would consist of installing of an addition-al 300 to 500 systems to meet the growth in de-mand anticipated during the remainder of thiscentury. FAA estimates the cost of 1,425 MLSground systems to be $1.332 billion (1981 dol-lars), and users will be required to spend an ad-ditional $895 million for avionics if they wish totake advantage of this service.

OTA finds that the FAA’s analysis of MLSbenefits and costs does not establish a clear anduniversal case for MLS as opposed to ILS, andthat for this reason the FAA plan for a firstphase to gain the operational experience beforethe full deployment of MLS is entirely reason-able. However, at the end of the initial phase, itwould be appropriate to conduct a comprehen-sive review of the MLS program before proceed-ing with further implementation. A part of thisreview should be development of additional in-crements or intermediate steps between the 25sites planned for Phase I and the 900 planned forPhase II. Another part of this review should bemore specific benefit-cost analyses that differen-tiate and specify the benefits at various airportsin terms of levels of traffic, the types of usersserved, and the resulting reductions in noise,delay, or fuel consumption.

FUNDING AND COST ALLOCATION ISSUES

Findings

The program of airport development andATC system improvement through 1991 previ-ously proposed by FAA would require an ex-penditure of $1.6 billion to $1.9 billion per year,or about 50 to 75 percent above the spendinglevel of recent years in real terms. Implicit in

these figures is a commitment to spend roughlyequal sums annually from 1992 to 2000 in orderto complete programs already initiated and toundertake further improvements of the airportand airways system. These figures may change,however, as a result of changes in the forthcom-ing NAS Plan.


Historically, such expenditures have been fi-nanced from the Airport and Airways TrustFund, which lapsed in October 1980 but had anuncommitted balance of about $3 billion at theend of fiscal year 1981. This sum would coverless than 20 percent of the 1982-91 programscontemplated by FAA.

Congress has two basic options to providefunding for the developing airports and airwaysover the coming years. One would be to coverthese expenditures wholly by appropriationsfrom general funds. The other involves fundingthrough user charges by reestablishment of thetrust fund in some form, including:

● Reestablishment of the trust fund with arevenue and user charges similar to thosewhich existed prior to October 1980. Thiswould not cover the 1982-91 program ofcapital spending if—as in the past—sometrust fund revenues were also spent forO&M.

● Reestablishment of the trust fund, retainingthe present forms of funding but increasinguser charges to make revenues match pro-jected expenditures. Rates could be raisedeither uniformly (across the board) or selec-tively (to alter the mix of contributionsfrom various user classes).

● Reestablishment of the trust fund, but witha different form of user charges. Existing ex-cise taxes might be replaced with user leviesthat would reflect more accurately the mag-nitude of the benefits received by variousclasses of users, or by a system that wouldcharge individual users in relation to thecosts they impose on the airport and air-ways system.

All of these options would be controversialand would exacerbate many long-standing issuespertaining to access to the system, user cost allo-cation, and subsidies to aviation. The search fora solution is further complicated by the fact thatthe cost of operating the airport and airwayssystem would also be rising at the same time.

The disagreements over funding airport andairways improvements are so wide, and thesums so large, that the debate could conceivablyextend over a number of years. To the degree

that such a stalemate delays the funding of theFAA’s proposed programs, some of the follow-ing courses of action might have to be consid-ered:

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●

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●

keep the existing equipment running as wellas possible, with administrative restrictionson traffic levels as needed to keep demandwithin capacity;cut back on the proposed plans, dispensingwith some improvements and funding onlythose for which there is the greatest or mostimmediate need;stretch out the procurement process over alonger period of time, in order to hold ex-penditures within the available revenues; orconsider alternative technologies or fundingmechanisms that shift more of the cost ofthe system to airspace users.

