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StudentResource

SubjectB2-14

Propulsion

Copyright © 2008 Aviation Australia

Allrightsreserved.Nopartofthisdocumentmaybereproduced,transferred,sold,orotherwisedisposedof,withoutthewrittenpermissionofAviationAustralia.



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CONTENTS

Definitions iii

Study Resources iv

Introduction v

TurbineEngineFundamentals 14.1.1-1

EngineFuelSystems 14.1.2-1

EngineIndicationSystems 14.2-1

EngineStarting&IgnitionSystems 15.13-1



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DEFINITIONS

Define

Todescribethenatureorbasicqualitiesof.

Tostatetheprecisemeaningof(awordorsenseofaword).

State

Specifyinwordsorwriting.

Tosetforthinwords;declare.

Identify

Toestablishtheidentityof.

List

Itemise.

Describe

Representinwordsenablinghearerorreadertoformanideaofanobjectorprocess.

Totellthefacts,details,orparticularsofsomethingverballyorinwriting.

Explain

Makeknownindetail.

Offerreasonforcauseandeffect.



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STUDYRESOURCES

JeppesenSandersonTrainingProducts:

A&PTechnicianPowerplantTextbook.

AircraftGasTurbinePowerplantsTextbook.

AircraftTechnicalDictionaryThirdEdition

AircraftInstrumentsandIntergratedSystems.

FADECforPart-662ndEdition(www.totaltrainingsupport.com)

B2-14StudentHandout



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INTRODUCTION

Thepurposeofthissubjectistofamiliariseyouwithconstruction,components,operationandmaintenanceofgasturbineenginesandassociatedinstrumentandelectronoicfuelcontrol

systemsusedinaircraft.

Oncompletionofthefollowingtopicsyouwillbeableto:

Topic 14 1 1 Turbine Engine Fundamentals

StateNewton’slawsofmotion.

Definepotentialenergy,kineticenergyandBraytoncycle.

Definetherelationshipbetweenthefollowing:

Force

Work

Power

Energy

Velocity

Acceleration.

Definetheconstructionalarrangementandoperationofthefollowingenginetypes:

Turbojet

Turbofan

Identify thecomponentsofanddefine theoperationof the following turbopropandturbo-shaftenginesystems:

Gascoupled/freeturbineand

Gearcoupledturbine(Reductiongearbox).

Topic 14 1 2 Engine Fuel Systems

Identifyenginefuelsystemcomponentsanddescribesystemlay-outsandoperations.

Describetheoperationofenginefuelmeteringsystems.

Describetheoperationofelectronicenginecontrol(FADEC).

Topic 14 2 Engine Indication Systems

Identifycomponentsofthe followingengine indicationsystemsanddescribesystemoperation:

ExhaustGasTemperature(EGT);

TurbineTemperature(Interstage(ITT),Inlet(TIT/TGT));

EngineThrust;

EnginePressureRatio(EPR);

TurbineDischarge/JetPipePressure;

OilpressureandTemperature;

FuelpressureandFlow;

EngineSpeed;

VibrationMeasurement;

EngineTorque;

Power; ManifoldPressureand

PropellerSpeed.



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Topic 15 13 Engine Starting and Ignition Systems

Describecomponentsofenginestartsystemsandtheiroperation.

Describecomponentsofengineignitionsystemsandtheiroperation.

Interpretthesafetyprecautionstobeobservedwhenperformingmaintenanceonengineignitionsystems.



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TOPIC 14.1.1 TURBINE ENGINE FUNDAMENTALS

NEWTON’S FIRST LAW OF MOTION

Newton’sFirstLawmaybestatedas:“Abodywillremainatrestorcontinueitsuniformmotioninastraightlineuntilacteduponbyanexternalnetforce.”

Newton'sfirstlawofmotionisalsooftenreferredtoasthelawofinertia.

Thelargerthemass,thegreatertheinertia.

NEWTON’S SECOND LAW OF MOTION

Newton’sSecondLawofmotionstates:“Theaccelerationofabodyisdirectlyproportionaltotheforceappliedtoitandisinverselyproportionaltothemassofthebody.”

When a force acts on an object, giving it motion, it gains momentum. Once an object has

momentum,ittakesforcetohaltthemotion.

Force=MassxAcceleration,orF=MxA,where:F=Forceinpounds,M=Massinlbs./ft/sec.², A=Accelerationinft/sec.²

So,theforcedevelopedbyagasturbineengineisproportionalto:

themassofairflowingthroughtheengine;

theaccelerationgiventothatmassofair.

NEWTON’S THIRD LAW OF MOTION

Newton’sThirdLawofmotionstates:“Foreveryaction,thereisanequalandoppositereaction.”

“Equal”meansequalinsizeand“opposite”meansoppositeindirection.

Rocketsandreaction-jetthrustersrelyonNewton’sThirdLawofMotionfortheireffect

Theactionofexhaustgases leavingaturbojetengineproducea reactioncalledthrust.ThisisNewton’sthirdlawofmotioninrespectofgasturbines.



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FORCE

Forceisdefinedasthecapacitytodowork,orthetendencytoproducework.

Itis alsoa vectorquantity thattends to produceaccelerationof abodyin the direction ofitsapplication.Itcanbemeasuredinunitsofpounds.

Turbojetandturbofanenginesareratedinpoundsofthrust.

Theformulaforforceis:Force=PressurexArea,orF=PxA

Where:F=Forceinpounds

P=Pressureinpoundspersquareinch(psi)A=Areainsquareinches.

