Pavement Concessions VER2 Text

Diploma of Airport Management

Airport Pavement Concessions(Preliminary)

INDEX Number S6780

Swan TAFE Diploma in Airport Management

Table of Contents

1. COURSE INTENT............................................................................................................................... 3

1.1 DEFINITIONS AND CONVERSION FACTORS............................................................................................31.2 EXTRACT FROM ERSA INTRO...........................................................................................................3

2. ICAO ANNEX 14 EXTRACT...............................................................................................................5

3. INTRODUCTION................................................................................................................................. 7

3.1 BACKGROUND................................................................................................................................... 73.2 THE NEED TO KNOW PAVEMENT STRENGTH.........................................................................................73.3 PUBLICATION OF PAVEMENT STRENGTH...............................................................................................73.4 DEVELOPMENT OF A STANDARD METHOD............................................................................................7

4. CONCEPT OF THE ACN-PCN METHOD............................................................................................8

5. HOW ACNS ARE DETERMINED........................................................................................................9

5.1 STANDARD VALUES AND DESCRIPTION OF TERMS................................................................................95.1.1 Subgrade Category.................................................................................................................95.1.2 Concrete Working Stress For Rigid Pavements.......................................................................95.1.3 Tyre Pressure....................................................................................................................... 105.1.4 Mathematically Derived Single Wheel Load...........................................................................105.1.5 Aircraft Classification Number (ACN).....................................................................................105.1.6 Center Of Gravity.................................................................................................................. 10

5.2 ABBREVIATIONS............................................................................................................................... 115.2.1 Aircraft parameters................................................................................................................115.2.2 Pavement and subgrade parameters.....................................................................................115.2.3 Tyre Pressures...................................................................................................................... 11

5.3 MATHEMATICAL MODELS.................................................................................................................. 115.4 COMPUTER PROGRAMMES................................................................................................................115.5 GRAPHICAL PROCEDURES................................................................................................................125.6 RIGID PAVEMENTS........................................................................................................................... 125.7 CALCULATION OF RIGID PAVEMENT REFERENCE THICKNESS AND ACN..................................................175.8 TYRE PRESSURE ADJUSTMENT TO ACN............................................................................................175.9 CALCULATION OF FLEXIBLE PAVEMENT REFERENCE THICKNESS AND ACN.............................................205.10 FLEXIBLE PAVEMENTS.................................................................................................................. 20

6. PROCEDURE FOR PAVEMENTS MEANT FOR LIGHT AIRCRAFT.................................................24

7. EXERCISES...................................................................................................................................... 24

8. GUIDANCE ON OVERLOAD OPERATIONS....................................................................................25

8.1 CRITERIA SUGGESTED BY ICAO.......................................................................................................258.2 PRACTICES OF VARIOUS COUNTRIES................................................................................................25

8.2.1 Canada................................................................................................................................. 258.2.2 United Kingdom.....................................................................................................................268.2.3 Australia................................................................................................................................ 26

9. ACNS FOR SEVERAL AIRCRAFT TYPES.......................................................................................29

9.1 INTRODUCTION................................................................................................................................ 30TABLE 1 - ACNS FOR SEVERAL AIRCRAFT TYPES ON RIGID AND FLEXIBLE PAVEMENTS....................................30

10. ACN CHARTS FOR VARIOUS AIRCRAFT.......................................................................................36

10.1 PAVEMENT STRENGTH CLASSIFICATION THE ICAO ACN / PCN METHOD........................4610.2 BACKGROUND........................................................................................................................ 4610.3 THE ICAO RATING SYSTEM...................................................................................................4610.4 HOW TO CALCULATE AN ACN...............................................................................................4710.5 HOW TO RATE A PAVEMENT.................................................................................................4810.6 LIMITATIONS OF ACN / PCN...................................................................................................48

1 Airport Pavement Concessions. Module 24945 VER 2

Diploma of Airport Management Swan TAFE

10.7 THE ACN REFERS ONLY TO RUTTING..................................................................................4910.8 LOAD RATING SYSTEM OR DESIGN METHOD......................................................................4910.9 SIGNIFICANCE OF OVERLOAD..............................................................................................49

11. PAVEMENT CONCESSIONS RPA GUIDANCE EXTRACT..............................................................50

12. PAVEMENT TYPES, RATING AND DESIGN PHILOSOPHY............................................................50

13. PAVEMENT OVERLOAD................................................................................................................. 51

14. CONCESSION DECISION CRITERIA...............................................................................................51

15. APPROVALS.................................................................................................................................... 51

Module S6780 VER 2 Airport Pavement Concessions. 2


1. COURSE INTENT.This segment of the Diploma in Airport Management is a educational unit intended to acquaint prospective aerodrome managers as to the basic factors involved in the ACN-PCN method, (Aircraft Classification Number – Pavement Classification Number).

It is not intended to enable aerodrome operators to make technical pavement evaluations. That is the province of a experienced professional airport civil engineer, however it does provide guidelines which enable judgments to be made, as to whether professional civil engineering advice is required.

1.1 Definitions and Conversion Factors.

The Newton is the SI unit (Standard International Unit) of force (Systeme International d’Unites ie metric system). This system is used in the sciences and by all countries using the metric system.

Force = mass x acceleration

Density = mass/volume. ie t/m3 or k/m3

Pressure = Force/Area :

ie Pounds per square inch, PSI or;Metric System, the Pascal …. kPa or MPa.

1 MPa = 1000 kPa.

A Newton is the force required to accelerate a mass of I kilogramme @ 1 metre per sec2.

1 Newton per m2 is 1 Pascal ….. the pressure unit.

1 Kilogramme = 2.2046 pounds.1 Pound = 0.453592 kilogrammes.NOTE.To convert MPa into PSI multiply MPa by 145.038.To convert inches to cm multiply inches by 2.54.

1.2 Extract from ERSA INTRO

11 RUNWAY DESIGNATION

RWY are normally numbered in relation to their magnetic direction rounded off to the nearest 10 degrees. Single runways are shown with the lower number on the left side. Parallel runways designated Left/Right are shown with the left runway listed first. Multiple runways are shown in ascending order from top to bottom.

12 RUNWAY BEARING

IN DEGREES MAGNETIC



13 RUNWAY DIMENSIONS

The RWY length is generally the TKOFF run (physical length) AVBL for both RWY directions. In ERSA, RWY dimensions are shown using one of the FLW methods:

a. ABBREVIATIONS: In this method, runway length is shown as multiples of I00 FT. (e.g. Lengths of 7000 FT to 7049 FT are shown as 70, lengths of 7050 FT to 7149 FT are shown as 71.)

b. UN-ABBREVIATED: Runway length and width are shown in both metres and feet, with the feet

bracketed. Metric values are not abbreviated.

SURFACE. Runway surface is shown as follows: a or A – asphalt or bitumen; b or B – concrete; c or C – other surfaces (always to be qualified by a note).

14 PAVEMENT STRENGTHS The ICAO standard method of reporting pavement strength known as Aircraft Classification Number/Pavement Classification Number (ACN/PCN) has been incorporated. NOTES: 1. Pavement strength data for RAAF aerodromes is tabulated in PLANNING PART 1 SECTION 2

CHAPTER 4. (MILITARY) 2. Omission of pavement strength indicates that the RWY is unrated. See AIP/AD 1.1 Section 6 for

operating limitations.

15. PAVEMENT TYPE FOR ACN-PCN DETERMINATION.

Pavement type Code Rigid pavement R Flexible pavement F

16. SUBGRADE STRENGTH CATEGORY.

Subgrade strength category Code High strength AMedium strength BLow strength CUltra low strength D

17. MAXIMUM ALLOWABLE TYRE PRESSURE.

1. Weight and tyre pressure limits are shown in kg and kPa (PSI) in the format 5700 450(65) and are gross limits, i.e. an ACFT may use that part of the movement area if the weight and tyre pressure are BLW the figures shown at the time of the operation. It the limitation is based on MTOW, this will be shown in the format MTOW 5700 KG, precluding an ACFT with MTOW in excess of the figure quoted FM OPR on the area specified.

2 Weight or tyre restrictions on RWY, TWY & aprons are shown in the remarks.

18. EVALUATION METHOD.Evaluation method Code Technical evaluation: representing a specific study of the pavement characteristics andapplication of pavement behaviour technology. T Using aircraft experience: representing a knowledge of the specific type and mass of aircraft satisfactorily being supported under regular use. U



Aircraft Classification Number – Pavement Classification Number (ACN-PCN) Method

2. ICAO Annex 14 Extract

Strength of Pavements

The bearing strength of a pavement shall be determined.

The bearing strength of a pavement intended for aircraft of apron (ramp) mass greater that 5,700Kg shall be made available using the aircraft classification number – pavement classification number (ACN-PCN) method by reporting all of the following information:a) the pavement classification number (PCN);b) pavement type for ACN-PCN determination;c) subgrade strength category;d) maximum tire pressure category or maximum

allowable tire pressure value; ande) evaluation method.

Note.- If necessary, PCNs may be published to an accuracy of one tenth of a whole number.The pavement classification number (PCN) reported shall indicate that an aircraft classification number (ACN) equal or less than the reported PCN can operate on the pavement subject to any limitation on the tire pressure, or aircraft all-up mass for specified aircraft type(s).

Note.- Different PCNs may be reported if the strength of the pavement is subject to significant seasonal variation.

The ACN of an aircraft shall be determined in accordance with the standard procedures associated with the ACN-PCN method.

Note.- The standard procedures for determining the ACN of an aircraft are given in the Aerodrome Design Manual, Part 3. For convenience several types currently in use have been evaluated on rigid and flexible pavements founded on four subgrade categories below and the results tabulated in that manual.

For the purpose of determining the ACN, the behaviour of a pavement shall be classified as equivalent to a rigid or flexible construction.

Information on pavement type for ACN-PCN determination, subgrade strength category, maximum allowable tire pressure category and evaluation method shall be reported using the following codes:

Pavement type for ACN-PCN determination:

Code

Rigid pavement R

Flexible pavement F

Note.- If the actual construction is composite or non-standard, include a note to that effect (see example 2 below).

subgrade strength category:

Code

High strength; characterised by K = 150 MN/m3 and representing all K values above 120 MN/m3 for rigid pavements, and by CBR 15 and representing all CBR values above 13 for flexible pavements.

