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LOW FUEL TEMPERATURES - BASICS, PRINCIPLES OF OPERATIONS AND A NEW SOFTWARE TOOL FOR OPERATIONAL PREDICTIONS

Lars KornstaedtAirbus Customer Services

Group Manager A380 PerformanceFlight Operations Support and Services

With the operation of the very long rangeA340-500 and -600 on polar routings, theexposure of Airbus aircraft to low fuel temper-ature issues has increased. This article summa-rizes some basics regarding the minimum fueltemperature that can be tolerated, reviews theprinciples of the FUEL LO TEMP procedures

on the Electronic Centralized Aircraft Monitor(ECAM) and in Flight Crew OperatingManual (FCOM) 3.02.28 and describes newsoftware now available to operators to predictfuel temperatures for a given flight at the timeof dispatch.

Low fueltemperaturesBasics, principles of operationsand a new software tool for operational predictions

Low fueltemperaturesBasics, principles of operationsand a new software tool for operational predictions

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LOW FUEL TEMPERATURES - BASICS, PRINCIPLES OF OPERATIONS AND A NEW SOFTWARE TOOL FOR OPERATIONAL PREDICTIONS

tanks. JetA1 can be considered ashaving unaltered freezing character-istics up to a content of 10% ofJetA, above that fraction resultsbecome unpredictable and the fuelfreezing point should be consideredto be that of JetA fuel.

ACTUAL FUEL FREEZING POINT

When known, the actual fuel freez-ing point of the fuel carriedonboard the aircraft may beretained as a criterion for cold fuelmanagement in flight, if a monitor-ing process, including mainte-nance, dispatch, and crew proce-dures, has been set up in a way thatis acceptable to local authorities.The process involves the retrievalof samples from the tanks throughthe drain holes after the refueling iscompleted. These are then putthrough jet fuel quality analysers,results can be radioed to the crew ifnot available before takeoff. Basedon a fuel freezing point measure-ment survey conducted by a com-pany involved in research in coldtemperature behaviour of petrole-um products at major US airports, atypical benefit of at least 3°C andup to 18°C can be expected fromthe implementation of such a mon-itoring process.

MINIMUM INLET TEMPERATURE

Engines have an oil cooling systemat their inlet, which uses the arrivingfuel as a heat sink, thus warming it.Various system architectures andhardware leads to a varying specifi-cation of the minimum temperaturethat a given engine type can copewith. The minimum temperature is expressed as a margin versus fuel freezing point - the minimum

engine inlet temperature is the actu-al fuel freezing point with the man-ufacturers margin added to it (seeright-hand table).

FUEL HEAT MANAGEMENT SYSTEMLIMITATION

During flight, the fuel temperaturequickly drops below the freezingpoint of water, of which a certainproportion is always contained inaviation fuels. This water can thenform ice crystals, which could trav-el to the engine inlet filters andclog them. To avoid this, the fuel iswarmed in the oil cooling system.For some engine types, a minimumfuel temperature below which take-off and/or flight is not permittedtherefore results from the enginecapability to warm up a water-satu-rated fuel flow, unless an anti-iceadditive is used.

OperationalproceduresGROUND PROCEDURES

Special attention should be paid tothe fuel contained in the outer tanksbefore a flight through forecast coldatmosphere is commenced. Thesetanks may remain completely orpartially filled with the cold reservefuel from the previous flight. This may have an unrelated, butundesirable, effect of leading towing surface icing after landing in ahumid environment. More impor-tantly, if partially filled, it may leadto high-proportion mixtures of dif-ferent fuel types. Both can be avoid-ed by performing a manual transferof the outer tank fuel inboard dur-ing the taxi-in. If the outer tanksremain full with fuel of a lower

LOW FUEL TEMPERATURES - BASICS, PRINCIPLES OF OPERATIONS AND A NEW SOFTWARE TOOL FOR OPERATIONAL PREDICTIONS

Minimum allowed fueltemperature

Operating the aircraft within thecertified environmental envelope isnot sufficient to prevent issueswith cold fuel. Indeed, safe engineoperation leads to two engine-spe-cific limitations: the minimuminlet temperature and the fuel heatmanagement system limitation.More information on fuel charac-teristics is given in the ‘Getting to Grips with Cold WeatherOperations’ brochure availablefrom Airbus.

