Encyclopedia of Aerospace Engineering. Part 3

1002 Pages·2016·205.61 MB·English

by  Blockley R.

Aircraft Fuel Systems
Roy Langton1, Chuck Clark1, Martin Hewitt2, and Lonnie Richards3
1ParkerAerospace,Irvine,CA,USA
2ParkerAerospace,Smithtown,NY,USA
3FuelSystems,AirbusUK,Filton,UK
the Figure 1 (discussed subsequently in Sections 2 and 3
1 Introduction 1 below), it is important to understand the wide variations in
environmentalconditionsandfuelpropertiesthatareimposed
2 PhysicalAttributesandFuelHandling 3
by the aircraft flight envelope and the resulting variations
3 FuelMeasurement,Management,andControl 13
in local pressure and temperature that must be tolerated by
Abbreviations 20
the equipment installed within the fuel tanks This is illus-
References 20 tratedinqualitativetermsinFigure2whichshowsthestatic
atmospherictemperatureextremesthataircraftareroutinely
exposedtoandthecorrespondingfuelpropertiesthathavea
significanteffectonthefunctionalityofaircraftfuelsystems
1 INTRODUCTION
andmustthereforebetakenintoaccountintheirdesignand
certification.
Theprimaryfunctionoftheaircraftfuelsystemistoprovide Fourofthemostimportantparametersoftoday’sjetfuels
acontinuoussourceoffuelattheappropriatepressuresand arefueldensity(asitvarieswithtemperature),vaporpressure,
temperaturestothepropulsionsystem(engines)throughout viscosity,andfreezepoint.
theaircraftflightenvelopeundernormaloperationandinthe
presenceoffailures. PART 1 PART 2
Sincethemodernaircraftfuelsystemcanbecomplexand
Physical attributes Fuel measurement
highly integrated, comprising six basic subsystem groups,
and fuel handling management and control
as indicated in Figure 1, this chapter has been divided into
twomainsectionstoprovideacleardescriptionoftheissues
Fuel storage Flight deck
involved:
AsindicatedinFigure1thereis,bynecessity,considerable
interactionbetweenthesesubsystemgroupsandthiswillbe
addressedastheissuesariseintheensuingdialog. Fuel handling Avionics
Before proceeding with a description of the functional-
ityandequipmentassociatedwithParts1and2outlinedin
Fuel tank inerting Sensors
Figure1. Systemtechnologygroups.
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2 AircraftSub-Systems
Fuel properties
40ºC
Hot day
Density
Standard day
Viscosity limit
0ºC
−40ºC
Cold day
Flash point −80ºC
Vapor
pressure
−120ºC
Fuel temperature Sea level 20000 ft 40000 ft 60000 ft
Figure2. Fuelcharacteristicsversusoperatingconditions.
Densityvariationmeansthatfuelathightemperaturewill Specificationsforjetfuelswerefirstintroducedintheearly
takeupmorevolumeandthereforeweighlessthanthesame 1940sinordertoestablishandcontrollimitstokeyproper-
volume at low temperature. Thus, aircraft loaded with cold tiesincludingcomposition,volatility,specificenergy,thermal
fuel will weigh more and have a greater range than the air- stability,lubricity,andcorrosioncharacteristics.
craft loaded with warmer fuel because the chemical energy The United States introduced the JP series of fuels in
stored in the fuel is a function of its mass rather than its support of military aircraft programs under the control and
volume. oversightoftheAmericanSocietyforTestingandMaterials
Vaporpressureisakeyfactorindeterminingthelimiting (ASTM).
operationalflightenvelopeofanaircraftsincehighevapora- In 1951 JP-4, a gasoline/kerosene blend was introduced
tionratesandultimatelyboilingcanoccurathighaltitudes. andbecamethemainstayofUSAirForceformanyyears.In
Viscosityisameasureof“pour-ability”whichbecomesan 1952, JP-5 was introduced which had a lower flash point
issueatlowtemperaturesandcanresultincriticalflowlosses and reduced volatility which were considered particularly
inpipingandpotentiallyresultinanengineshut-down.This importantparametersforuseonUSNavyships.