A

Medium strength; characterised by K = 80 MN/m3 and representing a range in K of 60 to 120 MN/m3 for rigid pavements, and by CBR 10 and representing a range in CBR of 8 to 13 for flexible pavements.

B

Low strength; characterised by K = 40 MN/m3 and representing a range in K of 25 to 60 MN/m3 for rigid pavements, and by CBR 6 and representing a range in CBR of 4 to 8 for flexible pavements.

C

Ultra low strength; characterised by K = 20 MN/m3 and representing all K values below 25 MN/m3 for rigid pavements, and by CBR = 3 and representing all CBR values below 4 for flexible pavements.

D

Maximum allowable tire pressure category:



Code

High; no pressure limit W

Medium; pressure limited to 1.50MPa

X

Low; pressure limited to 1.00MPa

Y

Very low; pressure limited to 0.50MPa

Z

Evaluation method:

Code

Technical evaluation: representing a specific study of the pavement characteristics and application of pavement behaviour technology.

T

Using aircraft experience; representing a knowledge of the specific type and mass of aircraft satisfactorily being supported under regular use.

U

Note.- The following examples illustrate how pavement strength data are reported under the ACN-PCN method.

Example 1.- If the bearing strength of a rigid pavement, resting on a medium strength subgrade, has been assessed by technical evaluation to be PCN 80 and there is no tire pressure limitation, then the reported information would be:

PCN 80/R/B/W/T

Example 2.- If the bearing strength of a composite pavement, behaving like a flexible pavement and resting on a high strength subgrade, has bee assessed by using aircraft experience to be PCN 50 and the maximum tire pressure allowable is 1.0MPa, then the reported information would be:

PCN 50/F/A/Y/U

Note.- Composite construction

Example 3.- If the bearing strength of a flexible pavement, resting on a medium subgrade, has been assessed by technical evaluation to be PCN 40 and a maximum allowable tire pressure is

0.8mpa, then the reported information would be:

PCN 40/F/B/0.8MPa/T

Example 4.- If a pavement is subject to a B747-400 all up mass limitation of 390,000Kg, then the reported information would include the following note:

Note.- the reported PCN is subject to a B747-400 all-up mass limitation of 390,000Kg.

Recommendation.- Criteria should be established to regulate the use of a pavement by an aircraft with an ACN higher than the PCN reported for that pavement in accordance with the standards laid down herein.

The bearing strength of a pavement intended for aircraft of apron (ramp) mass equal to or less than 5,700Kg shall be made available by reporting the following information:

maximum allowable aircraft mass; and

maximum allowable tire pressure.

Example: 4,000Kg / 0.50MPa.

Note.In Australia this is reported as:-

kg/kPa/PSI i.e.

4000/500(73PSI).


3. Introduction

3.1 Background.

In the early days of aviation the bearing strength of pavements was not a critical factor because aircraft wheel loads and tyre pressures were low. Generally dry natural ground conditions with minimal preparation were adequate. Over the years larger and larger aircraft with high wheel loads and tyre pressures have been developed necessitating the construction of pavements over the natural ground so that these high loads could be supported safely. Complex undercarriage arrangements have been employed to better distribute the ever increasing gross weight of aircraft on the pavement. For example the DC-3, a front line aircraft of the 1930’s, has a gross mass of 11500 kg supported on two single main wheels with tyre pressures of 310 kPa (45 psi). Today’s B747 has a gross mass of 380000 kg supported on sixteen mainwheels with tyre pressures of 1415 kPa (205 psi).

3.2 The need to know Pavement Strength

The question of whether a pavement is strong enough for a particular aircraft is of paramount importance for the safe operation of that aircraft. To enable an operator or pilot to determine if a runway is of adequate bearing strength he must know the strength of the runway and the loading characteristics of his aircraft. The consequences of using a pavement of inadequate strength depend on the severity of the overload and the frequency of operations. In extreme cases, they can range from catastrophic damage to bogging of the aircraft. Where the overload is not as severe, pavement damage of varying degrees can be causes which can result in runway closure and expensive repairs. It is therefore important that pavement strength and the loading characteristics of aircraft are made available to operators and pilots.

3.3 Publication of Pavement Strength

The information is made available by publication in the Aeronautical Information Publications.

A variety of methods have been used throughout the world to report pavement strengths and compare the loading characteristics of aircraft. Australia has used the load classification number method, and the pavement rating format of MTP/PDF/LCL (Maximum Tyre Pressure/Pavement Depth Factor/Load Classification Number) is familiar to the aviation industry.

3.4 Development of a Standard Method

The International Civil Aviation Organisation (ICAO) presviously recommended four methods for reporting pavement strength. For more than a decade ICAO has been working towards the development of a reporting method which will be accepted internationally and will remove the ambiguities and confusion of a number of different methods. An ICAO study group of pavement experts from a number of countries examined various methods and through its work the Aircraft Classification Number – Pavement Classification Number (ACN-pcn) Method was recommended. The ACN-PCN has now been approved as the ICAO standard and has been adopted by Australia.

Hence Annex 14, specifies that the bearing strength of a pavement intended for aircraft of mass greater than 5,700Kg shall be made available using the Aircraft Classification Number – Pavement Classification Number (ACN-PCN) method. To facilitate a proper understanding and usage of the ACN-PCN method the following material explains:

a) the concept of the method; and

b) how the ACNs of an aircraft are determined.

4. Concept Of The ACN-PCN Method

ICAO Annex 14 defines ACN and PCN as follows:

ACN A number expressing the relative effect of an aircraft on a pavement for a specified standard subgrade strength.

PCN A number expressing the bearing strength of a pavement for unrestricted aircraft operations.

At the outset, it needs to be noted that the ACN-PCN method is meant only for publication of pavement strength data in the Aeronautical Information Publications (AIPs). It is not intended for design or evaluation of pavements, nor does it contemplate the use of a specific method by the airport owner/authority either for the design or evaluation of pavements. In fact, the ACN-PCN method does permit member States (countries) to use any design/evaluation method of their choice. To this end, the method shifts the emphasis from evaluation of pavements to evaluation of load rating of aircraft (ACN) and includes a standard procedure for evaluation of the load rating of aircraft. The strength of a pavement is reported under the method in terms of the load rating of the aircraft which the pavement can accept on an unrestricted basis. The airport owner/authority can use any method of their choice to determine the load rating of their pavements. In the absence of a technical evaluation, the airport owner/authority may choose to go on the basis of the aircraft actually using the particular pavement. The airport owner/authority would then compute the ACN of the most critical aircraft, using one of the procedures described below, and convert this figure into an equivalent PCN and publish it in the AIP as the load rating of the pavement. The PCN so reported would indicate that an aircraft with an ACN equal to or less than that figure can operate on the pavement subject to any limitation regarding the aircraft tyre pressures.

The ACN-PCN method contemplates the reporting of pavement strengths on a continuous scale. The lower end of the scale is zero and there is no upper end. Additionally, the same scale is used to measure the load ratings of both aircraft and pavements.

To facilitate the use of the method, aircraft manufacturers will publish, in their documents information detailing the characteristics of their aircraft, ACNs computed at two different masses: maximum apron mass, and a representative operating mass empty, both on rigid and flexible pavements and for the four standard subgrade strength categories. Nevertheless, for the sake of convenience Annex 14, includes a table showing the ACNs of a number of aircraft. It is to be noted that the mass used in the ACN calculation is a “static” mass and that no allowance is made for an increase in loading through dynamic effects.

The ACN-PCN method also envisages the reporting of the following information in respect of each pavement:

a) pavement type;

b) subgrade category;

c) maximum tyre pressure allowable; and

d) pavement evaluation method used.

The above data are primarily intended to enable aircraft operators to determine the permissible aircraft types and operating masses, and the aircraft manufacturers to ensure compatibility between airport pavements and new aircraft under development. There is, however, no need to report the actual subgrade strength or the maximum tyre pressure allowable. Consequently, the subgrade strengths and tyre pressures normally encountered have been grouped into categories as indicated in 5.1.1.1 below. It is sufficient for the airport owner/authority to identify the categories appropriate to his pavement. (See also the examples included under Annex 14, 2.6.)

Pavement damage from rutting(surface) may occur from excessive tyre pressures. Excessive wheeel loadings may result in pavement failures such as general loss of shape or large

saucer shaped depressions.

5. How ACNs Are Determined

The flow chart, below, briefly explains how the ACNs of aircraft are computed under the ACN-PCN method.

5.1 Standard Values And Description Of Terms

5.1.1 Subgrade Category.

In the ACN-PCN method eight standard subgrade values are used (four rigid pavement K values and four flexible pavement CBR values), rather than a continuous scale of subgrade strengths. The grouping of subgrades with a standard value at the mid-range of each group is considered to be entirely adequate for reporting purposes. The subgrade strength categories are identified as high, medium, low and ultra low and assigned the following numerical values:

5.1.1.1 Subgrade Strength Category

High strength; characterised by K = 150 MN/m3 and representing all K values above 120 MN/m3 for rigid pavements, and by CBR 15 and representing all CBR values above 13 for flexible pavements.

Medium strength; characterised by K = 80 MN/m3 and representing a range in K of 60 to 120 MN/m3 for rigid pavements, and by CBR 10 and representing a range in CBR of 8 to 13 for flexible pavements.

Low strength; characterised by K = 40 MN/m3 and representing a range in K of 25 to 60 MN/m3 for rigid pavements, and by CBR 6 and representing a range in CBR of 4 to 8 for flexible pavements.

Ultra low strength; characterised by K = 20 MN/m3 and representing all K values below 25 MN/m3 for rigid pavements, and by CBR = 3 and representing all CBR values below 4 for flexible pavements.

5.1.2 Concrete Working Stress For Rigid Pavements

For rigid pavements, a standard stress for reporting purposes is stipulated ( = 2.75 MPa) only as a means of ensuring uniform reporting. The working stress to be used for the design and/or evaluation of pavements has no relationship to the standard stress for reporting.