EXAMPLE:The pressureacross theopeningofa jet tailpipe (exhaustnozzle) is6psiaboveambientandtheopeningis300squareinches.Whatistheforcepresentinpounds?

F=PxA

F=6x300

F=1,800pounds

Theforcementionedhere ispresent inaddition to reactive thrust inmost gas turbineenginedesigns.This“pressurethrust”willbediscussedlaterinotherchapter.

WORK

Mechanicalworkispresentwhenaforceactingonabodycausesittomovethroughadistance.Workisdescribedasusefulmotion.Aforcecanactonanobjectvertically(oppositetheeffectofgravity),horizontally(90degreestotheeffectofgravity),orsomewhereinbetween.Aforcecanalsoactonanobjectinadownwarddirection,inwhichcaseitwouldbeassistedbygravity.Thetypicalunitsforworkare“inchpounds”and“footpounds”.

Theformulaforworkis:Work=ForcexDistance,orW=FxD

Where:W=Workinfootpounds;F=Forceinpounds;D=Distanceinfeet.



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Forinstance,liftingthesameobjectthesameverticalheightrequiresthesamework,nomatterthepath.

POWER

The definition ofworkmakes nomention of time.Whether it takes five seconds tomoveanobjectorfivehours,thesameamountofworkwouldbeaccomplished.Power,bycomparison,doestakethetimeintoaccount.Toliftatenpoundobject15feetoffthefloorinfivesecondsrequiressignificantlymorepowerthanto lift itin fivehours.Workperformedperunitof time ispower.Power ismeasuredinunitsoffootpoundspersecond,footpoundsperminute,ormilepoundsperhour.

Theformulaforpoweris:Power=ForcexDistanceFxD

Where:P=Powerinfootpoundsperminute;D=Distanceinfeet;t=Timeinminutes.

EXAMPLE:A2,500poundengineistobehoistedaheightof9feetintwominutes.Howmuchpowerisrequired?

P=(FxD)/t

P=(2,500x9)/2=11,250ft.lbs/mm.



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Inphysics, acceleration isdefined asa change invelocity with respect to time.Observe thatdistancetraveledisnotconsidered,onlylossorgainofvelocitywithtime.Thetypical(Imperial)units foracceleration are feet per second/second (fps/s) and miles per hour/second (mph/s).Feetpersecond/secondaresometimesreferredtoasfeetpersecondsquared(fps2).

HORSEPOWER

Horsepower isamore commonand usefulmeasure ofelectricalpower.Yearsago using the

multiplierof1.5timesastronghorse’sability todousefulwork,itwasdeterminedthat33,000poundsofweightliftedonefootinoneminutewouldbethestandardintheEnglishsystem.Ifpower is in foot pounds/minute, it can be divided by 33,000 to convert to horsepower.Mathematically,theunitsoffootpoundsperminutewillcanceleachotherout,leavingonlythenumber.Horsepowerdoesnothaveunits,sincehorsepoweristheunit.Ifpowerisbeingdealtwithinunitsoffootpoundspersecond,550istheconversionnumber.Ifpowerisinmilepoundsperhour,375istheconversionnumber.

Theformulaforconvertingtohorsepoweris:Hp=Power(inft.lbs/mm.)/33,000.

EXAMPLE:Howmuchhorsepowerisrequiredtohoista2,500poundengineaheightof9feetintwominutes(thepreviousexamplewhichrequired11,250ft.lbs./minofpower)?

Hp=Power/33,000=11,250/33000=0.34orapproximately1/3Hp

SPEED and VELOCITY

Velocitydealswithhowfaranobjectmoves,whatdirection itmoves,andhowlongittookittomovethatfar.

Velocity isexpressed inthesameunitsasspeed,typically feetper second(fps)ormilesperhour(mph).Thedifferenceisthatspeeddoesnothaveaparticulardirectionassociatedwithit.Velocityisidentifiedasbeingavectorquantity,whilespeedisascalarquantity.

Theformulaforvelocityis:

Velocity=Distance÷time,orV=D÷t

ACCELERATION

TheSIunit–metre/second2.

Theformulaforcalculatingaccelerationis:



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Theaccelerationrateduetogravity,whenanobjectisin freefallwithnodrag,is32.2feetpersecond/second.Whenanobjectacceleratesatthisrate,it isexperiencingwhat isknownasaforceof1“g”.

Ifwedivided theaccelerationrate for theexamplefighterairplaneby32.2,wewoulddiscoverhowmany“g”forcesitisexperiencing(132÷32.2=4.1g’s).

Negativeaccelerationiscalleddeceleration.

ENERGYEnergyisusedtoperformusefulwork. Inthegasturbineenginethismeansproducingmotionandheat.Thetwoformsofenergywhichbestdescribethepropulsivepowerofthejetenginearepotentialandkineticenergy.

Potential Energy

Energystoredbyanobjectbyvirtueof itsposition.Forexample, anobject raisedabove thegroundacquirespotentialenergyequaltotheworkdoneagainsttheforceofgravity;theenergyisreleasedaskineticenergywhenitfallsbacktotheground.Similarly,astretchedspringhas

stored potential energy that is releasedwhen the spring is returned to its unstretched state.Otherformsofpotentialenergyincludeelectricalpotentialenergy.

Chemicalenergyisausefulbutobsolescenttermfortheenergyavailablefromelementsandcompoundswhen they react,as ina combustionreaction. Inpreciseterminology, there isnosuchthingaschemicalenergy,sinceallenergyisstoredinmatteraseitherkineticenergyorpotentialenergy.