characteristic which is typically specified as a design limit- Commercial(civil)aircraftfuelsbecameavailableinthe
ingparameteriscloselyrelatedtothefreezepointdiscussed 1950swiththeintroductionofJetAandJetA1.
below. JetAisakerosenegradeoffuelsuitableformostturbine
Towards the end of a long flight when the fuel quantity enginedaircraft.Ithasaflashpointabove38◦C(100◦F)and
remaining is low and ambient temperatures approach cold amaximumfreezepointof−40◦C.Itiswidelyavailablein
daylimits,thefuelbulktemperaturecanapproachthefreeze the United States and is supplied against the RSTM D1655
pointofcommonlyusedjetfuelscausingwaxtoprecipitate (JetA)specification.
out of solution. This wax can create obstructions and block JetA-1issimilartoJetA.Itisproducedtoastringentinter-
filtersandasaresultmayleadtoanengineshut-down.Forthis nationallyagreedstandardandiswidelyavailableinEurope
reason,thefuelbulktemperatureiscontinuouslymonitored andNorthAmerica.IthasthesameflashpointasJetAbut
andsafeoperatingmarginsmaintained.Ifsafeoperatingfuel withalowermaximumfreezepointof−47◦C.JetA-1meets
temperaturelimitsareapproachedthecrewisrequiredtotake the requirements of British specification DEF STAN 91-91
action (descend and/or increase Mach number) to alleviate (JetA-1).
thesituation.Thiscanbeaseriousproblemwhenoperating Another commercially available jet fuel is Jet B which
inpolarregionsifoperatingMachnumbermarginsaresmall is a distillate covering the naphtha and kerosene fractions.
sincedescendingmaynotnecessarilylocateanywarmerair. It can be used as an alternative to Jet A-1 but because it is
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AircraftFuelSystems 3
moredifficulttohandle(higherflammability),thereisonly initselfa“function”inthetraditionalsense,itnevertheless
significantdemandinverycoldclimateswhereitsbettercold imposesanumberoffunctionalconstraintsandrequirements
weather performance is important. In Canada it is supplied onthedesignandcertificationofboththefuelsystemandthe
againsttheCanadianSpecificationCAN/CGSB3.23. aircraftasawhole.
More recently, the US Military have introduced JP-8 jet Themostcommonlocationusedforfuelstorageiswithin
fuelwhichisaderivativeofJetAwiththeadditionofacor- thewingstructuretypicallybetweentheforwardandaftwing
rosion inhibitor and an anti-icing additive. This fuel meets spars.Thisspaceisnotreadilyusableforotheraircraftfunc-
therequirementsoftheUSMilitarySpecificationMIL-DTL- tionsduetogeometryandaccessconsiderations.
83133E. JP-8 also meets the requirements of the British From an aircraft and fuel system perspective, there are
SpecificationDEFSTAN91-87AVTUR/FSII. advantagesanddisadvantagesassociatedwiththeuseofwing
Aviation fuel additives are compounds added to the fuel structureforfuelstorage.Ontheplusside,theconsiderable
inverysmallquantities,usuallymeasurableonlyinpartsper fuel weight which acts in opposition to the wing lift force
million,toprovidespecialorimprovedqualities.Thequantity resultsinalowerthewingbendingmomentduringflightthan
tobeaddedandapprovalforitsuseinvariousgradesoffuel wouldbethecaseifthesamemassoffuelwerestoredinthe
arestrictlycontrolledbytheappropriatespecifications. fuselage.Inaluminumwingstructures,thiscanprovideasig-
Additivesincommonuseinclude: nificantpositivecontributiontothefatiguelifeoftheaircraft
structurebyreducingthemagnitudeofthestresscycleasso-
(cid:1)
Anti-oxidantstopreventtheformationofgumdeposits ciatedwitheachtake-offandlanding.Insomeapplications,
(cid:1)
Staticchargedissipaters fueliskeptinoutboardtanksuntiltheendofthecruisephase
(cid:1)
Corrosioninhibitors beforetransferringthefuelcontentstotheenginefeedtanks
(cid:1)
Anti-icingagents inordertomaximizethefatiguelifeofthewingstructure.