FUEL FREEZING POINT

The fuel characteristic that bestdescribes the lower limit in tem-perature for use in aircraft opera-tions is not actually the fuel freez-ing point, but the pumpabilitylimit. This limit is very close to thepour point, a temperature at whichthe fuel, cooled without stirring,will only just still pour from astandard glass cylinder. However,since the pour point is difficult todetermine accurately, the fuelfreezing point continues to be usedas a reference for low-temperaturecharacteristics.

Fuel is a mixture of different hydro-carbons that do not all solidify at thesame temperature. When fuel iscooled, an increasing proportion ofwax crystals form in the fuel. Thewax crystals can block fuel linesand filters, thus causing engineinstability, power loss and eventual-ly flameout, and their formationshould therefore be avoided.

The fuel freezing point is common-ly measured with the ASTM auto-mated optical test method D5901-03. ASTM International is an orga-nization which develops standards,amongst which are the fuel freez-ing point tests. This test method isbased on the observation of waxcrystals completely disappearingfrom a warmed fuel sample thatwas previously frozen. The methodis used widely except for Russianand other Eastern-European fuels,for which the GOST (Russian statestandard) method gives the temper-ature at which solid particles firstappear during the cooling process.The ASTM method thereforeaffords higher margins.

The specification fuel freezingpoint depends directly on the levelof distillation aimed for in its pro-duction. The higher the distillationgrade, the lower the yield of fuelfrom the crude oil. Fuel of differentspecifications is found in differentparts of the world, each with a dif-ferent specification fuel freezingpoint. The main focus is on JetA1,primarily available everywhereexcept the United States ofAmerica, and on JetA, which isused there (see left-hand table).

FUEL MIXTURES

When flying between areas wheredifferent fuel types are available,mixtures of fuel with different spec-ification fuel freezing points willoccur in the tanks. Experimentalevidence has shown that fuel mix-tures do not behave as ideal fluidsand that the resulting fuel freezingpoint is commonly adversely affect-ed. Airbus recommends avoidingmixtures in the outer, most exposedJetA -10°C

JP5 -46°C

JetA1/JP8 -47°C

RT/TS-1 -50°C

JetB -50°C

TH -53°C

JP4 -58°C

Fuel freezing point

Minimum inlet temperature

0°C

0°C

3°C

4°C

A320F 4°C

A340 5°C

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LOW FUEL TEMPERATURES - BASICS, PRINCIPLES OF OPERATIONS AND A NEW SOFTWARE TOOL FOR OPERATIONAL PREDICTIONSLOW FUEL TEMPERATURES - BASICS, PRINCIPLES OF OPERATIONS AND A NEW SOFTWARE TOOL FOR OPERATIONAL PREDICTIONS

freezing temperature than fuel local-ly available, it may be advantageousto keep that fuel there, since experi-ence shows that a higher initial fueltemperature only has a delayingeffect on the cooling process, butusually does not affect the minimumtemperature observed in-flight. Thebenefit of the lower freezing pointprevails in such a case.

IN-FLIGHT PROCEDURES

On aircraft equipped with a fueltemperature measurement system,an ECAM warning is triggeredwhen the fuel temperature dropsbelow the minimum acceptable tem-perature in that tank. The minimumacceptable temperature is not thesame for all tanks. The temperaturein the feed tanks (typically, but notexclusively, the inner tanks) may notdrop below the minimum acceptabletemperature limited by the engineinlet and the heat management sys-tem limitation, while the fuel in theremaining tanks may be allowed toreach the freezing point. The ECAMwarnings are calibrated to the spe-cification values of JetA and JetA1 fuel and request manual fueltransfers. The crew may delay theapplication of the procedures appro-priately, if the actual fuel freezingpoint of the fuel carried is known.