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Kinetic Energy

Theenergypossessedbyabodybecauseofitsmotion,equaltoonehalfthemassofthebodytimesthesquare of its speed,equal toonehalf themassof the bodytimes thesquareof itsspeed.

Formofenergythatanobjecthasbyreasonofitsmotion.Thekindofmotionmaybetranslation(motion along a path from one place to another), rotation about an axis, vibration, or anycombinationofmotions.Thetotalkineticenergyofabodyorsystemisequaltothesumofthekineticenergiesresultingfromeachtypeofmotion.

Thekineticenergyofanobjectdependson itsmassandvelocity.Forinstance, theamountofkineticenergyKEofanobjectintranslationalmotionisequaltoone-halftheproductofitsmass

mandthesquareofitsvelocityv,orKE=1/2mv².

Forexample,a500,000kgmassA380aircraftisflyingoverSydneyat250meterspersecond,whatisitskineticenergy?

KineticEnergy=½·500000·250²=15,625,000,000joules.



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BERNOULLI’S THEOREM

Bernoulli’sprincipledealswithpressureofgases.Pressurecanbechangedinthegasturbineenginebyaddingorremovingheat,changingthenumberofmoleculespresent,orchangingthevolumeinwhichthegasiscontained.

Bernoullidiscoveredthatairactsasanincompressiblefluidwouldactwhenflowingatsubsonicflowrates.

Theprincipleisstatedasfollows:“Whenafluidorgasissuppliedataconstantflowratethroughaduct,thesumofpressure(potential)energyandvelocity(kinetic)energyisconstant.”Inotherwords,whenstaticpressureincreases,velocity(ram)pressuredecreases.Orifstaticpressuredecreases, velocity (ram) pressure increases, meaning that velocity pressure will change inrelationtoanychangeinstaticpressure.

Ifairisflowingthroughastraightsectionofductingwhichthenchangestoadivergentshape,itskineticenergyintheaxialdirectionwilldecreaseastheairspreadsoutradially,and,asthetotal

energy at constant flow rate of the air is unchanged, the potential energy must increase inrelationtothekineticenergydecrease.

TherearemanyexampleswithinagasturbineengineoftheapplicationofBernoulli’sTheorem:

theairpassagesbetweenindividualbladesofacompressororturbine;

thediffusersectionofacentrifugalcompressor;

thecross-sectionalshapeofengineinletandexhaustducts;

theentiregasflowpaththroughtheengine.



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BERNOULLI’S HEOREM PRESSURE VELOCITY TEMP GRAPH

TheapplicationofBernoulli’sTheoreminatypicalsingle-spoolaxialflowturbo-jetengine.

Theanimationshows thechanges ofpressure,velocity, temperature (turbojet) duringgroundrun-up.

BRAYTON CYCLE

TheBraytoncycleisalsowidelyknownasa“constantpressurecycle”.Thereasonforthisisthatinthegas turbineengine,pressureis fairlyconstantacross thecombustionsectionasvolumeincreasesandgasvelocitiesincrease.

Combustiontakesplaceatconstantpressureingasturbineengines.



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The four continuousevents shown on thepressure- volumegraphare: Intake, compression,expansion(power),andexhaust.

Referringtothegraph,

AtoBindicatesairenteringtheengineatbelowambientpressureduetosuctionandincreasingvolumeduetothedivergentshapeoftheductinthedirectionofflow.

BtoCshowsairpressurereturningtoambientandvolumedecreasing.

CtoDshowscompressionoccurringasvolumeisdecreasing.

DtoEindicatesaslightdropinpressure,approximately3%,throughthecombustionsectionandanincreasingvolume.Thispressuredropoccursasa resultofcombustionheataddedandiscontrolled by the carefully sizedexhaust nozzleopening.Recall that there isabasicgaslawwhichstatesthatgaswill tendtoflowfromapointofhighpressuretoapointoflowpressure.Thepressuredropinthecombustorensuresthecorrectdirectionofgasflowthroughtheenginefromcompressortocombustor.Theairrushinginalsocoolsandprotectsthemetalbycentering

theflame.

EtoFshowsapressuredropresultingfromincreasingvelocityasthegasisacceleratedthroughtheturbinesection.

FtoGshowsthevolume(expansion)increasewhichcausesthisacceleration.Gcompletesthecycleasgaspressurereturnstoambient,orhigherthanambientatthenozzleifitischoked.

ENGINE STATIONS

Asystemofstandardstationnumberingmakes iteasier to findvarious locationsonandwithintheengine.

Numbers from 1 to 9 designate certain locations. For example, station 2 is always thecompressorinlet.

Inaddition tothestation numbers,prefixesareused toshowvariousparameters occurringatthesestationswithintheengine.



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Forexample,

TemperaturehastheprefixT.

o

Thetemperatureoccurringatstation5iscalledT5.

PressurehasaprefixPandcanbefurtherdividedinto:

o Pt–totalpressure;

o Ps–staticpressure.

Thestaticpressureatstation3isknownasPs3.

Engine Directional References

Forpurposesofidentifyingengineconstructionpoints,orcomponentandaccessoryplacement,

directionalreferencesareusedalongwithstationnumbers.Thesereferencesaredescribedasforwardattheengineinletandaftattheenginetailpipe,withastandard12hourclockorientation.Thetermsright-andleft-hand,clockwiseandcounterclockwise,applyasviewedfromtherearoftheenginelookingforwardtowardtheinlet.



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GAS TURBINE ENGINE TYPES

Gasturbineenginesareconsideredtobeoftwotypes:

a.