(cid:1)
Biocideadditivestopreventmicrobialgrowthformation On the negative side of the equation, since the wing
internal cavity is typically very thin (especially towards the
wing tip) the challenge to the fuel quantity gauging system
Aninterestingnewdevelopmentinfueladditivesdeveloped
designerisconsiderable.Thispointiscoveredinmoredetail
bytheUSMilitaryisathermalstabilityimproverusedwith
inSection3ofthischapter.Figure3showsthefuelstorage
thenewJP-8fueldesignated“JP-8plus100”whichprevents
arrangementsfortwocontrastingcommercialaircraft.Onthe
carbon deposits formation when operating at high tempera-
left is the Embraer 170; a small regional aircraft which has
tures.
onlytwofueltanksandatotalcapacityofabout9500kg.On
The following text describes the system functions and
theright(nottothesamescale)istheAirbus340–600long-
equipment associated with each of the subsystem groups
range transport. This aircraft has eight different fuel tanks
outlinedabove.
andatotalcapacityofabout160000kg.
Because of the large quantities of fuel carried by most
aircraft today, the location and geometry of the fuel stor-
2 PHYSICAL ATTRIBUTES AND FUEL
age tanks play a critical role in the aircraft design and its
HANDLING
operationalcapabilities.Onecriticalfailuremodeassociated
withfueltanklocationisthe“Uncontainedrotorburst”asso-
This section of the chapter addresses the fluid network and ciatedwiththeengines,and,toalesserextenttheAuxiliary
tanksandtherolethattheyplayinthefuelhandlingaspects PowerUnit.Thesedevicescontainhighspeedrotatingspools
of the aircraft fuel system. The fluid network facilitates containing enormous amounts of kinetic energy, which if
the movement of fuel during the refuel/defuel, fuel trans- released due to some mechanical or control system failure,
fer,fueljettison,andenginefeed(andAPUfeed)subsystem have the potential to cause major structural damage to the
functions. aircraftandintheprocesscouldresultinpenetrationofthe
Each of these subsystems is described in the following fueltankswiththeresultantlossoffueloverboard.
subsections. Inconductinganuncontainedrotorburstevaluation,itis
requiredtoassumethatanuncontainedrotorburstwillresult
in the emission of debris of infinite energy normal to the
2.1 Fuelstorage enginerotationalaxis.Thisdebrisenvelopeexpandsfromits
originalongathreedegreeconicalplane.Theoutcomeofthe
Storing large quantities of fuel aboard an aircraft is a chal- rotorburstanalysiscanhavefarreachingimplicationsontank
lenge to the aircraft designer and while fuel storage is not structural boundaries as well as on system architecture and
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4 AircraftSub-Systems
Figure3. Fuelstorageexamples.
fuel handling equipment installation. This also involves the Figure 5 shows a photograph of an F-16 Fighting Fal-
routingofelectricalsuppliestokeyfuelsystemequipmentin con with drop tanks installed and Figure 6 shows an F-15
ordertoallowoperationaftertheeventandhencecontinued Eagle with conformal tanks. The latter photograph clearly
feedandtransferasneededtotheremaininggoodengine(s). illustratestheaerodynamicsuperiorityoftheconformaltank
The above commentary applies primarily to commercial arrangement.
aircraft.Militaryaircraft,incontrast,typicallyexhibitmore
complex fuel storage arrangements as is clearly illustrated
2.1.1 Tankventing
byFigure4whichisaperspectiveviewofthefueltanksof
theExperimentalAircraftProgramprototypewhichwasthe Openventsystems
forerunneroftheEurofighternowdesignatedastheTyphoon. All commercial aircraft have open vent systems. The pro-
To augment the operational range of military aircraft, vision of adequate fuel tank venting throughout the aircraft
external tanks are used. These tanks are typically mounted operational flight envelope allows the tanks to “breathe” as
underthewingandconnectedtotheaircraft’sfuelsystemvia theaircraftclimbsanddescends.Withoutthisprovision,large
quickdisconnects.Thisallowsthetankstobedroppedduring pressure differences would develop between the ullage and
flightafterthefuelhasbeenused.Fuelisusuallytransferred outsideairresultinginverylargeforcesonthetankstructure.