Discussion

Capital expenditures for airport capacity im-provements and new ATC technology plannedfor the coming decade would result in a sharp in-crease in the FAA budget compared to the fund-ing levels of the past 10 years. The combinedexpenditures for airport grants-in-aid, for ATCfacilities and equipment (F&E), and for associ-ated research, engineering, and development(RE&D) were in the range of $0.95 billion to$1.35 billion per year (in constant 1980 dollars)between 1971 and 1980 (see fig. 29). * Capital ex-penditures for fiscal year 1982 to fiscal year 1991could total between $16 billion and $19 billion(1980 dollars), with $4.5 billion to $6 billionallocated to airport grants in aid, $10 billion to$11 billion for F&E, and $1.5 billion to $2 billionfor RE&D. The combined outlay in these cate-gories would amount to $1.6 billion to $1.9 bil-lion per year, a real increase of 50 to 75 percentover the 1971-80 average.

A large part of airport expenditures through-out the 1982-91 period would be allocated to ca-pacity increases at congested hub airports anddevelopment of GA reliever airports to takesome of the pressure off large and medium hubs.

*In fiscal year 1980, the total in these three categories was $950million; in fiscal year 1981, $885 million.


Figure 29.—Airport and Airways Trust FundExpenditure 1971-80*

1971 1972 1973 1974 1975 1976 1977 1978 1979 1980

Fiscal year

*Appropriations in various years for operating and maintenance expenses,totaling $3,370 million, are not shown. They amount to about 30 percent of alltrust fund expenditures.

SOURCE: FAA Monthly Management Report, March 1981.

In the near term, the bulk of the F&E expendi-tures would be for replacement of en route com-puters, the first stages of MLS and Mode S im-plementation, and modernization of flight serv-ice stations. Beyond 1990, the major F&E ex-penditures would be for completion of en routeautomation, initiation of terminal area automa-tion, and further deployment of MLS. Programssuch as MLS, Mode S, and terminal and en routeautomation would not be completed by 1991;there would be a foIlow-on requirement for anadditional funding in the 1990’s to carry theseprograms to completion and to initiate furtherATC technology improvements.

The FAA’s justification for these planned ex-penditures is that they will be needed to relieveairport congestion, to enable the ATC system tohandle higher traffic levels without compromiseof safety, and to improve the efficiency (produc-tivity) of the ATC system. Increasing productiv-ity is especially important in view of the pro-jected increase in aircraft operations and the re-sulting rise in ATC costs that would occur overthe next 10 years if automated en route, termi-nal, and flight service station equipment werenot installed.

Since establishment of the Airport and Air-ways Development Program in 1970, expendi-tures for airport improvements and ATC facil-ities and equipment, including the associatedRE&D, have been financed by the Airport andAirways Trust Fund. Between fiscal year 1971and fiscal year 1980, the trust fund provided $4billion in airport grants, $2.6 billion for F&E,and $0.7 billion for RE&D. During the same pe-riod, the trust fund also provided almost $2.2billion for O&M expenses of the ATC system.Expenditures from the trust fund have never ex-ceeded revenues, and as of the end of fiscal year1981 the trust fund had an uncommitted balanceof about $3 billion.

The principal source of revenue for the trustfund through fiscal year 1980 was an 8-percenttax on domestic airline tickets. Other taxes con-tributing to a lesser extent were a 5-percent way-bill tax on air cargo, a 7 cents per gallon tax onjet fuel and gasoline used by GA, a $3 interna-tional departure tax, an aircraft use tax for pro-peller aircraft, and taxes on airplane tires andtubes. In fiscal year 1980, these taxes contrib-uted $1.87 billion to the trust fund, with 85 per-cent coming from the domestic airline passengerticket tax.

On October 1, 1980, the legislative authoriza-tion of ADAP and the trust fund expired andCongress declined to pass reauthorizing legisla-tion. Since then, receipts from the passengerticket tax (reduced to 5 percent) have been re-mitted to the general fund. The air cargo way-bill, international departure, and aircraft usetaxes have been abolished. Revenues from thetax on aviation gasoline (4 cents per gallon) andtube and tire taxes have been remitted to theHighway Trust Fund.