ThrustProducingEngines;

b. TorqueProducingEngines.

Thetwoclassificationsofthrustproducingturbineenginesare:a.Turbojet;b.Turbofan.

Thetwoclassificationsoftorqueproducingturbineenginesare:a.Turboprop;b.Turboshaft.



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TURBOJET ENGINE

The turbojet, as first patented by Sir Frank Whittle, had an impeller compressor, annularcombustor, and a single stage turbine. Today it ispossible tosee manyvarieties of turbojetenginedesigns,butthebasiccomponentsarestillthecompressor,combustor,andturbine.

Theturbojetgetsitspropulsivepowerfromreactiontotheflowofhotgases.Airenterstheinletand its pressure is increased by the compressor. Fuel is added in the combustor and theexpansioncreatedbyheatforcestheturbinewheeltorotate.Theturbinesectioniscoupledtothecompressorsectionanddirectlydrivesit.Theenergyremainingdownstreamoftheturbineinthetailpipeacceleratesintotheatmosphereandcreatesthereactionwerefertoasthrust.

Theyhaverelativelyfewmovingpartsandcreatethrustbyacceleratingarelativelysmallmassofairwithalargeamountofacceleration.

Theyarelessefficientduetolossesfromnoiseandincompletecombustion.



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Engine Pressure Ratio

Whendiscussingaturbojetengineyoumustbefamiliarwiththetermenginepressureratio,or

EPR. An engine’s EPR is the ratio of the turbine discharge pressure to the engine inlet airpressure.EPRgaugereadingsareanindicationoftheamountofthrustbeingproducedforagivenpowerleversetting.Totalpressurepickups,orEPRprobes,measuretheairpressureattwopointsintheengine;oneEPRprobeislocatedat thecompressorinletandasecondEPRprobeislocatedjustaftofthelaststageturbineintheexhaustsection.EPRreadingsareoftenusedasverificationofpowersettingsfortake-off,climb,andcruise.EPRreadingsareaffectedbyandaredependentonpressurealtitudeandoutsideairtemperature(OAT).

TURBOFAN

Theturbofan,ineffect,isa ducted,multi-bladedpropellerdrivenbyagas turbineengine.Thisfanproducesapressureratioontheorderof2:1,ortwoatmospheresofcompression.Generally,

turbofanscontain20to40fixedpitchblades.



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Bycomparison,thefandiameterofaturbofanengineismuchlessthanthatofthepropelleronaturbopropengine,butitcontainsmanymorebladesandmovestheairwithagreatervelocityfromitsconvergentexhaustnozzle.

Turbofanhasmoreturbinestagesthanaturbojetinordertodrivethefanatthefrontorback.

Thereare:

Forwardfanengines

Aft-fanengines:doesn’tcontributetocompression.

Fan Bypass Ratio

ThepropulsiveefficiencyofaTurbofanengineismeasuredbyFanBypassRatio.

Fanbypassratioistheratioofthemassairflowwhichflowsthroughthefanduct,dividedbythemass airflowwhich flows through thecoreportion of theengine. Fan airflowpassesover theouterpartofthefanbladeandthenoutofthefanexhaustandbacktotheatmosphere.Coreengineairflowpassesovertheinnerpartofthefanbladesandisthencompressed,combusted,andexhaustedfromthehotexhaustduct.

Thefanorbypassairisnotusedforcombustionbutproducesthemajorityofthrust.



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Turbofanenginescanbe

Highbypass(4:1ormore)

Mediumbypass(2or3:1)

Lowbypass(1:1)

Mostturbofanengineshaveseparatelowpressureandhighpressurecompressorandturbinespools.

Generaloverviewofatypicalhighbypass-ratioturbofanengine(AdaptedfromPratt&Whitney).



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TURBOPROP

Betterpropulsiveefficiencyatlowspeedcomparedtoaturbojet,theextraturbinestagesareusedtodriveashaft.

Connectedtotheshaftisareductiongearboxandapropeller.

Thepropellermovesalargemassofairwitharelativelysmallamountofacceleration.

Turbopropenginesareveryfuelefficientatlowerairspeeds.

Thepropellerstartstobecomeaerodynamicallyinefficientathigherairspeeds.

TwomaintypesofTurbopropengines:

Fixedshaft(AlsocalledGearCoupledturbine);

Freeturbine.Thefixedturbineisconnecteddirectlytothecompressor,reductiongearbox,andpropellershaft,in another words, themain power shaft of a fixed shaft engine goes directly to a reductiongearboxwhichcandriveapropeller,forexample,GarrettTPE331fixedshaftturbopropengine.



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Free turbine turboprop engine

Alsocalledgascoupled.

Forexample,Pratt&WhitneyPT6freeturbineturbopropengine(Reverseflowcombustor).

Thefreeturbineisconnectedonlytothegearboxandpropellershaft.Thisisanindependentturbine that isnot connected to themain turbine.This arrangementallows thefree turbine toseek itsoptimumdesignspeedwhilecompressorspeedissetatitsdesignpoint(pointofbestcompression).

Someoftheadvantagesofthefreeturbineare:

1.Thepropellercanbeheldatverylowrpmduringtaxiing,withlownoiseandlowbladeerosion.

2.Theengineiseasiertostart,especiallyincoldweather.

3.Thepropelleranditsgearboxdonotdirectlytransmitvibrationsintothegasgenerator.

4.Arotorbrakecanbeusedtostoppropellermovementduringaircraftloadingwhenengineshutdownisnotdesired.

Disadvantage:Theenginedoesnothavetheinstantaneouspowerofreciprocatingengines.