intotheaircraftintegraltanksusingcompressedair. It is impractical to accommodate these forces via the wing
Amorerecentdevelopmentistheconformaltankwhich structural design because of the resultant weight penalty;
isdesignedtoconformaerodynamicallytotheshapeofthe therefore,thedesignoftheventsystemplaysacriticalrolein
aircraft. These tanks are not discarded during flight but can protectingthetankstructurefromstructuralfailureastheair-
beremovedorinstalleddependinguponthemissionrequire- crafttransitsbetweengroundandcruisealtitudes.Theemer-
ments.Conformaltanksaredesignedtohaveminimalimpact gency descent case following a loss of cabin pressurization
ontheaircraft’saerodynamicorstealthperformance. ataltitudeisusuallythedesigncaseforventsystemsizing.
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AircraftFuelSystems 5
Figure4. Fueltankarrangementforamilitaryfighteraircraft.CourtesyofBAESystems.
Duringtherefuelprocess,theup-liftedfueldisplacesair the dynamic pressure from the airstream providing a small
in the fuel tanks. For safety reasons, spillage of fuel to the increaseinullagepressureabovetheprevailingambient.
outsidemustbeavoided.Toaccomplishthisconsistentlyand Formostofthecommercialaircrafttheventboxislocated
reliably, a vent box (sometimes referred to as a surge tank) outboardofthewingfueltankstowardsthewingtip.During
isprovidedtocaptureanyfuelthatmayentertheventlines. flight any fuel that finds its way into the vent box is either
ThisboxisconnectedtotheoutsideairstreamviaaNational drainedorscavengedbackintotheinterfacingfueltank.
Advisory Committee for Aeronautics (NACA) scoop; also
knownasasubmergedairinlet.Thisscooprecoverssomeof
Figure 5. An F-16 with drop tanks. Courtesy of US Air Force Figure 6. An F-15E with conformal tanks. Courtesy of US Air
photo/MasterSgt.AndyDunaway. Forcephoto/SeniorAirmanMirandaMoorer.
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6 AircraftSub-Systems
Climb vent 2.1.2 Contamination
Withintheaircraftfuelstorageenvironment,fuelcontamina-
Left wing tank Right wing tank
tionisamajorissuesinceitmustbeconsideredasacommon
Left vent box Right vent box modefailureprospect.Fuelcontaminationcanresultinloss
of all propulsive power since the problem source affects all
engines.Perhapsthemostcommonsourceoffuelcontami-
nationiswater.Eventhoughwateringround-basedhydrant
sources is aggressively controlled via coalescing filtration
Dive vent
systems, dissolved water can be present and undetectable
Center tank
at concentration levels of up to 80ppm at typical ground
Figure7. Simplethreetankventsystemschematic.
ambient conditions. Also there remains a source of water
contaminationthatoccursduringnormaloperationthatmust
berecognizedandmanaged.Intheoperationalenvironment,
The positioning of the vent openings within each fuel
boththefuelandwingtankstructurecanbecomeextremely
tankmustaccountforthevaryingattitudesandacceleration
cold during the high altitude cruise phase. During descent,
forces that can occur both on the ground and in flight. For-
large quantities of outside air come into the ullage as the
ward acceleration and pitch-up attitudes will force fuel aft
pressuredifferencebetweentheoutsideairandtheullageis
and with swept wings fuel will be forced outboard. A vent
equalized.Duringdescentintotropicalclimates,thisaircan
must therefore be located at the wing root near the forward
beparticularlyhumidandasaresult,watercondensesonto
tankboundary.
thecoldstructure.