5.1.3 Tyre Pressure

The results of pavement research and re-evaluation of old test results reaffirm that except for unusual pavement construction (i.e., flexible pavements with a thin asphaltic concrete cover or weak upper layers), tyre pressure effects are secondary to load and wheel spacing. Consequently, tyre pressure may be categorised in four groups for reporting purposes as high, medium, low and very low and assigned the following numerical values:

High - No pressure limit

Medium - Pressure limited to 1.50 MPa

Low - Pressure limited to 1.00 MPa

Very low - Pressure limited to 0.50 MPa

5.1.4 Mathematically Derived Single Wheel Load

The concept of a mathematically derived single wheel load has been employed in the ACN-PCN method as a means to define the landing gear/pavement interaction without specifying pavement thickness as an ACN parameter. This is done by equating the thickness given by the mathematical model for an aircraft landing gear to the thickness for a single wheel at a standard tyre pressure of 1.25 MPa. The single wheel load so obtained is then used without further reference to thickness; this is so because the essential significance is attached to the fact of having equal thicknesses, implying "same applied stress to the pavement, rather than the magnitude of the thickness. The foregoing is in accord with the objective of the ACN-PCN method to evaluate the relative loading effect of an aircraft on a pavement.

5.1.5 Aircraft Classification Number (ACN).

The ACN of an aircraft is numerically defined as two times the derived single wheel load, where the derived single wheel load is expressed in thousands of kilograms. As noted previously, the single wheel tyre pressure is standardised at 1.25 MPa. Additionally, the derived single wheel load is a function of the subgrade strength. The aircraft classification number (ACN) is defined only for the four subgrade categories (ie., high, medium, low, and ultra low strength). The two (2) multiplying factor in the numerical definition of the ACN is used to achieve a suitable ACNs in terms of gross mass scale so that whole number ACNs may be used with reasonable accuracy.

5.1.6 Center Of Gravity

Because an aircraft operates at various mass and centre of gravity conditions the following conventions have been used in ACN computations.

the maximum ACN of an aircraft is calculated at the mass and c.g. that produces the highest main gear loading on the pavement, usually the maximum ramp gross mass and corresponding aircraft CG. The aircraft tyres are considered as inflated to the manufacturers recommendation for the condition;

relative aircraft ACN charts and tables show the ACN as a function of aircraft gross mass with the aircraft c.g. at a constant value corresponding to the maximum ACN value (usually, the aircraft c.g. for max ramp mass) and at the max ramp mass tyre pressure; and

specific condition ACN values are those ACN values that are adjusted for the effects of tyre pressure and/or c.g. location, at a specified gross mass for the aircraft.

5.2 Abbreviations

5.2.1 Aircraft parameters

MRGM Maximum ramp gross mass in kilograms

5.2.2 Pavement and subgrade parameters

Standard working stress for reporting, 2.75 MPa

t Pavement reference thickness in centimetres

Note: t is defined as the thickness of slab for rigid pavements, or total thickness of pavement structural system (surface to subgrade) for flexible pavements.

CBR California Bearing Ratio in per cent

Note. CBR values are always used in the standard format. Hence a CBR of 14 would be entered into the charts or formulae as 15.

5.2.3 Tyre Pressures

Ps Tyre pressure for derived single wheel load - 1.25 MPa

Pq Tyre pressure for aircraft at maximum ramp gross mass condition

Theoretical asphalt pavement Theoretical rigid pavement

Asphalt

Base Thickness (t) Concrete Thickness (t)

Sub-base Sub-baseSubgrade Subgrade

5.3 Mathematical Models.

Two mathematical models are used in the ACN-PCN method: the Westergaard solution for a loaded elastic plate on a Winkler foundation (interior load case) for rigid pavements, and the Boussinesq solution for stresses and displacements in a homogeneous isotropic elastic half-space under surface loading for flexible pavements. The use of these two, widely used, models permits the maximum correlation to world-wide pavement design methodologies, with a minimum need for pavement parameter values (that is, only approximate subgrade K, or CBR values are required).

5.4 Computer Programmes.

The two computer programmes developed using these mathematical models have been produced. The programme for evaluating aircraft on rigid pavements is based on the programme developed by Mr R G Packard of Portland Cement Association, Illinois, USA and that for evaluating aircraft on flexible pavements is based on the US Army Engineer Waterways Experiment Station Instruction Report S-77-1, entitled “Procedures for Development of CBR Design Curves”. It should be noted that the aircraft classification tables included in this manual, completely eliminate the need to use these programmes in respect of most of the aircraft currently in use.

5.5 Graphical Procedures.

Aircraft for which the manufacturers have published pavement thickness requirement charts can also be evaluated using the graphical procedures described below.

NOTE.

There is no relationship between the pavement reference thickness and the actual pavement thickness.The reference thickness relates to the ACN (aircraft) and the pavement thickness is the result of design methodology.

5.6 Rigid Pavements.

This procedure uses the conversion chart shown in Figure 1 and the pavement thickness requirement charts published by the aircraft manufacturers. The Portland Cement Association computer programme was used in developing Figure 1. This figure relates the derived single wheel load at a constant tyre pressure of 1.25 MPa to a reference pavement thickness. It takes into account the four standard subgrade K values detailed above, and a standard concrete working stress of 2.75 MPa. The figure also includes an ACN scale which permits the ACN to be read directly. The following steps are used to determine the ACN of an aircraft:

using the pavement requirement chart published by the manufacturer obtain the reference thickness for the given aircraft mass, K value of the subgrade, and the standard concrete stress for reporting, ie. 2.75 MPa;

using the above reference thickness and Figure 1, obtain a derived single wheel load for the selected subgrade; and

the aircraft classification number, at the selected mass and subgrade K value, is two times the derived single wheel load in 1,000Kg. Note that the ACN can also be read directly from the chart and further that tyre pressure corrections are not needed when the above procedure is used.

NOTE.

The charts in this text relating to ACN graphical determinations, can be enlarged to A3 for increased accuracy.

Figure 1 - ACN Rigid Pavement Conversion Chart

This chart is a rigid pavement ACN conversion chart which is supplied with a grid – for ease of use.

Fig 1-4

Fig 1-5

5.7 Calculation of Rigid Pavement reference thickness and ACN.

Example.

Find the ACN of B727-200 Standard at 78,500 Kg on a rigid pavement resting on a medium strength subgrade (K = 80 MN/m3). Note that if the K value was 65 MNm –3 the standard K value is still used because this standard value of 80 incorporates all values between 60 and 120 MNm –3 . The tyre pressure of the main wheels is 1.15 MPa.

Using the graphical method select chart 1-4. Note that if the aircraft is listed at Table 1, then the ACN can be directly calculated.

The allowable working stress in MPa on the right hand side of the page is not to be selected – it is not the standard working stress of 2.75 MPa which is required (Fig 1-5 is correct in this regard). The chart shows a line at 400 PSI which is 2.75 MPa converted to PSI.From that working stress move horizontally to the aircraft mass line, in this chart the 78,500 mass is the line which is not given a listed mass. Note that if the aircraft mass does not equate to a listed line then a graph interpolation is required.Hence from that intersection vertically to the selected sub-grade strength. Then horizontally to the pavement thickness value.On this chart it is easier to use the line marked in inches to ascertain the value of t. By inspection each gradation on the line equates to 0.2 inches, therefore again by inspection the t value is 12.5 inches which converts to 31.75 cm. This is the reference pavement thickness t.Moving to chart Fig 1, a reference thickness of 31.75 cm on a medium strength subgrade gives a DSWL of 25, hence a ACN of 50.

5.8 Tyre Pressure Adjustment To ACN

Aircraft normally have their tyres inflated to the pressure corresponding to the maximum ramp gross mass and maintain this pressure regardless of the variations in take-off masses. There are times, however, when operations at reduced masses and reduced tyre pressures are productive and reduced ACNs need to be calculated. To do this for rigid pavements, a chart has been prepared by the use of the PCN computer programme PDILB and is given in Figure 2. The example included in the chart itself explains how the chart is used.

The figure included at 1-7 can be used to to convert rigid pavement ACN’s at different tyre pressures. The use of the chart is shown at the example given.

Using the data given in Figure 2 from the intersection of a ACN of 50 at the standard tyre pressure line of 1.25 MPa (1250 kPa) move vertically to the intersection of the horizontal 1500 kPa line. Then following the arc shape intersect the ACN/PCN line at 53.

Figure 2 - ACN Tyre Pressure Adjustment - Rigid Pavements OnlyNOTE .This chart may only be used for revising ACN’s at standard TP, and not reference thicknesses.

5.9 Calculation of Flexible pavement reference thickness and ACN.

5.10 Flexible Pavements.

This procedure uses the conversion chart shown in Figure 2 and the pavement thickness requirement charts published by the aircraft manufacturers based on the United States Army Engineers CBR procedure. The former chart has been developed using the following expression: NOTE DSWL is in kilogrammes.

Where t = reference thickness in cm.DSWL = a single wheel load with 1.25 MPa tyre pressureCBR = standard subgrade (Note the chart uses four standard values 3, 6, 10 and 15)Cl = 0.5695C2 = 32.035

Example.If a aircraft has a DSWL of 25, hence a ACN of 50, and is resting on a flexible pavement with a subgrade value CBR of 9, what is the value of T.

t = 25000 - 25000 0.5695 x 10 32.035 x 1.25 = 61.4 cm

The reason for using the latter charts is to obtain the equivalence between the group of landing gear wheels effect to a derived single wheel load by means of Boussinesq Deflection Factors. The following steps are used to determine the ACN of an aircraft:

a) using the pavement requirement charts published by the manufacturers determine the reference thickness for the given aircraft mass, subgrade category, and 10,000 coverages;

b) enter Figure 3 with the reference thickness determined in step a) and the CBR corresponding to the subgrade category and read the derived single wheel load; and

c) the ACN at the selected mass and subgrade category is two times the derived single wheel load in 1000 kg. Note that the ACN can also be read directly from the chart and also further that tyre pressure corrections are not needed when the above procedure is used.

DSWL - DSWLt = C1 x CBR C2 x PS

Figure 3 - ACN Flexible Pavement Conversion ChartNote. When using the formula embedded in the chart t is in cm and CBR standard values.

Figure 4

This chart is a flexible pavement ACN conversion chart supplied with grid lines for ease of use.

Example.

Find the ACN of DC-10-10 at 157,400Kg on a flexible pavement resting on a medium strength subgrade (CBR 10). The tyre pressure of the main wheels is 1.28 MPa.