TURBOSHAFT

Turboshaftengines are gasturbine engines that operate somethingother than apropeller bydeliveringpowerto ashaft.Turboshaftenginesaresimilar toturbopropengines,and insomeinstances,both use the same design. Like turboprops, turboshaftengines usealmost all theenergyintheexhaustgasestodriveanoutputshaft.Thepowermaybetakendirectlyfromtheengineturbine,ortheshaftmaybedrivenbyitsownfreeturbine.Likefreeturbinesinturbopropengines,afreeturbineinaturboshaftengineisnotmechanicallycoupledtotheengine’smainrotor shaft, soitmay operate at its ownspeed.Freeturbinedesigns areused extensively incurrentproductionmodelengines.



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yused topowerhelicoptersandauxiliarypowerunitsaboard

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ctureshowingisaGeneralElectricT-64Turboshaftengine.

Turboshaftenginesarefrequentllargecommercialaircraft.

http://en.wikipedia.org/wiki/File:Helicopter_engine_CH-53G.jpg



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ENGINE COMPONENTS

Therearesevenbasicsectionswithineverygasturbineengine.Theyarethe

airinlet.

compressorsection.

combustionsection.

turbinesection.

exhaustsection.

accessorysection.

systemsnecessaryforstarting,lubrication,fuelsupply,andauxiliarypurposes,suchasanti-icing,cooling,andpressurization.

Additional terms you often hear include hot section and cold section. A turbine engine’s hotsection includesthecombustion,turbine,andexhaustsections.Thecoldsection,ontheotherhand,includestheairinletductandthecompressorsection.

Air Inlet Duct

Theairinlettoaturbineenginehasseveralfunctions,oneofwhichistorecoverasmuchofthetotalpressureofthefreeairstreamaspossibleanddeliverthispressuretothecompressor.Thisisknownasram recoveryorpressurerecovery. Inaddition torecoveringandmaintainingthepressure of the free airstream, many inlets are shaped to raise the air pressure aboveatmosphericpressure.

Another function of the air inlet is to provide auniform supply ofair to the compressor so thecompressor can operate efficiently. Furthermore, the inlet duct must cause as little drag aspossible.It takesonlyasmallobstructionto theairflowinsideaducttocauseaseverelossofefficiency.Ifaninletductistodeliveritsfullvolumeofairwithaminimumofturbulence,itmustbemaintainedasclosetoitsoriginalconditionaspossible.Therefore,anyrepairstoaninletduct



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mustretaintheduct’ssmoothaerodynamicshape.Tohelppreventdamageorcorrosiontoaninletduct,aninletcovershouldbeinstalledanytimetheengineisnotoperating.

FOREIGN OBJECT DAMAGE

Toensure the operatingefficiencyofan air inlet duct, periodic inspection for ForeignObjectDamage(FOD)andcorrosionisrequired.

Preventionofforeignobjectdamage(FOD)isatoppriorityamongturbineengineoperatorsandmanufacturers.

COMPRESSOR SECTION

Theprimaryfunctionofa compressoris toforceairintotheengineforsupportingcombustionandprovidingtheairnecessarytoproducethrust.

Onewayofmeasuringa compressor’s effectiveness is tocompare thestaticpressureof thecompressordischargewiththestaticairpressureattheinlet.Ifthedischargeairpressureis30



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timesgreater than theinletair pressure, that compressorhas acompressorpressure ratioof30:1.

The compressor section has also several secondary functions. For example, a compressorsuppliesbleedair tocoolthehotsectionandheatedair foranti-icing.In addition,compressorbleedair isusedforcabinpressurization,airconditioning,fuelsystemdeicing,andpneumaticenginestarting.

Therearetwobasictypesofcompressorsusedtoday:

thecentrifugalflowcompressor,and

theaxialflowcompressor.

Eachisnamedaccordingtothedirectiontheairflowsthroughthecompressor,andoneorbothmaybeusedinthesameengine.

CENTRIFUGAL FLOW COMPRESSORS

Thecentrifugalcompressor,sometimescalledaradialoutflowcompressor,isoneoftheearliestcompressordesignsand isstillused today insomesmallerenginesandauxiliarypowerunits(APU’s).

Centrifugalcompressorsconsistofanimpeller,adiffuser,andamanifold.



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AXIAL FLOW COMPRESSORS

Anaxialflowcompressorhastwomainelements,arotorandastator.Therotorconsistsofrowsof blades fixed on a rotating spindle. The angle and airfoil contour of the blades forces airrearwardinthesamemannerasapropeller.Thestatorvanes,ontheotherhand,arearrangedinfixedrowsbetweentherowsofrotorbladesandactasdiffusersateachstage,decreasingairvelocityandraisingpressure.

Eachconsecutiverowofrotorbladesandstatorvanesconstitutesapressurestage.Thenumberofstagesisdeterminedbytheamountofairandtotalpressureriserequired.



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DIFFUSER

Asairleavesanaxialflowcompressorandmovestowardthecombustionsection, itistravelingatspeedsupto500feetpersecond.Thisisfartoofasttosupportcombustion,thereforetheairvelocitymust be slowed significantly before it enters the combustion section. The divergentshapeofadiffuserslowscompressordischargewhile,atthesametime,increasingairpressuretoitshighestvalueintheengine.Thediffuserisusuallyaseparatesectionboltedtotherearofthecompressorcaseandaheadofthecombustionsection.