Similarly forward deceleration and pitch down attitudes
Another common source of fuel tank contamination
will force fuel forward and for swept wings inboard. To
involvesmicrobialgrowth.Thisoccursasaresultofspores
accommodate this situation a vent must be positioned out-
intheairfromtheventsystemthatcangrowwhenthefuel
board and aft in the fuel tank. Figure 7 shows a simple
tankenvironmentalconditionsarefavorable.Thissituationis
schematicofaventsystemforathreetankaircraft.
exacerbatedbythefactthatfueltanksareexpectedtooperate
Intheexampleshown,eachtankisventedsuchthatthere
withoutinspectionforlongperiods.
isabreathingpathtotheoutsideairforalllong-termflight
Water management is perhaps the most challenging
attitudesthatoccurduringnormalflightoperations.Inorder
contaminationissueinthecommercialaircraftindustrytoday.
to prevent or minimize the opportunity for the vent lines to
Withthehighutilizationratesoftoday’scommercialaircraft,
becomefilledwithfuel,floatactuatedventvalvesareoften
thereistypicallynotenoughtimebetweenmissionstoallow
installedthatclosetheventlineswhenfuelispresent.
any free water to settle into the tank sumps from where it
can be drained via the water drain valves during routine
Closedventsystems
maintenance.
Theflightenvelopesofmostmilitaryaircraftinvolveopera-
Duringoperation,freewatercanfreezeandaffectthenor-
tionatextremelyhighaltitudes(e.g.,above60000ft.(1ft.=
malfunctionofthefuelsystemcomponents.Thecertification
0.3048m)) where local ambient pressures are below 1psi
of a new fuel system requires test evidence that the system
(1psi = 6894.75Pa) absolute. Even the best recovery pres-
can operate safely under icing conditions, (SAE ARP 1401
sure in the fuel tank ullage space for an open vent tank of
AircraftFuelSystemandComponentIcingTest).
anaircraftcruisingatMach0.8at60000ft.willbelessthan
1.5psiabsolute.
This situation is aggravated by the fact that ambient
2.2 Fuelhandlingfunctionsandequipment
temperatures (and recovery temperatures during subsonic
operation) can be extremely low at these altitudes causing
2.2.1 Refuellinganddefuelling
fuelwaxingconcernsfortraditionalcommercialfuels.
As a result, military aircraft fuels often have vapor pres- Aircraft refuelling is performed prior to almost every flight
suresthatcanbeequaltoorhigherthantheullagepressures whereas defuelling is primarily performed during mainte-
attheseoperatingconditionsmakingitimpossibletopump nanceactionswhich,ideally,areonlyrequiredonanextended
the fuel to the engines. A closed vent system arrangement schedule basis. Gravity, or over-the-wing refuelling is lim-
is therefore employed to provide a means of increasing the ited to very small general aviation aircraft where the time
ullagepressureabovethelocalambientpressurewhenoper- to refuel the aircraft is not critical. All present day jet air-
atingatveryhighaltitudes. craft, both commercial and military, use pressure refuelling
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AircraftFuelSystems 7
withthepossibleexceptionofverysmallunmannedmilitary Defuellingtheaircraftisnormallyrequiredonlyformain-
aircraft. tenanceoftheaircraftalthoughdefuelingofanin-serviceair-
Typicallytherefuellingsystemmustprovide: craftmayberequiredtoreducetheamountofon-boardfuel.
Defuelling is normally performed by suction applied at
(cid:1)
Fast refuel times, usually between 15 and 40 minutes theaircraft’sgroundrefuellingadapterorbyusingon-board
dependinguponthesizeandmissionoftheaircraft. transferandenginefeedpumpstopumpthefuelofftheair-
(cid:1)
Accurate loading of the required fuel quantity, often via craft. Suction defueling provides the fastest off load rates
anautomatedsystemonboardtheaircraftthatallowsthe whereastheuseofonaircraftpumpswillgetmoreofthefuel
refueloperatortopresetthetotalfuelloadrequiredatthe offtheaircraftbutatamuchslowerrate.