Using the chart at figure 1-6. From the given CBR value of 10 rise vertically to the intersection of the 157,400 kg aircraft mass, hence horizontally to the reference pavement thickness of 57 cm.

Using this thickness in figure 4 we arrive at a ACN of 43.

This can be checked mathmatically by using the formula embeded in the chart at figure 3.

ACN = 57 2 1000 0.878 – 0.0124910

ACN = 43----------------------------------------------------------------------------------------------------------------------------------------------

For flexible pavements, the CBR equation

was used to equate thickness and solve for the reduced pressure ACN in terms of the tyre pressure ACN at the reduced mass giving the following expression:

1 1 C1 x CBR C2 x PNew

ACN New Pressure = ACNOld Pressure x1 1

C1 x CBR C2 x POld

Example.A aircraft has a ACN of 33 on a flexible pavement resting on a subgrade who’s CBR is 10. If the tyre pressure is raised to 203 PSI what is the new ACN. The standard tyre pressure is 1.25 MPa. 203 PSI converts to 1.4 MPa. C1 = 0.5695 C2 = 32.035

33 x 1 - 1 0.5695 x 10 32.035 x 1.4

1 - 1 0.5695 x 10 32.035 x 1.25

equals 0.1755926 - 0.022297 0.1755926 - 0.0249726

DSWL DSWLt = C1 x CBR C2 x PS

New ACN = 33 x 1.01776 = 33.6 say 34.

6. Procedure For Pavements Meant For Light Aircraft

The ACN-PCN method described above is not intended for reporting strength of pavements meant for light aircraft, that is those with mass less than 5,700Kg. Annex 14 specifies a simple procedure for such pavements. This procedure envisages the reporting of only two elements: maximum allowable aircraft mass and maximum allowable tyre pressure. It is important to note that the tyre pressure categories of the ACN-PCN method are not used for reporting maximum allowable tyre pressure. Instead, actual tyre pressure limits are reported as indicated in the following example:4,000Kg/0.50MPaAustralia reports this as 4000/500(73PSI)

7. Exercises

Exercise 1:

Find the ACN of B727-200 Standard at 78,500 Kg on a rigid pavement resting on a medium strength subgrade (K = 80 MN/m3). The tyre pressure of the main wheels is 1.15 MPa.

Solution:

The ACN of the aircraft from Table 1 of this document is 50

Exercise 2

An AIP contains the following information related to a runway pavement:PCN of the pavement = 80Pavement type = rigidSubgrade category = medium strengthTyre pressure limitation – noneRating determined by aircraft usage

Write this information as it could appear in the AIP.

Determine whether the pavement can accept the following aircraft at the indicated operating masses and tyre pressures:

Aircraft Mass Tyre pressure

Airbus A 300 Model B2 142,000Kg 1.23MPaB747-100 334,751Kg 1.55MPaConcorde 185,066Kg 1.26MPaDC-10-40 253,105Kg 1.17MPa

Solution

PCN 80/R/B/W/U

ACNs of these aircraft from Table 1 of this document are 44, 51, 71 and 53, respectively. Since the pavement in question has a PCN of 80 it can accept all of these aircraft.

Exercise 3:

Find the ACN of DC-10-10 at 157,400Kg on a flexible pavement resting on a medium strength subgrade

(CBR 10). The tyre pressure of the main wheels is 1.28 MPa.

Solution:

The ACN of the aircraft from Table 1 of this document is

57 - (196,406 - 157,400) x (57 - 27)(196,406 - 108,940)

= 57 - 39,006 x 30 87,466

= 57 - 13.4 = 43.6 say 44

8. GUIDANCE ON OVERLOAD OPERATIONS

8.1 Criteria Suggested By ICAO

Overloading of pavements can result from either loads too large or a substantially increased application rate or both. Loads larger than the defined design or evaluation load shorten the pavement design life whilst smaller loads or less frequent loading extend the pavement design life. With the exception of massive overloading, pavements in their structural behaviour are not subject to a particular limiting load above which they suddenly or catastrophically fail. Behaviour is such that a pavement can sustain a definable load for an expected number of repetitions during its design life. As a result, occasional minor overloading is acceptable, when expedient, with only limited loss in pavement life expectancy and relatively small acceleration of pavement deterioration. For those operations in which magnitude of overload and/or the frequency of use do not justify a detailed analysis the following criteria are suggested:

for flexible pavements occasional movements by aircraft with ACN not exceeding 10 per cent above the reported PCN should not adversely affect the pavement;

for rigid or composite pavements, in which a rigid pavement layer provides a primary element of the structure, occasional movements by aircraft with ACN not exceeding 5 per cent above the reported PCN should not adversely affect the pavement;

if the pavement structure is unknown the 5 per cent limitation should apply; and

the annual number of overload movements should not exceed approximately 5 per cent of the total annual aircraft movements.

Such overload movements should not normally be permitted on pavements exhibiting signs of distress or failure. Furthermore, overloading should be avoided during any periods of thaw following frost penetration or when the strength of the pavement or its subgrade could be weakened by water. Where overload operations are conducted, the appropriate authority should review the relevant pavement condition regularly and should also review the criteria for overload operations periodically since excessive repetition of overloads can cause severe shortening of pavement life or require major rehabilitation of pavement.

8.2 Practices Of Various Countries

8.2.1 Canada

The technical assessment of a proposed overload operation is based on the overload ratio concept. The overload ratio is a measure of the load imposed by the aircraft relative to the nominal design strength of the

pavement. For flexible pavements, the overload ratio imposed by an aircraft is determined by calculating the subgrade bearing strength required for the existing thickness of pavement. This calculated subgrade bearing strength is then divided by the actual subgrade bearing strength to form the overload ratio. For rigid pavements, the overload ratio is defined as the flexural stress imposed in the slab by the aircraft divided by the design flexural stress of 2.75 MPa.

On the basis of these overload ratios, aircraft operations are classified as follows:

Overload Ratio Operational Classification

less than 1.25 Unrestricted1.25 to 1.50 Limited1.50 to 2.00 Marginal

greater than 2.00 Emergency use only

The approval of operations classified as limited or marginal involves the risk of an accelerated rate of pavement deterioration and shortened service life. This risk increases with increasing value of overload ratio and frequency of operations. The decision to approve such operations therefore depends on the willingness of the airport authority to fund pavement rehabilitation measures earlier than may otherwise be necessary. Normal practice at airports operated by Transport Canada is to permit aircraft operations falling into the limited and marginal classifications, unless otherwise dictated by age and condition of the pavement, or funding constraints.

Similar considerations apply to permitting operations by aircraft with tyre pressures higher than restrictions reported. Provided the overload ratio is less than 1.50, aircraft are normally permitted to operate with tyre pressures one range higher than the tyre pressure range for which the pavement was designed.

8.2.2 United Kingdom

Individual aerodrome authorities in the United Kingdom are free to decide on their own criteria for permitting overload operations as long as pavements remain safe for use' by aircraft. However, the following guidance is provided:

a 10 per cent difference in ACN over PCN involves an increase in pavement working stresses which are generally considered acceptable provided the following conditions are satisfied:

the pavement is more than twelve months old;

the pavement is not already showing signs of loading distress; and

overload operations do not exceed 5 per cent of the annual departures and are spread throughout the year.

overload operations representing a difference in ACN over PCN of from 10 per cent to 25 per cent justify regular inspections of the pavement by a competent person in addition to satisfying the above criteria. There should be an immediate curtailment of such overload operations as soon as distress becomes evident and the higher loading should not be reimposed until appropriate pavement strengthening work has been completed;

overload operations representing a difference in ACN over PCN of from 25 per cent to 50 per cent may be undertaken under special circumstances. They call for scrutiny of available pavement construction records and test data by a qualified pavement engineer and a thorough inspection by a pavement engineer before and on completion of the movement to assess any signs of pavement distress; and

overload operations in excess of an ACN over PCN of 50 per cent should only be undertaken in an emergency.

8.2.3 Australia

8.2.3.1 General

The Aircraft Classification Number/Pavement Classification Number (ACN/PCN) method is used to specify the strength of pavements and hence permissible aircraft masses.

The operation of an aircraft above the maximum masses and tyre pressures is not permitted unless a pavement concession is approved by the airport owner/authority.

8.2.3.2 Information Published For Rated Pavements

The parameters published to specify the strength of a pavement suitable for use by aircraft above 5700kg maximum all up mass are:

a) Pavement Classification Number. PCN

b) The pavement type:

Rigid Pavement R

Flexible Pavement F

c) The subgrade strength in four standard categories:

High Strength A

Medium Strength B

Low Strength C

Ultra-low Strength D

d) The maximum tyre pressure in kilopascals (KPa)

e) The method by which the pavement has been evaluated:

By technical evaluation TFrom aircraft experience U

Eg 55/F/A/1400/T

The parameters published for pavements suitable for use by aircraft not above 5700kg maximum all up mass are:

a) The permissible aircraft gross mass in kilograms.

b) The maximum tyre pressure in kilopascals.

Eg 3500/550 (recall Australian AIP ERSA methodology.)

8.2.3.3 Determination of Pavement Strength Suitability – Rated Pavements

Compare the aircraft tyre pressure with the maximum listed for the pavement.

a. If the tyre pressure does not exceed that listed then the tyre pressure is acceptable.

b. If the tyre pressure exceeds that listed; the permissible pressure may be increased using the factor obtained in the figure below up to a limit of 1400KPa. If the pressure requirements are then met, and provided that not more than four movements within a seven day period are proposed for aircraft above

5700kg maximum gross mass, the tyre pressure is acceptable.

c. An aircraft may use a pavement if its ACN for the appropriate type and subgrade strength does not exceed the published PCN.

Mathematically the above graph is :-Aircraft Mass < 5 tonnes multiply aerodrome tyre pressure by 2.Aircraft Mass between 5 and 15 tonnes multiply aerodrome tyre pressure by 2.5 – M

10Aircraft Mass > 15 tonnes multiply aerodrome tyre pressure by 1.

Example. – for aircraft above 5700 kg MTOW.

A aerodrome listing in ERSA (COCOS ISLAND) gives a runway as PCN30/F/A/700(102PSI)U.

If a aircraft wishing to use this aerodrome has a ACN greater than 30 the aircraft operator requires a pavement concession (PC) from the aerodrome operator.

If the aircraft mass is greater than 15 tonnes and the tyre pressure is above 700 kPa the aircraft operator requires a PC.