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COMBUSTION SECTION

A combustion section is typically located directly between the compressor diffuser and turbinesection.All combustion sections contain the samebasic elements: one or more combustionchambers(combustors),afuelinjectionsystem,anignitionsource,andafueldrainagesystem.

Thecombustionchamberorcombustorina turbineengine iswherethefuelandairaremixedandburned.Atypicalcombustorconsistsofanoutercasingwitha perforatedinnerliner.Theperforationsarevarioussizesandshapes,allhavingaspecificeffectontheflamepropagationwithintheliner.

Thefuelinjectionsystemmeterstheappropriateamountoffuelthroughthefuelnozzlesintothecombustors. Fuel nozzlesare located in the combustionchamber case or in thecompressoroutletelbows.Fuelisdeliveredthroughthenozzlesintothelinersinafinelyatomizedspraytoensurethoroughmixingwiththeincomingair.Thefinerthespray, themorerapidandefficientthecombustionprocessshouldbe.

Atypical ignitionsourceforgasturbineengines isthehigh-energycapacitordischargesystem,consistingofanexciterunit,twohigh-tensioncables,andtwosparkigniters.Thisignitionsystemproduces60to100sparksperminute,resultinginaballoffireattheigniterelectrodes.Someofthesesystemsproduceenoughenergytoshootsparksseveralinches,socaremustbetakentoavoidalethalshockduringmaintenancetests.

A fuel drainage system accomplishes the important task of draining the unburned fuel afterengine shutdown. Draining accumulated fuel reduces the possibility of exceeding tailpipe orturbine inlet temperature limitsdue toanengine fire after shutdown. Inaddition,draining theunburned fuel helps to prevent gum deposits in the fuel manifold, nozzles, and combustionchamberswhicharecausedbyfuelresidue.

Inordertoallowthecombustionsectiontomixtheincomingfuelandair,ignitethemixture,and

coolthecombustiongases,airflowthroughacombustorisdividedintoprimaryandsecondarypaths.Approximately25to35percentoftheincomingairisdesignatedasprimarywhile65to75percentbecomessecondary.Primary,orcombustionair,isdirectedinsidethelinerinthefrontendofacombustor.

Thesecondaryairflowinthecombustionsectionflowsatavelocityofseveralhundredfeetpersecondaroundthecombustor’speriphery.Thisflowofairformsacoolingairblanketonbothsides of the liner and centers the combustion flames so theydo not contact the liner.Somesecondary air is slowedandmetered into the combustor through the perforations in the linerwhereitensurescombustionofanyremainingunburnedfuel.Finally,secondaryairmixeswith



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B2-14 Propulsion

theburnedgasesandcoolairtoprovideanevendistributionofenergytotheturbinenozzleatatemperaturethattheturbinesectioncanwithstand.

TURBINE SECTION

After the fuel/air mixture is burned in the combustor, its energymust be extracted. A turbinetransformsaportionofthekineticenergyinthehotexhaustgasesintomechanicalenergytodrivethecompressorandaccessories.

ThepictureshowingisaPW400094-InchFanEngine.



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B2-14 Propulsion

Ina turbojetengine,the turbineabsorbsapproximately60 to80%of the totalpressureenergyfromtheexhaustgases.Theturbinesectionofaturbojetengine islocateddownstreamof thecombustionsectionandconsistsoffourbasicelements;acase,astator,ashroud,andarotor.

EXHAUST SECTION

Thedesignofaturbojetengineexhaustsectionexertstremendousinfluenceontheperformanceofanengine.Forexample,theshapeandsizeofanexhaustsectionanditscomponentsaffectthe temperature of theairentering the turbine, or turbine inlet temperature, themass airflowthrough the engine, and the velocity and pressure of the exhaust jet.Therefore,an exhaustsectiondeterminestosomeextenttheamountofthrustdeveloped.

A typical exhaust section extends from the rear of the turbine section to the point where theexhaust gases leave the engine. An exhaust section is comprised of several componentsincludingtheexhaustcone,exhaustductortailpipe,andexhaustnozzle.



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ACCESSORY SECTION

Theaccessorysection,oraccessorydrive,ofagasturbineengineisusedtopowerbothengineand aircraft accessories such as electric generators, hydraulic pumps, fuel pumps, and oilpumps. Secondary functions include acting as an oil reservoir, or sump, and housing theaccessorydrivegearsandreductiongears.

Theaccessorydrivelocationisselectedtokeeptheengineprofiletoaminimumforstreamlining.

Typicalplaceswhereanaccessorydriveislocatedincludetheengine’smidsection,orthefrontorrearoftheengine.



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Issue Revision Page28of28

B2-14 Propulsion

B:January2008 2

ENGINE MOUNTS

Enginemount designand construction for gas turbine engines is relativelysimple.Sincegasturbineenginesproducelittletorque,theydonotneedheavilyconstructedmounts.Themountsdo,however,supporttheengineweightandallowfortransferofstressescreatedbytheenginetotheaircraftstructure.

Onatypicalwingmountedturbofanengine,theengineisattachedtotheaircraftbytwotofourmountingbrackets.However,becauseofinducedpropellerloads,aturbopropdevelopshighertorque loads, so engine mounts are proportionally heavier. By the same token, turboshaftenginesusedinhelicoptersareequippedwithstrongerandmorenumerousmountlocations.

-EndofthisTopic-



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B2-14 Propulsion AA Form TO-19

B2-14.1.2EngineFuelSystemsIssueB:January2008 Revision1 Page1of34

TOPIC14.1.2:ENGINEFUELSYSTEMS

Fuel Control and Metering Systems

Gasturbineenginesconvertthelatentenergyof fuel intoheattoprovidetheenergyfortheoperationoftheengineandthrustfortheaircraft.