The figure ‘tank transfer schemat-ic’ shows an example for the A340-500 and -600, the FCOM3.02.28 FUEL LO TEMP proce-dures request manual transfers:• From outers to inners, when

the fuel freezing point isreached in the outers

• Forward from the trim tank,when the fuel freezing point is reached in the trim tank

• From center to inner, when the minimum acceptabletemperature is reached in the inners

TAT INCREASE

In addition to manual transfers, theprocedure also recommends toincrease the TAT (Total AirTemperature), which is the temper-ature measured on the structure ofthe aircraft. The TAT is derivedfrom the outside or Static AirTemperature (SAT) and depends onthe Mach (Ma) number:

TAT = SAT x (1 + 0.2 x Ma2)

There are two means of increasingthe TAT: increasing speed toincrease aerodynamic warming,and reducing altitude to fly inwarmer air. The TAT formula high-lights that speed increase only has amarginal effect, while the exchangerate SAT to TAT is one to one.

To give an impression of the effectin time, both means are shown inseparate variations from a basicscenario, which considers anA340-600 taking off at MaximumTake Off Weight (MTOW) flyingfrom the US East Coast to EastAsia on a great circle routing atLong Range Cruise (LRC) speedand on an optimum altitude pro-file. The aircraft is using JetA fuelwith a specification fuel freezingpoint of -40°C. A statistical outsidetemperature profile is consideredfor this example, based on a 95%reliability for January, meaningthat at any point during an actualflight in that month, there is only a5% chance of encountering lowertemperatures.

TAT increase - ‘fly faster’

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Distance (nm)

Distance (nm)

The first variation consists in fly-ing at maximum managed speed(Ma 0.845 for this type) on a fuel-optimized profile. The figure “flyfaster” shows the relevant parame-ters plotted against the ground dis-tance, the basic scenario alwaysshown in shades of blue and thevariation case in shades ofred/orange. The speed increaseleads to a shift in the optimum pro-file, which is mirrored by the SATprofile, but even where the aircraftflies at the same altitude, a smallincrease in TAT can be observed.This increase delays the momentwhen the temperature drops belowfreezing point in the outer tanks bysome 300 nautical miles (nm), butdoes not avoid the need for themanual transfer procedure fromthe outers to the inners. It does,however, succeed in maintainingthe inner tanks temperature aboveor at the freezing temperature,which is sufficient for the RollsRoyce Trent 500 engines thatequip this aircraft type. Thiscomes at a cost: fuel burn at iden-tical takeoff weight is increased by7.9 tonne (t).

The second variation considersflying the whole distance at FlightLevel (FL) 290. The figure ‘flylower’ shows the large increase inSAT, which is mirrored by the TAT.The lower flight level succeeds indelaying the manual transfer by800 nm and subsequently main-taining a comfortable margin of thefuel freezing point in the innertanks. The cost of this variation is afuel burn increase by 5.5t.

Comparison of fuel temperatures in outerand inner 1/4 tanks for two speeds

Tank transfer schematic

Comparison of fuel temperatures in outer and inner 1/4 tanks for two altitude profiles

TAT increase - ‘fly lower’

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operability on a variety of plat-forms. At this time, stand-alonesolutions are being developed forPC and UNIX under RS6000, inaddition to the basic implementa-tion, which is a complete solutionincluded in the PerformanceEngineer’s Programs (PEP) soft-ware suite.

An important aspect lies in thelarge variety of flight plans to beimported. This has been tackled inthe PEP environment through thedefinition of a standard format,which uses XML (eXtensibleMarkup Language) and is a subsetof the standard defined for theA380 Onboard InformationSystem (OIS). The translation ofthe airline pilot log to XMLrequires the development of a cus-tomized tool. All the relevant infor-mation for this task is provided inthe Performance Programs Manual(PPM), but assistance and even thedevelopment of such a tool is avail-able as an Airbus service.

SECTOR ANALYSIS

The Sector Analysis is designed tohelp assess the exposure of a city-pair to cold fuel issues on a statis-tical basis. For this task, somesimplifications are made. Therouting is assumed to be a GreatCircle one; the performance on

the route is established with theAirbus Flight Planning (FLIP)software, which is also part of thePEP.