If the aircraft mass is below 15 t, and when the aerodrome allowable tyre pressure (TP) multiplied by the allowable permanent tyre pressure concession is less than 1400 kPa meets the required TP, but more than 4 movements in a 7 day period is countenanced then, a PC is still required.

If the aircraft mass is below 15 t, and when the aerodrome allowable tyre pressure (TP) multiplied by the allowable permanent tyre pressure concession is less than 1400 kPa meets the required TP, and 4 movements in a 7 day period is countenanced then, a PC is not required and the aircraft operator has no requirement to inform the aerodrome operator of the movements other than the normal parking, aerodrome operator fees , Customs or Quarantine etcetera requirements.

Should the above operation cause or give rise to concern to the aerodrome operator in terms of pavement considerations, it is encumbent upon the aerodrome operator to raise these concerns with the Authority – CASA.

A AIS document AIP AD 1.1 lists the above requirements. A document named Aerodrome Pavement Concession System – An Explanation for Aerodrome

Operators was previously published by the Commonwealth Department of Transport and Communications. This Department is now named the Department of Transport and Regional Services and on request may provide the above document.

In terms of Cocos Island, a aircraft of mass 10 tonnes, ACN of 30 and TP of 800 kPa wishes to use the aerodrome once per week.

Evaluation.

a) ACN equal to the PCN - therefore acceptable.b) Tyre pressure concession – 1.5 x 700 (1050) is greater than the aircraft tyre pressure and less

than 1400 kPa - therefore acceptable.c) One movement per week meets the frequency criteria – therefore accepable.

Conclusion.

The aircraft operator does not require a pavement concession.

8.2.3.4 Determination of Pavement Strength Suitability – Unrated Pavements

An aircraft may operate on an unrated pavement, provided the aircraft gross mass and tyre pressure do not exceed that determined in the figure below:

As a general rule if a aircraft has a TP of 65 PSI or less, providing the pavement is ‘”dry to depth” , then the movement is suitable for a unrated and/or unsealed pavement for unrestricted movements.

8.2.3.5 Pavement Concessions

The operator of an aircraft requiring a pavement concession should apply to the airport operator for approval to operate.

The airport operator has sole discretion regarding whether, or not, to issue a pavement concession and under what conditions the aircraft may use the aircraft pavements.

9. ACNS FOR SEVERAL AIRCRAFT TYPES

9.1 Introduction

Several aircraft types currently in use have been evaluated on rigid and flexible pavements using computer programmes and the results tabulated in the table below. The two all-up masses shown in column 2 for each aircraft type are respectively the maximum apron (ramp) mass and a representative operating mass empty. To compute the ACN for any intermediate value, proceed on the assumption that the ACN varies linearly between the operating mass empty and the maximum apron mass.

Table 1 - ACNs for several aircraft types on rigid and flexible pavements

ACN for ACN forRigid pavement subgrade - MN/m3 Flexible pavement subgrade - CBR

AircraftType

All-upmass

(kg)

Load onone maingear leg

(%)

Tyrepressure

(MPa)High150

Medium80

Low40

Ultra low 20

High15

Medium10

Low6

Very low3

1 2 3 4 5 6 7 8 9 10 11 12

A300 B2 137 000 47.0 1.20 35 42 50 58 39 43 53 68Airbus 85 910 18 21 25 29 20 22 24 34

A300 B2 142 000 47.0 1.29 35 45 53 61 40 45 55 71Airbus 85 910 19 22 26 30 21 22 25 34

A300 B4 150 000 47.0 1.39 41 49 57 65 43 49 59 76Airbus 88 180 20 22 26 31 21 22 25 35

A300 B4 157 000 47.0 1.48 45 53 62 70 46 52 63 80Airbus 88 330 20 22 26 31 21 22 25 36

A300 B4 165 000 47.0 1.29 46 55 64 73 49 56 68 84Airbus 88 505 17 20 25 29 20 21 25 36

A300-600 165 000 47.0 1.29 46 55 64 73 49 56 68 84Airbus 87 100 17 19 24 28 19 21 24 35

A300-600R 170 000 47.4 1.35 49 58 68 78 52 58 71 89Airbus 85 033 17 19 23 28 19 20 23 34

A300-600R 171 700 47.4 1.35 50 59 69 79 52 59 72 90Airbus 85 033 17 19 23 28 19 20 23 34

A310-200 132 000 46.7 1.23 33 39 46 54 36 40 48 64Airbus 76 616 15 18 21 24 18 19 20 27

A310-200 138 600 46.7 1.30 35 42 51 58 39 43 52 68Airbus 76 747 16 18 21 25 18 19 20 28

A310-200 142 000 46.7 1.33 37 44 52 60 40 44 54 70Airbus 75 961 15 17 20 23 17 18 20 27

A310-300 150 000 47.0 1.42 42 49 58 66 44 49 59 76Airbus 77 037 13 14 17 20 15 15 16 24

A310-300 157 000 47.4 1.49 45 54 63 71 47 53 64 81Airbus 78 900 14 15 18 22 15 15 16 25


AircraftType

All-upmass

(kg)


(%)

Tyrepressure

(MPa)High150

Medium80

Low40

Ultra low 20

High15

Medium10

Low6

Very low3

1 2 3 4 5 6 7 8 9 10 11 12

A320-100 66 000 47.1 1.28 37 40 42 44 33 34 38 44Airbus Dual 37 203 19 20 21 23 18 18 19 27

A320-100 68 000 47.1 1.34 39 41 43 45 35 36 40 46Airbus Dual 39 700 20 22 23 24 19 19 20 23

A320-100 68 000 47.1 1.12 18 21 24 28 18 19 23 32Dual Tandem 40 243 9 10 12 14 9 10 11 14

A320-200 73 500 47.0 1.45 44 46 48 50 38 40 44 50Airbus Dual 39 748 20 22 23 25 19 19 20 24

A320-200 73 500 47.0 1.21 18 22 26 30 19 21 26 35Dual Tandem 40 291 9 10 11 13 9 10 11 14

BAC 1-11 39 690 47.5 0.93 25 26 28 29 22 24 27 29Series 400 22 498 13 13 14 15 11 12 13 15

BAC 1-11 44 679 47.5 0.57 22 25 27 28 19 24 28 31Series 475 23 451 10 11 12 13 9 10 12 15

BAC 1-11 47 400 47.5 1.08 32 34 35 36 29 30 33 35Series 500 24 757 15 16 16 17 13 13 15 17

BAe 146 37 308 46.0 0.80 18 20 22 23 17 18 20 24Series 100 23 000 10 11 12 13 10 10 11 13

BAe 146 37 308 46.0 0.52 16 18 19 21 13 16 19 23Series 100 23 000 9 10 11 12 8 9 11 13

BAe 146 40 600 47.1 0.88 22 23 25 26 19 21 23 27Series 200 23 000 11 12 13 14 10 10 11 13

BAe 146 40 600 47.1 0.61 19 21 23 24 16 20 22 27Series 200 23 000 10 11 12 12 8 10 11 13

B707-120B 117 027 46.7 1.17 28 33 39 46 31 34 41 5457 833 12 12 15 17 13 14 15 20

B707-320B 148 778 46.0 1.24 38 46 54 62 42 47 57 7264 764 13 14 17 20 15 15 17 22

B707-320C 152 407 46.7 1.24 40 48 57 66 44 49 60 76(Freighter) 61 463 13 14 16 19 14 15 17 21

B707-320C 152 407 46.7 1.24 40 48 57 66 44 49 60 76(Convertible) 67 269 14 15 18 21 16 17 19 24

B707-320/420 143 335 46.0 1.24 36 43 52 59 40 44 54 6964 682 13 14 17 20 15 15 17 22

B720 104 326 47.4 1.00 25 30 37 42 29 31 39 5150 258 10 11 13 16 11 12 14 18

B720 B 106 594 46.4 1.00 25 30 37 42 29 31 39 5152 163 10 11 13 16 11 12 14 18

B727-100 77 110 47.6 1.14 46 48 51 53 41 43 49 5441 322 22 23 25 26 20 20 22 26

B727-100C 73 028 47.8 1.09 43 45 48 50 39 40 46 5141 322 22 23 25 26 20 21 22 26


AircraftType

All-upmass

(kg)


(%)

Tyrepressure

(MPa)High150

Medium80

Low40

Ultra low 20

High15

Medium10

Low6

Very low3

1 2 3 4 5 6 7 8 9 10 11 12

B727-200 78 471 48.5 1.15 48 50 53 56 43 45 51 56(Standard) 44 293 24 26 27 29 22 23 25 29

B727-200 84 005 48.0 1.02 49 52 55 58 45 48 55 60(Advanced) 44 270 23 24 26 28 21 22 24 29

B727-200 86 636 47.7 1.06 51 54 58 60 47 50 56 61(Advanced) 44 347 23 25 26 28 22 22 24 28

B727-200 89 675 46.9 1.15 54 57 60 62 49 51 58 63(Advanced) 44 470 23 25 27 28 21 22 24 28

B727-200 95 254 46.5 1.19 58 61 64 67 52 55 62 66(Advanced) 45 677 24 25 27 29 22 22 25 29

B737-100 44 361 46.2 0.95 23 24 26 27 20 22 24 2826 581 12 13 14 15 12 12 13 15

B737-200 45 722 46.4 0.97 24 25 27 29 22 23 26 3027 170 13 14 15 16 12 12 14 16

B737-200 52 616 45.5 1.14 29 31 32 34 26 27 30 3427 125 13 14 15 16 12 12 13 15

B737-200 52 616 45.5 0.66 24 26 28 30 21 25 29 3427 125 11 12 13 14 10 11 13 15

B737-200/200C 53 297 46.4 1.16 30 32 34 35 27 28 31 36(Advanced) 29 257 15 16 17 18 14 14 15 17

B737-200/200C 56 699 46.3 1.23 33 34 36 38 29 30 34 38(Advanced) 28 985 15 16 17 18 14 14 15 17

B737-200 58 332 46.0 1.25 34 36 38 39 30 31 35 39(Advanced) 29 620 15 16 17 18 14 14 15 17