refuel station. This facility allows the airline to select a Defuelingisalsoanecessaryfunctionfollowinganacci-
fuelloadthatmatchestheupcomingflightrequirements dentwheretheaircraftisdamagedandfuelmustberemoved
thusavoidingtheoperationalpenaltyassociatedwithcar- beforeitcanbesafelymovedforrepair.Forthisreason,con-
ryingtheextraweightofunneededfuel. sideration must be taken during the design phase regarding
(cid:1)
Accuratelocationofthefuelonboardtoensurecompli- thelocationoftherefuel/defuelpointstoallowaccessinthe
ancewithaircraftCGlimits unlikelyeventofawheels-uplanding.
(cid:1)
Protection against overboard spillage of fuel out of the
tankventsystem.
2.2.2 EngineandAPUfeed
The pressure source for the refuelling process is either via Duringflightthefeedsystemmustensurethatfuelpressure
a mobile fuel truck or, at most large airports, via an under- at the engine interface (and to the APU when operating) is
groundhydrantsystem. maintainedabovethefuelvaporpressure(i.e.,abovethefuel
Mostaircraftonlyrequireasinglepressurerefuellingpoint boilingpoint)byapredefinedmarginthroughouttheopera-
since the flow rate required to meet the refuelling time is tional envelope of the aircraft for all possible combinations
easily achievable. Larger commercial aircraft may require of fuel types and fuel temperatures. The system must also
multiple refuelling points to meet the contracted refuelling ensurethatcontaminationofthefuelwithairorwaterdoes
time.Whenmultiplerefuellingpointsarerequired,theyare notexceedthelimitssetbytheenginemanufacturer.
typicallylocatedonbothsidesoftheaircraft.Anaircraftwith Airworthinessregulationsrequirethateachenginehasits
multiplerefuellingpointscan,ofcourseberefuelledfroma owndedicatedfeedtank.Thelocationofthefeedtankrelative
singlepointprovidedthattheresultingadditionalrefueltime to the engines can have significant impact on the design of
isacceptable. thefeedsystem.Forexample,anaircraftwithrear-mounted
Therefuelstation,usuallyinboardontheleadingedgeof engines having feed tanks located at the inboard section of
thewing,hasarefueladapterand,alongside,arefuelpanel thewingwillresultinlongfuelfeedlinesandthereforethe
indicatorforusebythegroundcrew.Figure8belowshows potentialformoreflow-relatedpressurelosses.Largevaria-
the refuel station of an Embraer 190 aircraft in the refuel tions in fuel pressure at the engine interface can also occur
process. duringaircraftpitchattitudeexcursions.
Thetraditionallocationoftheenginesbelowthewingis
thereforepreferredbythefuelsystemdesignersincefeedline
lengthscanbekeptshortandtheeffectsofaircraftpitchvari-
ationswillbeminimal.Thelocationofthefeedtankabove
theengineswillalsoprovideasmallbutbeneficialpressure
head.
Engine (and APU) feed pressure is provided by boost
pumps (also referred to as feed pumps) which fall into the
followingtwocategories:
(cid:1)
Motor-drivenpumps
(cid:1)
Ejectorpumps(alsoreferredtoas“Jetpumps”)
Motor driven pumps comprise two main elements, an elec-
tricmotorandapumpingelementasshownintheschematic
Figure8. Embraer190refuelstation. diagramofFigure9.Thecentrifugalpumpingelementshown
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8 AircraftSub-Systems
Figure9. Motordrivenfuelpumpconcept.
inthefigureisusedalmostexclusivelyinaircraftfuelboost of the device is extremely good. The ejector pump is also
pumpapplicationssinceitisideallysuitedtothetaskwhich inherentlyverygoodatpumpingdowntoverylowfuelinlet
requireshighvolumeflow,lowpressureriseandhighrelia- levels.
bility. In ejector feed pump applications, the motive flow high
For ease of maintenance, fuel pumps are usually of pressure source must come from the engine and therefore
the “Cartridge in canister” arrangement as indicated in the a separate motor-driven pump is required to provide boost
schematic of Figure 9. Thus, the cartridge containing the pressureforenginestarting.Oncetheengineisrunning,and
motor and pumping element can be removed without hav- motiveflowisestablished,this“Boot-strap”typeoffeedcir-
ingtogainaccessto,ordrain,thefueltank(seeKarassiket cuitwillbesustainable.Themotor-drivenstart-uppumpcan
al.2008formoredetailsonthissubject). thenbeshutdown.