For the statistical weather data, aconnection has been built to theNOAA GUACA (National Oceanicand Atmospheric Administration - Global Upper Air Climatic Atlas),which was compiled from dataobtained in the 80’s and 90’s fromthe ECMRWF (European Centrefor Medium Range WeatherForecasts). This database needs tobe installed specifically to allowextraction of statistical data formonthly or seasonal exposureanalysis.

Information and the database areavailable from the NOAA website:http://navy.ncdc.noaa.gov/prod-ucts/compactdisk/guaca.html.

The FLIP computation results in aspecific output, which can be direct-ly imported into the FTP PEP userinterface, through which the sametype of analysis can be obtained asfor a Flight Plan Analysis.

APPLICABILITY

The FTP software is available forall A330/A340 Family aircraft andwill be released for entry into ser-vice of the A380 and the A350.

LOW FUEL TEMPERATURES - BASICS, PRINCIPLES OF OPERATIONS AND A NEW SOFTWARE TOOL FOR OPERATIONAL PREDICTIONSLOW FUEL TEMPERATURES - BASICS, PRINCIPLES OF OPERATIONS AND A NEW SOFTWARE TOOL FOR OPERATIONAL PREDICTIONS

Fuel temperatureprediction softwareAs described in the previous sec-tion, while it is possible to managecold fuel issues once in flight, anun-forecasted occurrence can leadto over-consumption and evendiversion. In order to give airlinesthe ability to analyse a flight for itsexposure to fuel freezing before itis flown, Airbus has designed theFuel Temperature Prediction (FTP)software.

PRINCIPLES

The FTP software relies on exter-nal data sources to obtain:• Navigation data (waypoints,

distance, time)• Weather data (temperatures)• Performance data (altitude,

speed, fuel on board)

From this data, it determines thenominal fuel distribution andmovement during the flight andsimulates the effect of the variousheat exchange processes, like con-vection, radiation, external heatsources and mass transfer. Theresult is a detailed output of thefuel temperatures in each tank ateach waypoint, a summary thatindicates whether the flight can bedispatched under the given condi-tions, whether any manual trans-fers were triggered during the sim-ulation, and what the lowest tem-perature forecast in each tank is.This information allows the dis-patcher to decide in an informedmanner, which areas may have tobe avoided on a critical flight.

OPERATING MODES

The FTP can be used in two waysdepending on the original source of the navigation, weather and per-formance data: the Flight PlanAnalysis and the Sector Analysis.

FLIGHT PLAN ANALYSIS

The Flight Plan Analysis uses anairline operational flight plan (orpilot log) as a basis for the compu-tation. Operational flight plans aregenerated with ComputerizedFlight Planning (CFP) systemsavailable from a number ofproviders. They run on differenthardware, like PCs, UNIX or main-frame computers, or even remotelyover Internet or similar connections,and they produce a document,which is highly customized by theoperators to their specific needs.

One of the design constraints forthe FTP software was therefore its

Fuel level tank

For some years monitoring of fueltemperatures for flights in coldatmospheres, such as trans-polar flights,has been normal and operationalprocedures to deal with this have beenput in place. With the very long rangeA340-500 and A340-600, Airbusconsidered the increased exposure ofthese aircraft to low temperatures shouldbe addressed at an earlier stage in theprocess than through the existing in-flightmanagement procedures detailed in theFCOM and triggered by ECAM warnings.

This led to a review and from this came Airbus’ recommendation thatoperators implement a fuel qualitymonitoring programme for exposed flightsfrom airports where only high freezingtemperature fuel is available and thecreation of the Fuel TemperaturePrediction (FTP) programme. The FTPenables the prediction and avoidance ofcold fuel issues at the time of aircraftdispatch and is recommended foroperators to support their operations onroutes where such issues can arise.

CONTACT DETAILS

Lars KornstaedtAirbus Customer ServicesGroup Ma nager A380 Performance Flight Operations Supportand ServicesTel: +33 (0)5 61 93 36 75Fax: +33 (0)5 61 93 29 68fltops.perfo@airbus.comlars.kornstaedt@airbus.com Conclusion

Schematic of heatexchange mechanisms