B737-300 61 462 45.9 1.34 37 39 41 42 32 33 37 4132 904 18 18 20 21 16 16 17 20

B737-300 61 462 45.9 1.14 35 37 39 41 31 33 37 4132 904 17 18 19 20 15 16 17 20

B737-400 64 864 46.9 1.44 41 43 45 47 35 37 41 4533 643 19 20 21 22 16 17 18 21

B737-500 * 60 781 46.1 1.34 37 38 40 42 32 33 37 4131 312 17 17 19 19 15 15 16 19

B747-100 323 410 23.4 1.50 41 48 57 65 44 48 58 77162 385 17 19 22 25 19 20 22 28

B747-100B 334 749 23.1 1.56 43 50 59 68 46 50 60 80173 036 18 20 24 28 20 21 24 30

B747-100B 341 553 23.1 1.32 41 49 58 68 46 51 62 82171 870 17 19 22 26 20 21 23 30

B747-100B 260 362 24.1 1.04 27 32 40 47 33 36 43 59SR 164 543 16 17 21 25 19 20 23 30

B747SP 302 093 22.9 1.30 35 42 51 59 40 44 52 71


AircraftType

All-upmass

(kg)


(%)

Tyrepressure

(MPa)High150

Medium80

Low40

Ultra low 20

High15

Medium10

Low6

Very low3

1 2 3 4 5 6 7 8 9 10 11 12

147 716 14 16 19 22 17 17 19 25

B747SP 318 881 21.9 1.40 37 44 52 60 41 45 54 72147 996 14 15 18 21 16 17 18 23

B747-200B 352 893 23.6 1.37 45 53 64 73 50 55 67 88172 886 18 20 24 28 21 22 24 31

B747-200C 373 305 23.1 1.30 46 55 66 76 52 57 70 92166 749 16 18 21 25 19 20 22 29

B747-200F/300 379 201 23.2 1.39 47 57 68 78 53 59 73 94156 642 16 17 20 24 18 19 21 26

B747-400 395 987 23.4 1.41 53 63 75 85 57 64 79 101178 459 19 21 25 29 21 22 25 32

B757-200 109 316 45.2 1.17 27 32 38 44 29 32 39 5260 260 12 14 17 19 14 14 17 22

B767-200 143 789 46.2 1.31 33 38 46 54 37 40 47 6578 976 15 17 20 24 18 19 21 26

B767-200-ER 159 755 46.9 1.21 37 44 54 63 43 47 57 7780 853 16 18 21 25 19 19 22 28

B767-300 159 665 47.5 1.21 38 45 54 63 43 48 58 7886 070 17 19 23 27 20 21 24 32

B767-300-ER 172 819 46.9 1.31 43 51 61 71 48 53 65 8687 926 18 20 24 28 21 22 24 32

B767-300-ER 185 520 46.0 1.38 47 56 66 76 51 57 70 9288 470 18 20 24 28 21 22 24 31

Caravelle 52 000 46.1 0.75 15 17 20 22 15 17 19 23Series 10 29 034 7 8 9 10 7 7 9 11

Caravelle 55 960 46.0 0.88 16 19 22 25 17 19 21 26Series 12 31 800 8 9 10 12 8 9 10 12

Concorde 185 066 48.0 1.26 61 71 82 91 65 72 81 9878 698 21 22 25 29 21 22 26 32

Canadair 95 708 47.5 1.12 25 30 35 40 27 30 36 47CL 44 40 370 9 10 11 13 9 10 11 14

Convair 87 770 46.6 1.03 26 31 36 41 27 31 36 44880 M 40 195 9 10 12 14 10 10 12 15

Convair 115 666 48.5 1.28 41 48 54 60 40 45 53 64990 54 685 15 17 19 22 15 16 19 24

DC-3 11 430 46.8 0.31 6 7 7 7 4 6 8 97 767 4 5 5 5 3 4 5 6

DC-4 33 113 46.8 0.53 13 15 17 18 11 14 16 2022 075 8 9 10 11 7 9 10 12

DC-8-43 144 242 46.5 1.22 41 49 57 65 43 49 59 7461 919 15 16 18 21 15 16 18 23


AircraftType

All-upmass

(kg)


(%)

Tyrepressure

(MPa)High150

Medium80

Low40

Ultra low 20

High15

Medium10

Low6

Very low3

1 2 3 4 5 6 7 8 9 10 11 12

DC-8-55 148 778 47.0 1.30 45 53 62 69 46 53 63 7862 716 15 16 19 22 15 16 18 24

DC-8-61/71 148 778 48.0 1.30 46 54 63 71 48 54 64 8068 992 17 19 22 25 18 19 21 28

DC-8-62/72 160 121 46.5 1.29 47 56 65 73 49 56 67 8365 025 15 16 19 22 16 16 18 24

DC-8-63/73 162 386 47.6 1.34 50 60 69 78 52 59 71 8772 002 17 19 23 26 18 19 22 29

DC-9-15 41 504 46.2 0.90 23 25 26 28 21 22 26 2822 300 11 12 13 14 10 11 12 14

DC-9-21 45 813 47.2 0.98 27 29 30 32 24 26 29 3223 879 12 13 14 15 11 12 13 15

DC-9-32 49 442 46.2 1.07 29 31 33 34 26 28 31 3425 789 14 15 15 16 12 13 14 16

DC-9-41 52 163 46.7 1.10 32 34 35 37 28 30 33 3727 821 15 16 17 18 13 14 15 18

DC-9-51 55 338 47.0 1.17 35 37 39 40 31 32 36 3929 336 17 17 18 19 15 15 16 19

MD-81 63 957 47.8 1.17 41 43 45 46 36 38 43 4635 571 20 21 23 24 18 19 21 24

MD-82/88 68 266 47.6 1.27 45 47 49 50 39 42 46 5035 629 21 22 24 25 18 19 20 24

MD-83 73 023 47.4 1.34 49 51 53 55 42 46 50 5436 230 21 22 24 25 18 19 21 24

MD-87 68 266 47.4 1.27 45 47 49 50 39 42 46 5033 965 19 21 22 23 17 18 19 22

DC-10-10 196 406 47.2 1.28 45 52 63 73 52 57 68 93108 940 23 25 28 33 26 27 30 38

DC-10-10 200 942 46.9 1.31 46 54 64 75 54 58 69 96105 279 22 24 27 31 24 25 28 36

DC-10-15 207 746 46.7 1.34 48 56 67 74 55 61 72 100105 279 22 24 27 31 24 25 28 36

DC-10-30/40 253 105 37.7 1.17 44 53 64 75 53 59 70 97120 742 20 21 24 28 22 23 25 32

DC-10-30/40 260 816 37.6 1.21 46 55 67 78 56 61 74 101124 058 20 21 25 29 23 23 26 33

DC-10-30/40 268 981 37.9 1.24 49 59 71 83 59 64 78 106124 058 20 21 25 29 23 23 26 33

MD-11 274 650 39.2 1.41 56 66 79 92 64 70 85 114127 000 23 25 28 32 25 26 29 37

DCH 7 19 867 46.8 0.74 11 12 13 13 10 11 12 14DASH 7 11 793 6 6 7 7 5 6 6 8


AircraftType

All-upmass

(kg)


(%)

Tyrepressure

(MPa)High150

Medium80

Low40

Ultra low 20

High15

Medium10

Low6

Very low3

1 2 3 4 5 6 7 8 9 10 11 12

FOKKER 27 19 777 47.5 0.54 10 11 12 12 8 10 12 13Mk500 11 879 5 6 6 7 4 5 6 7

FOKKER 50 20 820 47.8 0.59/ 10 11 12 13 8 10 12 14HTP 12 649 0.55 6 6 7 7 5 5 6 8

FOKKER 50 20 820 47.8 0.41 9 10 11 12 6 9 11 14LTP 12 649 5 5 6 7 4 5 6 8

FOKKER 28 29 484 46.3 0.58 14 15 17 18 11 14 16 19Mk1000LTP 15 650 6 7 8 9 5 6 7 9

FOKKER 28 29 484 46.3 0.69 15 16 18 18 13 15 17 20Mk1000HTP 16 550 8 8 9 10 6 7 8 10

FOKKER 100 44 680 47.8 0.98 28 29 31 32 25 27 30 3224 375 13 14 15 16 12 13 14 16

HS125-400A 10 600 45.5 0.77 6 6 7 7 5 5 6 7 -400B 5 683 3 3 6 3 2 3 3 3

HS125-600A 11 340 45.5 0.83 7 7 7 8 5 6 7 8 -600B 5 683 3 3 3 3 2 3 3 3

HS748 21 092 43.6 0.59 10 11 11 12 8 9 11 1312 183 5 5 6 6 4 5 6 7

IL-62 162 600 47.0 1.08 42 50 60 69 47 54 64 7966 400 14 15 18 20 16 17 18 24

IL-62M 168 000 47.0 1.08 43 52 62 71 50 57 67 8371 400 16 17 19 22 17 18 20 26

IL-76T 171 000 23.5 0.64 38 38 38 39 37 40 45 5383 800 11 14 16 16 15 16 18 22

IL-86 209 500 31.2 0.88 25 31 38 46 34 36 43 61111 000 13 14 16 19 16 17 19 23

L-100-20 70 670 48.2 0.72 30 33 36 38 27 31 33 3834 205 14 15 16 17 12 14 15 16

L-100-30 70 670 48.4 0.72 30 33 36 38 27 31 33 3934 701 14 15 16 17 12 14 15 17

10. ACN Charts For Various Aircraft

Main Gear Tyre Pressure 1410KPa




Main Gear Tyre Pressure 1260MPa






A document named Aerodrome Pavement Concession System – An Explanation for Aerodrome Operators was previously published by the Commonwealth Department of Transport and Communications. This Department is now named the Department of Transport and Regional Services and on request may provide the above document. This document provides a expanded listing of the charts detailed above.

10.1 PAVEMENT STRENGTH CLASSIFICATION THE ICAO ACN / PCN METHOD

An Australian Perspective.

10.2 BACKGROUND

In 1974 the International Civil Aviation Organisation (ICAO) called for the development of a single internationally accepted method of reporting pavement strength. An ICAO study group (Australia was represented by Mr Don Hayman) looked at various methods, and through its work the Aircraft Classification Number / Pavement Classification Number (ACN / PCN) method was proposed as an amendment to Annex 14 (the ICAO document which contains international standards and recommended practices relating to aerodromes). The method is fully described in ICAO's Aerodrome Design Manual, Document 9157-AN/901 Part 3, PAVEMENTS, Chapter 1, entitled Procedures for Reporting Pavement Strength.