The function of the fuel system is to provide the engine with fuel, in a form suitable forcombustion and to control its flow to the required rates necessary for easy starting,accelerationandstablerunning,inallengineoperatingconditions.

Fuel System Layout

Foragasturbineenginetodeliverthepowerrequired,itneedsasystemthatsuppliesfuelinsufficientquantitiestoallowforvaryingconditions,altitudesandpowersettings.

Layoutofaircraftandenginefuelsystemsvarywiththetypeandsizeofaircraft,however,mostsystemsincludethefollowingcomponents:

Fueltank.

Boostpump.

Fuelflowtransmitter.

Lowpressureshutoffvalve.

Lowpressuretransmitter.

Fuelheater.

Fuelfilter.

Highpressurefuelpump.

Fuelcontrolunit.

Highpressureshutoffvalve.

Pressurisinganddumpvalve.

Fuelburners.

Fuelpressuredifferentialswitch.

The block diagram in Figure 1.2-1 shows the fuel system layout of a typical gas turbineengine. At the lowest point of the fuel tank (1), an electrically driven boost pump (2)incorporatingameshfilterdeliverslowpressurefuelthroughfuelflowtransmitter(3)tothe

lowpressureshutoffcock(4)locatedontheenginefirewall.Fromtherefuelflowsthroughthelowpressuretransmitter(5)tothefuelheater(6)andontothefilter(7).Fuelisthendeliveredtothehighpressurepump(8)throughtheFCU(9)totheandhighpressureshutoffcock(10). Itthenflowstothepressurisinganddumpvalve(1.2)andontofuelmanifoldsandburners(12).

A fuel pressure differential switch (13) takes a pressure reading from near the fuel flowtransmitter(3) and frombetweenthe fuel filter (7) and highpressurepump(8) togiveanindicationthat the fuel filter isbecomingblockedbyiceorforeignmaterial inthefuel thusenablingthepilottoselectfuelheatingtoremoveicefromthefilter.



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Figure1.2-1

Fuel System Components

Fuel Flow Transmitter

Fuelflowmetersareusedinfuelsystemstoshowtheamountoffuelconsumedperhourbytheengine,thusallowingthepilottoaccuratelycalculatetheavailableflighttimeremaining. Asfuelflows throughthemeter,it spinsasmallturbinewheelandadigitalcircuit readsthenumberofrevolutionsinaspecifiedperiodandconvertsthistoafuelflowrate.

Low Pressure Shut Off Valve

Low pressure shut off valves on modern aircraft, normally mounted behind the enginefirewall,areusedtoisolatetheenginefuelsystemfromtheairframeincaseoffireorsystemmaintenance.Thetwocommontypesofshutoffvalvesare:

Motordrivengatevalve.

Solenoidoperatedvalve.

Motor Driven Gate Valve

Thisvalveshown inFigure1.2-2usesa reversibleelectricmotor linked toaslidingvalveassembly.Themotormovesthevalvegateinandoutofthepassagethroughwhichthefuelflows,thusshuttingofforturningonthefuelflow.

Figure1.2-2



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Solenoid Operated Valve

Asolenoidvalvehasanadvantageoveramotordrivenvalve,beingmuchquickertoopenorclose.ThevalveinFigure1.2-3isasolenoidoperated,poppettypevalve.Whenelectricalcurrentmomentarilyflowsthroughtheopeningsolenoidcoil,amagneticpullisexertedonthe

valvestemthatopensthevalve.Whenthestemriseshighenough,thespringloadedlockingplungerisforcedintothenotchinthevalvestem.Thisholdsthevalveopenuntilcurrentismomentarilydirected to the closing solenoid coil. Themagneticpull of this coil pulls thelockingplungeroutofthenotchinthevalvestem,thespringclosesthevalveandshutsofftheflowoffuel.

Figure1.2-3

High Pressure Shut Off Valve

Thehighpressure(HP)shutoffcockisavalvemountedinthefuelcontrolunit(FCU)andisusedtogiveadefiniteshutoffofthefuellinefromtheFCUtothefuelburnernozzles.

The HP cock may be connected directly to the engine power lever and operates frommaximumthrottle(HPcockopen)toidlethrottle(HPcockopen)thenthroughagatetocutoff(HPcockclosed).

However,onturbopropelleraircraftitisnormallyconnectedinconjunctionwiththepropeller

feather control lever to give amovement through gatesofengine run (HP cock open) toenginestop(HPcockclosed)thenpropellerfeather(HPcockclosed).



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Pressurising and dump valve

A fuelpressurisingand dumpvalve isnormallyrequiredonenginesusingduplextype fuelnozzles,todividethefuelflowintoprimaryandmainmanifolds,andtodrainfuelfromthese

manifoldsonshutdown.Pressurising Valve

Thefuelpressurisingvalvecontrolsthefuelflowsrequiredforstartingandaltitudeidling,allfuelpassesthroughtheprimarymanifold.Asfuelflowincreases,thevalvebeginstoopenthemainmanifolduntilatmaximumflowthemainmanifoldispassingapproximately90%ofthefuel.

Dump Valve

Thedumpvalvegivesthecapabilityto“dump”ordrainfuelfromthefuelmanifoldsaftershutdown.Manifolddumpingisaprocedurewhichsharplycutsoffcombustionandalsopreventsfuelboiling,orafterburning,asaresultofresidualengineheat.Thisboilingtendstoleave

soliddepositswhichcouldclogfinelycalibratedpassageways.Operation

The construction and operation of pressurisation and dump valves varies with differentmanufacturers, however, the following is a description of the operation of a typicalpressurisationanddumpvalve,showninFigure.1.2-4.