There are now several bills before Congressthat would restore the trust fund. These pro-posals include provision for airline passengerticket taxes between 4 and 6.5 percent, taxes onGA fuel, an air cargo waybill tax of 2 to 5 per-cent, and a $1 to $5 international departure tax. *

● Generally, the Administration’s proposal provides for highertax rates than any of the House or Senate bills. The tax rate for GAjet fuel under the Administration’s proposal would be 20 cents per-gallon initially, rising to 65 cents per gallon by fiscal year 1986.The tax on aviation gasoline would rise from 12 cents per gallon infiscal year 1982 to 36 cents per gallon in fiscal year 1986. In con-gressional proposals, the tax on fuel ranges from 4 cents to 8.5cents per gallon.


Many Members of Congress have voiced strongopposition to reestablishing the trust fund or in-creasing the present user taxes so long as there isa large uncommitted balance in the trust fund.Sponsors of the various bills have pointed outthat reauthorization of trust fund taxes in someform will be necessary to provide revenue forprojected airport and ATC capital improve-ments. They also point out that the trust fund isconsistent with the position of the present Ad-ministration that, e.g., whenever the FederalGovernment provides a service directly to a par-ticular industry, those who receive the benefitshould bear the cost.

Regardless of the action taken on these pro-posals, the Administration and Congress will, inthe long run, have to grapple with the questionof how to finance planned airport and ATC cap-ital expenses. The balance in the trust fund nowwould cover less than 20 percent of the outlaysby FAA for 1982-91. If these funds were to be ex-pended at the fiscal year 1981 rate of $1.6 billionannually and no new taxes were authorized, thetrust fund would be exhausted by the end of1983. Even if the most ambitious of the currenttax proposals were to be enacted and if trustfund moneys were also used to defray aboutone-quarter of O&M expenses (as they were infiscal year 1980), trust fund revenues wouldprobably be insufficient to meet planned capitalexpenditure and O&M costs beyond 1987 or1988.

Some of the implications of providing fundingfor FAA airport and airways programs by ap-propriations from the general fund or, alterna-tively, by reauthorization of the trust fund arediscussed below.

General Fund

Capital expenditures for airports and airwayscould be financed from general revenuesthrough annual appropriations. There arenumerous precedents for this in other areasalthough it runs counter to the 10-year Federalpolicy of financing airport and airways im-provements through a dedicated trust fund sup-ported by user charges. Funding from generalrevenues has the basic advantage of giving theCongress close control of FAA capital programs

through the annual appropriations process. Onthe other hand, financing from general revenueshas several major disadvantages: it introducesadditional uncertainty in to the funding processand might make it difficult to plan and imple-ment long-range programs, which might be can-celed or delayed during periods of budget auster-ity, perhaps to the detriment of the nationalairspace system. A corollary disadvantage isthat the FAA’s capital programs might have tocompete with operational expenses for a share ofthe FAA budget and (if a choice had to be made)operational expenditures would probably re-ceive first consideration since they cannot be de-ferred or curtailed as easily as capital expendi-tures.

Perhaps the greatest objection to general fundfinancing, however, has been that it would con-stitute a subsidy of aviation by the public, manyof whom would receive no direct benefit: one-third of the adult population in the United Stateshas never flown, and fewer than 10 percent usecommercial or general aviation on a regular ba-sis. Such an approach, it is argued, would alsocontradict the economic precept that the users ofa special service should bear the cost of thatservice—a view that the present Administrationhas advocated strongly. It is argued by some,however, that the general public also benefits inmany indirect ways from services provided tothe aviation community, including mail serviceand air freight as well as use of the system bymilitary aircraft.

Trust Fund

Financing airport and airways improvementsfrom a trust fund, either like that which existedprior to October 1980 or in a modified form, isan approach favored by many observers. It pro-vides a continuing and stable source of fundsearmarked for capital programs, and it securesthose funds directly from users of the system.On the other hand, it has the general disadvan-tage of any sort of trust fund: the statutoryrestrictions on the purposes for which moneysmay be used might limit Congress’ flexibility inmeeting other, perhaps more pressing, needs.The long-standing controversy over use of Air-port and Airways Trust Fund monies for meet-


ing annual O&M expenses of FAA is a clear illus-tration of this.