The amendment to Annex 14 became effective on 24th July 1981. The ACN / PCN method is an ICAO standard and is now used by about 75% of the world's airports. It replaced four different methods that had been in use by various ICAO member States for some time.

ICAO formed another international pavements committee in 1993 to review the ACN / PCN rating system, particularly as applied to new and future large aircraft such as the Boeing 777. The committee consists of pavement specialists from Boeing, McDonnell Douglas, US FAA, IATA, Airports Council International, Canada, France, the UK, the Netherlands and Australia. Mr Bruce Rodway represented Australia on this committee that was established as Boeing felt that the ACN calculated for the B-777 did not reflect the impact of that aircraft’s gear configuration on aircraft pavements.

10.3 THE ICAO RATING SYSTEM

The ACN / PCN system is the means by which airport owners and operators regulate the use of their pavements having regard to pavement strength and maintenance strategies in relation to revenue from landing fees. New runways are usually designed to last for 15 to 20 years under the expected aircraft traffic before needing major maintenance such as an asphalt overlay. An amount of minor routine maintenance is expected to be necessary during the life of the pavement. Airport owners are free to design the pavements by any method they choose, and are also free to assign a load rating of their choice. Airport operators publish a PCN and a subgrade strength category. The aircraft manufacturers provide the aircraft operators, with aircraft load data. This is in the form of an ACN for all possible aircraft operating weights. If the aircraft's ACN at its intended operating weight is less than the PCN of the pavement it is assured of access to the airport subject to the tyre pressure also being below the published value. If the ACN is greater than the PCN, however, access is at the discretion of the airport owner. The airport owner may allow unrestricted operations at a requested operating weight, or may restrict the frequency of operations or may refuse access. These permits to operate an aircraft with an ACN that is higher than the PCN are termed Pavement Concessions. The owners may require that the aircraft operate at a lower load, thereby reducing the aircraft's ACN to an acceptable number. This would also reduce the aircraft's earning capacity. The airport owner can take into account any likely additional maintenance cost or reduced pavement life (ie reduced time between asphalt overlays) that might result from overload operations and balance these and other factors against the extra revenue obtained through landing charges.

The aircraft manufacturer must be calculate an ACN using a fixed technical method and is intended to indicate the relative pavement damaging effect of each aircraft. An aircraft’s weight, its wheel layout, its tyre pressure and the subgrade strength fix an aircraft’s ACN. The airport owner has no say in what the ACN of an aircraft is as it is a technical fact. By contrast, the PCN functions as a pavement management tool, and its selection is largely a business decision. Airport operators have considerable scope in rating their pavements and should take into account the observed performance of aircraft pavements under aircraft of known ACNs if such information is available. They should also take into account the thickness and strength of their pavements. The airport owner should have regard to the size and numbers of aircraft he wishes to attract to his airport, and also take into account the amount he are prepared to spend to maintain the pavements. The airport owner can, for example raise the PCN of the pavements to allow unrestricted access

of a new heavier aircraft if he wishes, and consciously accept the fact that the pavement maintenance bill may increase as a result of the heavier aircraft using the pavements.

10.4 HOW TO CALCULATE AN ACN

The case of flexible pavements will be used for illustration but similar principles apply to concrete pavements. In order to describe the method it is necessary to first define the Pass to Coverage Ratio (PCR) because the amount of damage to the pavement does not depend upon the number of aircraft that pass along it, but upon the coverages. A point on the pavement surface is said to receive a coverage when any part of a tyre's contact area passes over it. The PCR is defined as the number of passes of an aircraft that is statistically required for the most frequently covered point on the pavement to receive one coverage. The PCR depends upon wheel configuration, tyre width and the degree of aircraft wander about the pavement centreline. For example, the PCR for a B747 on a taxiway is about 1.7 whereas that of a Fokker F-28 is about 3.5. That is, the most frequently 'hit' point on the taxiway is about twice as likely to be hit when a 747 passes as it is when an F-28 passes along the taxiway. To explain the ACN calculation it is also necessary to first define the Equivalent Single Wheel Load (ESWL), which is the load on a single wheel that will cause pavement damage equal to that caused by the aircraft's actual multiwheel gear at its actual gear load. An aircraft's ACN is a number expressing the pavement damage caused by the aircraft relative to that caused by other aircraft. The number depends not only upon the operating mass of the aircraft and its tyre pressure, but also upon the pavement subgrade strength. Airport owners and pilots obtain the ACN of an aircraft from published tables or graphs. An aircraft has many ACNs, for example, the ACN for a Boeing 747-200 ranges from 18 when it is operating empty on a rigid pavement over high strength subgrade to 92 when it is operating full on a flexible pavement overlaying a very low strength subgrade. The corresponding range of ACNs for a Douglas DC-3 is 4 to 9.

Few people if ever need to calculate an ACN from first principles. To do this involves doing a full pavement thickness design using a specific pavement design method. In the case of flexible pavements, it is the CBR. based method as detailed in Instruction Report S-77-1, Procedures for Development of CBR Design Curves by the U.S. Army Corps of Engineers. It is necessary to know the mass, the tyre pressure and the wheel arrangement of the aircraft. Because ACNs are reported for four subgrade strengths, represented by CBR values of 3, 6, 10 and 15, the thicknesses of flexible pavement required for 10,000 coverages of the aircraft (Note: Not unlimited coverages) over CBR values of 3, 6, 10 and 15 must all be calculated. These thicknesses are referred to as reference thicknesses.

Once the reference thickness is known, the four ACN values for a particular aircraft weight are then read from the figure below.

The figure above is simply a CBR design chart which gives the Pavement Thickness required for 10,000 coverages, computed using the Corps' S-77-1 method, for single wheel loadings of 1.25 MPa tyres. Only results for CBR values 3, 6, 10 and 15 are included on the chart. The design method assumes that the deflection at subgrade level is an indicator of pavement damage. That is, it is assumed that the rate at which ruts develop at the pavement surface depends upon the magnitude of the load-induced deflections at subgrade level. Thus the above figure gives the load required on a single wheel inflated to 1.25 MPa which would produce the same deflection at subgrade level as would the actual multiwheeled aircraft at its actual load. This load is termed the Derived Single Wheel Load (DSWL). The ACN of an aircraft is defined as twice the derived single wheel load, expressed in thousands of kilograms. No special significance should be attached to the factor of two as it is just an arbitrary scaling factor.

10.5 HOW TO RATE A PAVEMENT

In contrast to an aircraft, which has a variety of ACNs depending upon its weight, tyre pressure, subgrade strength and pavement type (rigid or flexible), a pavement has just one Pavement Classification Number (PCN). The airport owner is free to rate aircraft pavements by any method. In selecting a PCN owner should consider not only the pavement strength and condition, but the size and number of aircraft the owner wishes to accommodate over the pavement's design life, and also his pavement maintenance strategy. Guidance to the rating of pavements is provided in Chapter 3 of the ICAO Aerodrome Design Manual, which deals with pavement evaluation.

The ACN / PCN method does not constrain designers to use any particular method to design pavements. However, the Corps' S-77-1 design method must be used to compute an ACN, and the concept of derived single wheel load is an inherent part of that method. Thus the significance of a PCN is best explained in terms of the Corp's design method. Taking the simple case of a pavement used by one aircraft type, a PCN of 60 would mean that, according to the Corps of Engineers design method, the pavement would sustain 10,000 coverages of an aircraft of ACN of 60 before becoming unacceptably rutted. If aircraft using the facility have lower ACNs, the pavement will sustain more than 10,000 coverages before deteriorating to the same rutted condition, and conversely, will sustain fewer than 10,000 if aircraft with ACNs greater than 60 use it.

The most common mistake made in rating runways is to think that only the actual pavement thickness and the subgrade CBR need be known. When this is done, the actual thickness is treated incorrectly as a reference thickness and an ACN is read from the figure above. The ACN is then adopted as the PCN of the pavement. This is wrong! If the pavement has been designed, its thickness has been made sufficiently large to cater for all the aircraft that are expected to use the runway during the design period, typically the 15 to 20 years between resurfacing. That is, the thickness depends upon both the sizes and numbers of all the expected aircraft. To illustrate the rating error, consider the simple situation of a pavement designed to cater for 100,000 coverages of a B737-400 at 50 tonnes. The required pavement thickness (ie the design thickness) over a CBR 6 subgrade is about 780mm. The required thickness for 10,000 coverages is only 630mm (which is, by definition the reference thickness). The figure above tells us that the ACN for a reference thickness of 630mm over CBR 6 (Code C subgrade) is 30. We could also have got this by looking up the ACN chart for the B737-400. If, however we confused the design thickness with the reference thickness we would read from the above figure an ACN of 45 for a thickness of 780mm. An ACN of 45 corresponds to a B767 at 135 tonnes; a far more damaging aircraft than the pavement was designed to cater for. So to rate the runway as PCN 45 would be to invite unrestricted use by the B767 and other comparable aircraft, and the intended pavement life would be much shortened. The correct rating for the pavement would be PCN 30, equal to the ACN of the aircraft it was designed to serve.

10.6 LIMITATIONS OF ACN / PCN

Equal coverages (not passes) of aircraft that have the same ACN are expected to cause equal pavement damage. The relative damage caused by each aircraft is not, however, proportional to the ACNs. For example, an aircraft of ACN 24 does not cause twice the damage of an aircraft of ACN 12. Therefore the ACN / PCN method provides no means of directly quantifying the effect on pavement life of a mix of aircraft, nor does it directly quantify the effect of overloading. This is recognised as a limitation of the system. A number of investigators have devised techniques to overcome this limitation and to enable airport managers

to assess the relative effects of various aircraft on a pavement in terms of the amount of pavement life that is consumed by each passage of each aircraft.

For example, the relative damage can be calculated using the method of layered elastic analysis. This has been done for a pavement with a PCN of 42 on a subgrade CBR of 6. Using the computer program APSDS, and the damage due to each aircraft relative to that caused by each pass of the Dash-8 is given in the last column of the table below, which also gives the ACN and PCR of each aircraft.