Ejector pumps (see Figure 10) are often used to pro- Motive flow can be provided from two alternative
vide the primary fuel boost function in smaller transport sources:
aircraft applications such as business jets and regional air-
(cid:1)
craft. The attraction of this pump type is the fact that the Adedicatedpumpmountedontheenginegearbox
(cid:1)
ejector pump has no moving parts and hence the reliability Amotiveflowoutletontheenginefuelmeteringunit
Figure10. Theejectorpumpconcept.
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AircraftFuelSystems 9
Inordertominimizetheunusablefuel,scavengepumpsare relativelylongtime,itisneverthelessachallengetothefuel
often employed to suck up fuel from the remote corners of systemdesignerandforthisreasonmustbeconsideredearly
fuel tanks and to discharge this fuel at the inlet to the main inthesystemdesigntradestudiesthatexaminefueltransfer
feed pump(s). Ejector pumps are used for this purpose and requirementsandsolutions.
themotiveflowforthesescavengedevicesmaybetakenfrom Thedesignissuehereistominimizetheweight,cost,and
the motor-driven boost pump outlet if other motive sources maintainability issues associated with a system where the
are not available. In some cases the boost pumps may be probabilityofbeingusedinserviceisverysmall.Therefore
locatedina“Collectorcell”withinthefeedtank.Thescav- additionalequipmenttosupportthejettisonfunctionwillbe,
engeejector(s)wouldthendischargeintothiscell.Withthis forthemostpart,aweightandoperationalpenaltythatmust
arrangement, the collector cell is maintained full and, per- bebornebythefleetthroughoutitsoperationallife.
hapsslightlypressurizedsothatduringnegativegtransients,
themainboostpumpinletremainssubmerged.
2.2.4 Integratedfuelhandlingsystemsolution
In multi-engined aircraft, an engine crossfeed system is
required to allow fuel from the feed tank of a failed engine Inordertobetterexplainthefunctionsandequipmentassoci-
to be consumed by the other engine or engines. In some atedwiththefuelhandlingaspectsofaircraftfuelsystems,a
of today’s long-range transport aircraft where ETOPS cer- hypotheticaltwin-engineaircraftfuelhandlingsystemispre-
tification is required, a dual crossfeed valve arrangement is sentedhereinschematicformasFigure11.Thisschematic
employed so that the crossfeed function remains available depicts a three-tank fuel storage arrangement with only the
followingafailureofanyoneofthetwocross-feedvalves. centertankandtheleftwingshownforclarity.
Alloftheequipmentrequiredtosupportthefuelhandling
functionsoutlinedaboveareincludedintheschematic.The
2.2.3 Fueltransferandjettison
feedsystemisshowninsolidblackwhiletherefuelandjet-
Afueltransfersystemisneededinapplicationswheremul- tisonsubsystemswhichsharethesameplumbingareshown
tipletanksareusedforfuelstoragetoensurethatfueliscon- ingray.Thetransfersystempipingisshowninshadedgray.
sumedfromthevarioustanksinaccordancewithapredeter- Theaircraftisrefuelledbyconnectingtherefuelsourceto
minedschedule.Thisschedule(orfuelburnsequence)takes the adaptor at the refuel station. Two refuel shut-off valves
intoaccountmanyoperationalconsiderationsincluding: connectedtothemainrefuelgalleryallowupliftedfuelinto
(cid:1) eachfueltank.Thefuelenterseachwingtankintheoutboard
(cid:1) AircraftCGvariationwithfuelburn compartment and flows into the inner compartment and the
(cid:1) Wingloadalleviationrequirements collector cell via check valves installed at the compartment
Feedtankmaximumandminimumfuelquantitiesduring boundaries. These check valves allow fuel to move easily
transfer inboardwhilerestrictingflowintheoutboarddirection.