If Congress elects to continue the trust fundapproach, as most of the pending bills pertainingto funding FAA’s capital programs now pro-pose, there are several options open:

● Reauthorize the Airport and Airways TrustFund as it existed before October 1980, Thisfund, supported by various user excisetaxes, would provide for some or all ofFAA’s capital expenditures over the comingdecade. Whether it could also meet someportion of operating expenses would de-pend on the rates established for the varioususer taxes. Much of the current debate inCongress is on this specific point: i.e., theappropriate amount of taxation to be im-posed on each class of airspace user.

• Retain the tax mechanisms of the formertrust fund but substantially alter the schemeof taxation, so that each category of userswould pay a share more nearly proportion-ate to the benefits they received. In the trustfund as constituted before October 1980,commercial aviation (domestic and interna-tional air carriers and air cargo airlines)contributed 93 percent of the revenues but,according to cost allocation studies by DOTand FAA, received a smaller share of thebenefits—in effect, cross-subsidizing GA.Since nearly all of the revenues from com-mercial aviation were derived from the taxon airline tickets, the subsidy to GA was ac-tually provided by airline passengers, notairlines. The Administration’s recent pro-posal would redress this imbalance some-what by greatly increasing the tax on fuelfor GA aircraft, but it would probably stillfall short of levying charges on GA com-mensurate with the benefits received, espe-cially by business aircraft operating in andout of hub airports.

Private GA operators and the makers ofGA aircraft have vigorously opposed suchtax schemes, on the grounds that VisualFlight Rules (VFR) and IFR users imposegreatly different costs on the ATC system,and that high fuel taxes would reduce air-craft utilization in the short run and reduce

sales of GA aircraft in the long run. Theyalso state that the ATC system was de-signed to meet the needs of air carriers, anda few hub airports, with facilities and serv-ices that GA users neither asked for, norwant, nor need. In this sense, some GAusers claim that they subsidize commercialair traffic. A third, and perhaps more fun-damental, objection raised by GA is thatthere is no accurate method of determiningthe value of the benefits received by GA orany other class of airspace user, and henceno sound basis for establishing an appropri-ate level of taxation.Levy charges on users, either based on theactual use they make of the airport and air-ways system or based on the burden theyplace on the system to provide varioustypes of services. The United States maybethe only major nation that does not routine-ly charge for the use of its airspace; manycountries in Europe and elsewhere in theworld levy charges for the use of terminaland en route airspace (based on distance,time, and type of service provided), in addi-tion to landing fees like those collected inthis country to defray the costs of airportconstruction, maintenance, and operation.The chief conceptual problem is how toquantify user benefits or determine the costof a service. Two major attempts by FAAand the Department of Transportation(DOT) to develop such a methodology, thecost allocation studies of 1973 and 1978, ’ 2

met with major objections from variousaviation groups on the grounds that costscould not be determined with sufficient ac-curacy and that an equitable formula for al-locating costs had not been developed.

Assuming that the methodological problemscould be overcome, there would still remainpractical problems of how to assess user charges.The simplest and most direct method would be a

‘Airport and Airway Cost Allocation Study; Determi~?ation,Allocation, and Recovery of System Costs (Washington, D. C.,U.S. Department of Transportation, September 1973).

‘Financing the Airport and Air-may System: Cost Allocation andRecovery, FAA-AVP-78-14 (Washington, D. C.: Federal AviationAdministration, November 1978).

.


charge for service at the time a flight plan isfiled. While this would capture fees from IFRusers, it might encourage some GA operators tofly “off the system” (i.e. VFR to smaller airports)in order to avoid airport and airway charges,perhaps to the detriment of safety. It would alsocreate a bookkeeping and administrative taskfor FAA in levying charges for use of the system.