Aircraft Mass (Tonnes) ACN PCR Relative Damage

B737 65T (near maximum) 41 3.7 10,000B737 55T 33 3.9 3,000DC9 62T 42 3.4 9,000DC9 50T 32 3.8 2,000B727 95T (maximum) 62 2.9 120,000B727 68T 42 3.4 11,000BAel46 45T (maximum) 27 3.7 800BAel46 41T 24 3.9 500F-28 33T (maximum) 20 3.5 200F-50 20T (maximum) 11 4.1 5Dash 8 15.5T (maximum) 9.5 5.4 1B767 128T 42 2.0 27,000B747 255T 42 2.2 26,000B747 397T (maximum) 76 1.7 560,000

10.7 THE ACN REFERS ONLY TO RUTTING

An ACN is calculated using the Corp's CBR design method. This method assumes a failure mode that consists of surface rutting caused by overstressing the subgrade. Design life is considered to have expired when the surface ruts to an extent where it is unacceptably rough for aircraft traffic, or fails to shed water, thereby leading to loss of friction and possibly aquaplaning. The pavement is then typically resurfaced with asphalt and the design life begins again. Pavement failure due to fatigue cracking of the bituminous surfacing layer, or cracking of other bound layers, or deformation within the surface and basecourse layers is not addressed by the method, and must be separately considered by the designer.

10.8 LOAD RATING SYSTEM OR DESIGN METHOD

The ACN / PCN method should be recognised to be an aircraft load rating system. It is not a pavement design system. The ACN of an aircraft is not pavement specific. In other words it does not depend upon the detailed makeup of the pavement structure; whether it is made of high quality fine crushed rock, or natural gravel, whether it has a thick asphalt surface or a simple sprayed seal. It is a function only of the weight, tyre pressure and wheel arrangement of the aircraft, and the CBR of the subgrade. The simplicity of the ACN / PCN rating system is possible only because the CBR design method does not depend upon the detailed structure of the pavement. Where the critical failure mechanism is fatigue of the asphalt surfacing layer or cracking of other bound layers, the damage caused by an aircraft depends very much upon the detailed structure of the pavement in question and separate specific design attention should be given to the problem. Also, the airport owner should take account of the detailed structure of the pavement when deciding the PCN to adopt and publish. In other words, the details of a pavement's structure do not affect the ACN but should affect the PCN because the selection of a PCN is a design matter.

10.9 SIGNIFICANCE OF OVERLOAD

Except for extreme overload situations the risk to an aircraft is minimal, whereas the rate of deterioration of a

pavement will be increased above the design intent by any overloads, particularly where high frequencies are involved. The ACN / PCN system functions in practice as a pavement management tool for an airport owner, not to protect aircraft. As stated earlier, an owner might allow a particular aircraft to use the airport only if it operates at a restricted weight, and/or at reduced tyre pressure and/or at a limited frequency. Permanent concessions are often granted to allow aircraft with an ACN higher than the PCN to use a pavement for commercial reasons.

11. PAVEMENT CONCESSIONS RPA guidance extract.

A pavement concession is a privilege which may be granted to an aeroplane operator allowing the landing, take-off , taxiing or parking of an aeroplane on a runway, taxiway or apron which would not otherwise be available. It is an instrument of pavement management, enabling an aircraft to load an aircraft pavement beyond the rated load capacity under specific conditions generally related to aircraft tyre pressure, mass and frequency of operations, and the condition of pavement and subgrade.

Aerodrome runways, taxiways and aprons are designed to be able to withstand a specific number of loadings by the critical aircraft (of designated mass and tyre pressure), without needing major pavement maintenance. The need may arise to permit aircraft operations imposing more severe loadings an the pavement than the design loading, for instance, for commercial considerations such as the introduction of new aircraft types to the aerodrome or for special transport requirements such as disaster relief and medical evacuations.

The aerodrome operator is responsible for determining in each case whether a pavement concession is to be granted and any special conditions applicable. There is no mandatory requirement for an aerodrome operator to grant a pavement concession.In normal circumstances the issue of pavement concessions is based on economic considerations. The aerodrome operator should weigh the advantage to be derived from allowing some pavement overload (e.g. extra revenue, humanitarian gain or political advantage), against the penalties involved (e.g. pavement repairor the need to carry out maintenance earlier than would otherwise be the case) , and decide accordingly.The handling and issue of pavement concessions is to be in accordance with the procedures set out below.

12. Pavement types, rating and design philosophy

The majority of Australian aerodrome pavements are classified as flexible because they are designed to deflect under load conditions and then rebound when the load is removed. They consist of one or =re layers of mechanically compacted granular material with a bituminous spray seal or bituminous concrete surfacing. The pavement materials can vary considerably from natural gravel’s to manufactured fine crushed rock. Rigid Portland cement or concrete pavements, designed not to flex under the load imposed by large transport aircraft, are usually confined to aprons at major aerodromes where static loading imposes a much greater load penalty on the pavement.Three major factors determine the life of a properly designed and constructed pavement: the size of the load imposed, the number of times that the load is imposed, and the strength of the subgrade. These factors are inter-related and are influenced by external conditions (principally the weather) at individual aerodromes. A pavement designed for a particular magnitude of aircraft loading and number of times it issubjected to this load is normally equally suitable for more severe loading at a reduced number of movements or less severe loading at an increased number of movements. Further, during periods of low rainfall the subgrade is generally drier, and subgrade strengths correspondingly higher, than the design value. As a result, the pavements have greater strength than their design values, at those times.

To identify the strength of a pavement, the Aircraft Classification Number - Pavement Classification Number (ACN- PCN) method is to be adopted. The pavement strength rating lists the PCN, pavement type (rigid or flexible), subgrade strength category, minimum allowable tyre pressure and evaluation type (subjective or experienced). The published rating (see AIP/AGA-3) implies the pavement is adequate for unrestricted

operations of the design aircraft. operations by aircraft imposing more severe loadings on the pavement than are allowed by the pavement rating are not permitted without a pavement concession.

13. Pavement overload

Aircraft can impose loads on pavements outside the pavement rating limits in either of two ways : the aircraft tyre pressure can be above the maximum allowable, or the ACN can be above the PCN while the pressure is equal to or below the maximum allowable.

Pavement damage in the form of surface rutting may occur from excessive tyre pressure.

Excessive wheel loads may result in pavement failures such as a general loss of shape or large saucer-shaped depressions.

14. Concession decision criteria

An application for an aircraft to operate outside the limits allowed by a pavement rating usually involves assessment of the following factors.

To ensure the safety of the operation where the proposed overloading of a pavement is so great that damage to the aircraft is likely to the extent that the safety of the occupants is in doubt, a pavement concession is not to be approved regardless of the circumstances.

The probability of pavement damage should be assessed considering:

the operating mass and tyre pressure, and the type of the critical design aircraft;

pavement field test data;

current pavement conditions;

history of pavement performance under normal and concession use including any damage caused by previous operations;

whether the proposed operations are far a single operation, and of long term or short term duration;

whether there has been recent prolonged rainfall causing loss of subgrade strength.

Assessment of the social and economic importance of the operation involves consideration of:

the availability of alternative aircraft which would not overload the pavement concerned;

humanitarian or compassionate reasons, e.g. urgent medical evacuation, flood or disaster relief;

the political desirability, e.g. Head of State visits, Ministerial flights, etc.;

significant commercial importance to the community;

essential or desirable military operations.

The consequences of any pavement damage may include:

cost of repair after pavement damage and source of funding;

resources available to repair any damage;

disruption to routine operations caused by any damage or repairs.

15. Approvals

Humanitarian flights should rarely be refused unless there is doubt about the safety of the operation.

Other flights are often approved if the financial consequences of increased maintenance, pavement rehabilitation or strengthening together with the disruption to operations during repairs are acceptable. Flights may also be allowed even where pavement damage is expected provided the aircraft operator has agreed to pay the cost of repairs, and provided the disruption for repair (if needed) is acceptable.

Pavement concession format and procedures,

Where the aerodrome operator has decided to approve a pavement concession, this is to be done using the following.

In order to achieve industry-wide standardisation, concessions are to be issued under one of the following titles.

The 'ad hoc' pavement concession is to be used to cover a small number of operations, and issued on a flight-by-flight basis, or covering several flights on the one concession.

The ‘standing’ pavement concession is to. be used to cover a relatively large number of operations carried out over an extended period . They allow a specified number of Movements over a fixed period.

The 'schedule period' pavement concession is to be issued to airlines to provide a pavement concession for the whole of the duration of a scheduling period. These often nominate the number of movements, based on approved timetable schedules plus an allowance for special services.

The 'advisory' pavement concession is to be used, in response to requests for pavement concessions at some future but unspecified date. The request is actioned, but issue of the pavement concession is delayed pending advice of the actual date from the operator.

The handling of pavement concession requests is to follow existing industry procedures as set out below in order to minimise overall administrative costs to the industry.

Requests for pavement Concessions are to be directed initially to the CASA Regional Office for the State where the aircraft operator (or its Head Office) is based. The request is then to be forwarded to the operator of the aerodrome concerned for decision. (Where the operator is CASA/DOT&RS the request is to be forwarded to the Regional Office of the State where the aerodrome is located). Advice of the decision made is to be referred back to the CASA Regional Office, for onward transmission to the aircraft operator.

At major airports aircraft operators often contact the aerodrome operator direct.

Comment.

This segment of the Diploma in Airport Management is a educational unit intended to acquaint prospective aerodrome managers of the basic factors involved in the ACN-PCN method, (Aircraft Classification Number – Pavement Classification Number).

It is not intended to enable aerodrome operators to make technical pavement evaluations. That is the province of a experienced professional airport civil engineer, however it does provide guidelines which enable judgments to be made, as to whether professional civil engineering advice is required.

If the pavement is not “dry to depth” , or the pavement seal is old and/or brittle, or shows any sign of distress, it is highly advisable to seek professional civil engineering advise before issuing “pavement concessions” .

Conclusion.

It should be realised that the concept of “pavement concessions” is largely a question of economics or in some cases commercial reality for the aerodrome operator.

The “safety” of such operations remains, as usual, squarely in the domain of the aircraft owner/operator.

The information contained in this “text” must be considered as guidance material only.

As such TAFE MIDLAND .... the “college” accepts no liability should this text or other guidance material noted herein, be interpreted wrongly or prove to be dated or incorrect.

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