The feed system has two boost pumps mounted on the
Control of the fuel transfer system can be either under the lowerskinofeachcollectorcell.Eachpumphasanassoci-
direct control of the flight crew or via an automated Fuel atedpressureswitchtoprovideverificationoffunction.These
ManagementSystem(SeeSection3ofthischapter). switchesandassociatedpipingarenotshownforclarity.The
Therequirementforfueljettisonisbroughtaboutbythe feedpressureoutputfromtheboostpumpsprovidesamotive
differencebetweenthemaximumtake-offweight(MTOW) pressuresourceforthescavengeejectorpumpswhichmain-
andmaximumlandingweight(MLW)whichbecomesmore tainthecollectorcellsfullaslongasthereisfuelinthefeed
and more significant as the size of aircraft increases. This tank.Thereisalsoasuctionfeedcheckvalveinthecollec-
differencerepresentstheworstcasefueljettisonrequirement torcelltoallowtheenginetosuckfuelfromthetankinthe
byassumingafullyloadedaircraftdevelopinganemergency unlikelyeventoflossofbothfeedpumps.Inthissituationthe
situation immediately after take off that necessitates a need suction capability of the engine fuel system will be limited
fora“Soonaspossible”landing. toaltitudesofabout20000ft(6096m).Theactualvalueof
Sucheventscouldbeanenginefailure,afireeitherinthe thisoperationallimitwillbeestablishedduringflighttesting
engine or in a critical location of the aircraft, or any other oftheaircraftaspartofthecertificationprocess.
failurethatthreatensthesafetyofthepassengersandcrew,if In the example shown the main boost pumps are needed
immediateactionisnottakentofacilitatealandingasquickly to provide boost pressure for engine starting which would
aspossible. requiretheAPUtobeoperatingtoprovideACelectricpower.
Typical worst case jettison times are in the range Alternatively,asmallauxiliaryDCpumpcouldbeprovided
30–45min and even if this may seem to the reader as a toallowenginestartingonbatterypower.
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
Thisarticleis©2010JohnWiley&Sons,Ltd.
ThisarticlewaspublishedintheEncyclopediaofAerospaceEngineeringin2010byJohnWiley&Sons,Ltd.
DOI: 10.1002/9780470686652.eae463
10 AircraftSub-Systems
Figure11. Genericfuelhandlingsystemforatwin-engineaircraft
Acrossfeedvalveconnectstheleftandrightenginefeed AsindicatedintheschematicofFigure11,therearesev-
manifolds to cover the engine out case and to provide a eralisolationvalvesusedinthefuelsystemfluidnetwork.
cross-flowlateralbalancingfunctionifthereisasignificant Twotypesofisolation(orshut-off)valvesareincommon
difference between the left and right engine fuel consump- useinaircraftfuelsystems:
tion.
(i) Themotor-operatedvalveand
The transfer system uses the transfer/jettison pumps
(ii) Thehydro-mechanicallyoperatedvalve.
mounted in the center tank to keep the wing tanks topped
upasfuelisconsumed. Figure12showsexamplesofeachoftheabovevalvetypesin
These same pumps plus a dedicated pump in each wing schematicform.Inthecaseofthehydro-mechanicalvalvethe
tanksupportthejettisonfunction. controldeviceallowsfueltobleedintothetankthuslowering
Figure12. Shut-offvalveschematics
EncyclopediaofAerospaceEngineering,Online©2010JohnWiley&Sons,Ltd.
Thisarticleis©2010JohnWiley&Sons,Ltd.
ThisarticlewaspublishedintheEncyclopediaofAerospaceEngineeringin2010byJohnWiley&Sons,Ltd.
DOI: 10.1002/9780470686652.eae463