A second possibility would be to require allaircraft to have a transponder and to use surveil-lance data to compute charges based on the timein the system and the type of service received.While this would free users from financial trans-actions when they file flight plans, it would stillimpose on the ATC system a requirement forrecording and billing user charges. In addition,the universal requirement for a transponderwould be viewed by many owners of small GAaircraft as an extreme form of regimentation. Athird possibility involves approximation of usercosts through a combination of fixed and var-iable assessments on aircraft owners: fixedcharges could be collected in the form of annualtaxes based on aircraft occupants (includingflight crew) according to aircraft characteristicsor type of use.

Operating CostsA corollary problem that Congress will have

to deal with is how to meet the operating costs of

the system. (Many of the planned capital im-provements are intended to reduce these costs inthe long term. ) If these costs are covered primar-ily by appropriations from general revenues (thepractice of many years), the taxpayers would besubsidizing special services for a mode of trans-portation that only a few use directly, althoughthey may receive some indirect benefit. If paidwholly or largely by disbursements from thetrust fund, as the Administration proposes andmany Members of Congress oppose, the pres-sures on the trust fund would be greatly intensi-fied. Over two-thirds of the FAA’s annual budg-et goes to meet operating costs, but disburse-ments from the trust fund have covered onlyabout 15 percent of these expenses in the past.To take a more substantial portion of opera-tional expenses from the trust fund, as it is pres-ently structured, would exhaust the current sur-plus in a very short time. To prevent this, and atthe same time provide for needed capital invest-ments, the taxes supporting the trust fund wouldhave to be increased to yield significantly morerevenue than contemplated by any of the legisla-tive proposals before the Congress at this time.A tax increase of this magnitude would raise allof the issues cited earlier in connection with cap-ital funding options and greatly exacerbate theconflict among the various stakeholders in theaviation community.

PENDING LEGISLATION

Areas of congressional interest in the airportand air traffic control system include systemmodernization (especially system automationand the replacement of the en route computers),airport development, trust fund usage, and usercharges. This section briefly reviews congres-sional activities in the past 2 years, outlines thepositions taken by various congressional com-mittees on key issues, and identifies the majorlegislation now before Congress.

System Modernization

Major capital expenditures like the en routecomputer replacement have been the subject ofseveral congressional hearings and investiga-

tions. A recurring question has been the FAA’sability to plan and manage such a complex pro-curement.

In October 1980, the investigations staff of theSenate Committee on Appropriations released areport criticizing the FAA’s management of theexisting ATC computer system. The report citedweaknesses in the reporting of equipment out-ages, a lack of planning, and the absence of awell-defined approach to managing system oper-ations and software changes. The investigatorsrecommended the Congress withhold funding

for computer replacement until the FAA haddemonstrated a better understanding of the ca-pabilities and limitations of the existing system.


The report outlined specific actions FAA shouldtake to improve its performance and evaluationmethods. 3

After two sets of hearings on the safety as-pects of computer outages, the House Commit-tee on Government Operations raised many ofthe same questions in October 1981. Their re-port found that the FAA’s management informa-tion system did not provide accurate data onwhich to base important decisions about the reli-ability of the computer. The committee alsoquestioned the FAA’s ability to plan and managethe development and procurement of a newcomputer system. The report directed theGeneral Accounting Office (GAO) to initiate a“comprehensive investigation of the FAA’s plan-ning, management, and acquisition of auto-mated information systems. ”4 The GAO finalreport, due in October 1982, will cover FAAplanning and management for acquisitions inthree areas: ATC system automation, manage-ment information systems, and peripheralequipment.

The Subcommittee on Transportation of theHouse Committee on Science and Technology,which has shown a continuing interest in theATC computer question, has stated that the cur-rent computer system needs to be replaced andthat unnecessary delay in doing so would posesafety risks and increase the chances of furtherbreakdowns. in reviewing the alternatives forreplacing the system, the subcommittee’s reportof August 1981 favored a full modernization ofthe computer system, as opposed to an interimreplacement followed by a long-range procure-ment. The full committee recommended thatFAA publish a management plan detailing thecosts, schedules, milestones, and funding plansfor the computer replacement.’

‘U.S. Congress, Senate Investigations Staff, FAA ErI Route AirTraffic Co)~trol Computer System, submitted to the Subcommitteeon Transportation and Related Agencies, Committee on Com-merce, Science and Transportation, Rpt. No. 80-5, October 1980.

‘U.S. Congress, House, Committee on Government Operations,Air Traffic Cot~trol Computer Failures, Rpt. No. 97-137, June 11,1981.

‘U.S. Congress, House, Committee on Science and Technology.Subcommittee on Transportation, Aviation and Materials, Air~ra~fic Co)ltrol EtI Route Cot)~pt/trr h~(~cft~rtli:(ltlc)tl, Rpt. N o .97-12, August 1981.

To give further emphasis to these findings andrecommendations subcommittee chairman, Rep-resentative Dan Glickman introduced H. Res.202 in October 1981, which expressed the senseof the House that FAA should consult with theCommittee on Science and Technology as itdevelops plans for the future ATC system. Italso directed FAA to make regular reports to thecommittee, commencing with a system descrip-tion in December 1981 and a preliminary sub-system description in June 1982. This resolutionwas passed by the House on October 19, 1981.

Airport Development Aid

The Federal role in airport development waspreviously governed by the Airport and Air-ways Development Act of 1970, which expiredin October 1980 when the Congress could notagree to new authorizing legislation. Projects ex-tending into fiscal year 1981 were funded, but noauthorizations have been made for future years.In writing new authorizing legislation in 1981,the question of “defederalization” has been amajor issue. Defederalization would removelarge and medium hub airports from eligibilityfor ADAP funding, on the grounds that theseairports generate enough revenues to be self-sup-porting without Federal aid.

The Senate version of the authorizing legisla-tion, S.508, would make the top 69 air carrierairports ineligible for airport development andplanning grants. The Administration position,as contained in H.R. 2930 called for a moremodest defederalization measure, making thetop 42 airports ineligible for aid. These airportswould be permitted to impose a limited passen-ger facility charge (head tax) to make up lostrevenues (head taxes are currently forbidden atall airports that have received Federal aid). Thereport on S.508 by the Senate Committee onCommerce, Science, and Transportation sup-ports the defederalization concept and notes thatADAP funds make up a fairly small proportionof the total capital and operating budgets oflarger airports. If they were made ineligible, thereport points out, more Federal funds would beavailable for small airports unable to generatetheir own funds. Because the Senate bill limits


the total authorization to $450 million annuallyfor 5 years (1981-86), it is necessary to makethose funds available to those who need themmost.6

The House version of the authorizing legisla-tion, H.R. 2643, contains no provision for de-federalization, and members of the Committeeon Public Works who sponsored the House ver-sion have expressed opposition to the concept.Questions of equity are involved: opponents ofdefederalization are concerned that passengersusing major airports would have to bear a dou-ble tax—the Federal ticket tax in addition to anylocal passenger facility charge. Further, the tick-et tax on passengers at large airports alreadygenerates the bulk of revenues in the Airport andAirways Trust Fund, and it seems unfair to for-bid these airports the use of those funds. TheHouse bill proposes a $450 million annual au-thorization for 3 fiscal years. ’

Trust Fund UsageThe uncommitted balance in the trust fund

(about $3 billion at the end of fiscal year 1981)has long been a cause of controversy in Congressand among users. The Senate Committee onCommerce, Science, and Transportation attrib-utes this balance to the fact that the OMB underprevious administrations has sought to keeptrust fund revenues high and expenditures low.The current administration has proposed draw-ing down the balance significantly by funding 85to 100 percent of the FAA’s operations andmaintenance costs out of the trust fund, in addi-tion to capital costs. For example, the adminis-tration budget recommended financing expendi-tures such as aviation security and aircraft in-spection from the trust fund. Both Senate andHouse Committees on Appropriations,however, have continued to allow theseregulatory and police functions to be fundedfrom general funds.8

‘U.S. Congress, Senate, Committee on Commerce, Science, andTransportation, Report to Accompany S.508, Airport and AirwaySystem Development Act of 1981, S. Rpt. 97-97, May 15, 1981.

‘U.S. Congress, House, Committee on Public Works and Trans-portation, Report to Accompany H.R. 2643, Airport and AirwayImprovement Act of 1981, H. Rpt. 97-24 (Part II), May 19, 1981.

‘U.S. Congress, House, Committee of Conference for the De-partment of Transportation and Related Agencies for the fiscalyear ending Sept. 30, 1982, Conference Report to Accompany

H.R. 4209, H. Rpt. 97-331, NOV. 13, 1981.

The Senate Committee on Commerce, Sci-ence, and Transportation stated that the airportand airway system provides benefit to the gener-al public and therefore the general fund shouldcontinue to contribute to its operation. g Al-though many in Congress agree that somethingshould be done to reduce the balance, someMembers feel that taking operating costs out ofthe trust fund constitutes “raiding” the users’funds, which were collected for the purpose ofimproving the airways system, to subsidize ac-tivities that should be paid for out of generalrevenues. The DOT appropriations bill for fiscalyear 1981, in both House and Senate versions,appropriated funds from the trust fund to coverabout one-third of operating costs, about doublethe average share of the past 10 years. H.R.2643, as reported by the Committee on PublicWorks, authorizes a ceiling of 50 percent on op-erating costs to be taken from the trust fund infuture years; S.508 authorizes a ceiling of aboutone-third on operating costs to be taken fromthe trust fund.

User Taxes

Current proposals for reestablishing the trustfund call for no major changes in the user taxstructure. In general, the House, Senate, andadministration positions on user charges havesimply been differences in the level of tax in thetraditional categories:

● The administration proposal, embodied inS. 1047, calls for the greatest increase inuser taxes. It differentiates between GA gastaxes and GA jet fuel taxes, taxing gas at 12cents per gallon (rising to 36 cents in fiscalyear 1986) and jet fuel taxes at 20 cents (ris-ing to 65 cents). The passenger ticket taxwould be set at 6.5 percent, the waybill taxat 5 percent, and an international facilitiescharge of $3 per passenger would be author-ized.

● Another bill, S. 1272, cosponsored by sev-eral members of the Senate Committee onCommerce, Science, and Transportation,calls for an 8.5 cent tax for all GA fuels, a 3percent ticket tax, a 2 percent waybill tax,and a $1 international facilities charge.

‘Senate Commerce, Science and Transportation, S. Rpt. 97-97op. cit.


● A House bill, H.R. 4800, calls for a 12.5 sharp increases in user charges until some use iscents tax on all GA fuels, a 5 percent ticket made of the existing balance.10

tax and a $5 international facilities tax..

These measures are still under considerationby the Senate Committee on Finance and HouseCommittee on Ways and Means, and it is uncer-tain how they will appear after committee mark-up. Part of the difficulty in reaching a decisionon the tax level is the current uncommitted bal-ance in the trust fund and the unwillingness ofboth past and present administrations to spendthe money for its specified purposes. Membersof the Senate Commerce Subcommittee on Avia-tion and the House Science and TechnologySubcommitteeand Materials

The uncertainty about the costs and timing offuture capital expenditures also clouds thediscussion of tax levels. The options appear tobe: 1) increase taxes to maintain a substantialbalance in the trust fund in anticipation of largefuture expenditures, recognizing that the currentbalance could not cover the proposed programof system modernization; or 2) allow the trustfund to be depleted, knowing that revenues willhave to be greatly increased later if these futureexpenditures are to be paid for by user taxes.

on Transportation, Aviation,have stated they do not favor ‘OAL,~utjOtl 13aily,

oNov. 19, 1981, p. 102.

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