Whenthepowerleverisopened,apressuresignalfromthefuelcontrolunitmovesthedumpvalveagainstthespringpressureclosingthedumpportandopeningthepassagewaytothemanifolds.Ataspeedslightlyaboveidle,thefuelpressurewillbesufficienttoovercomethepressurisingvalvespringforce,andfuelwillalsoflowtothemainmanifold.

Onshutdownwhenthefuellever ismovedtoOFF,thepressuresignalholding thedump

portclosedandthefuelpassageopen,islost.Springpressureclosesthefuelpassageandopensthemanifoldstothefueldump,orreturnline.

Figure1.2-4



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Fuel Heater

AA Form TO-19


Drain Valves

Thedrainvalvesareused for drainingfuel fromvariouscomponentsof the enginewhereaccumulated fuel is most likely to present operating problems. This valve is normallyoperatedbypressuredifferential.

Fuelaccumulatesinthebottomofthelowercombustionchamberfollowingshutdownorafalsestart.Whentheairpressureinthecombustionchamberreducestonearatmospheric,the valveopens and allows the accumulatedfuel todrainaway. It is imperative that thisvalveisingoodworkingorder,otherwiseahotstartduringthenextstartattempt,oranafterfireonshutdownislikelytooccur.

Low Pressure Transmitter

Foraircraftfittedwithmorethanonefueltank,itisdesirabletohaveameansofwarningthepilotthatfuelinthesupplyingtankisexhausted(ortheboostpumpisnotoperating)andthatthefuelselectormustbesettodrawfuelfromanothertank.Thelowfuelpressureswitchisheldopenbynormalfuelpressures,buttheswitchcloseswhenthepressurefalls.Thisturns

onthewarninglightinthecockpit.

Turbinepoweredaircraft thatoperateathigh altitudesandlowtemperatures forextendedperiodsoftimehavetheproblemofwatercondensingoutofthefuelandfreezingonthefuelfilters.Topreventthis,theseaircrafthaveafueltemperaturegaugeandorafilterdifferentialpressurewarninglightthatilluminateswheniceobstructsthefilter.

Thepurposeofthefuelheateristoprotectthefuelsystemfromiceformationandtothawicethatformsonthefuelfilterscreen.Thisisachievedbyusinghotairthathasbeenheatedbythecompressorsectionoftheengine.AfuelheaterisdepictedinFigure1.2-5.

Figure1.2-5

Fuel / Oil Cooler

Thefuel/oilcoolerisdesignedtocoolthehotenginelubricatingoilbyusingthefuelflowingtothe engine passing through a heat exchanger. A thermostatic valve controls the oil flowwhichmaybypasstheheatexchangerifnocoolingisrequired.



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Fuel Filter

Becausethehighpressurefuelpump,fuelcontrolunit,pressurisationvalve,dumpvalveandtheburnersaremanufacturedtoveryfinetolerancesandfittedwithmanysmallorifices,a

filterisinstalledtoprotectthefuelcontrolcomponentsfromcontaminates.Thefiltermustbecapableofremovingparticlesmeasuringassmallas10microns.

High Pressure Fuel Pump

Enginemountedfuelpumpsarerequiredtodeliveracontinuoussupplyoffuelattheproperpressureatalltimesduringoperationoftheaircraftengine.Thefuelpumpsmustbecapableofdeliveringmaximumneededflowathighpressuretoobtainsatisfactorynozzleatomisationandaccuratefuelregulation.Thetwocommontypesofenginedrivenfuelpumpsnormallyusedare:

Spurgear.

Pistontype.

Spur Gear

Gear type pumps have approximately straight line flow characteristics, whereas fuelrequirements fluctuate with flight or ambient air conditions. Hence a pump of adequatecapacityatallengineoperatingconditionswillhaveexcesscapacityovermostoftherangeofoperation. This isacharacteristicwhichrequiresthe use ofapressurerelief valve fordisposingofexcessfuel.AtypicalconstantdisplacementgearpumpisillustratedinFigure1.2-6.Thefuelentersthepumpattheimpellerwhichgivesaninitialpressureincreaseanddischargesfueltothetwohighpressuregearelements.Eachoftheseelementsdischargesfuelthroughacheckvalvetoacommondischargeport.Shearsectionsareincorporatedinthedrivesystemofeachelement.Thus,ifoneelementfails,theothercontinuestooperate.Thecheckvalvespreventcirculationthroughtheinoperativeunit.Oneelementiscapableof

supplyingsufficientfuelformoderateaircraftspeeds. Areliefvalveisincorporatedinthedischargeportofthepumptoallowfuelinexcessofthatrequiredbytheenginetoberecirculatedtoininletsideofthehighpressureelements.

Figure1.2-6



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Piston

The variable displacement pump (Figure 1.2-7) system differs from the constantdisplacementpumpsystem. Pumpdisplacement ischangedtomeet thevaryingfuelflowrequirements,thatis,theamountoffueldischargedfromthepumpcanbemadetovaryat

anyonespeed.Thisisduetotheinclinationofthecamplate,movementoftherotorimpartsareciprocatingmotiontotheplungers,thusproducingapumpingaction.Thestrokeoftheplungersisdeterminedbytheangleofinclinationonthecamplate.Thedegreeofinclinationisvariedbythemovementofaservopistonthatismechanicallylinkedtothecamplateandisbiasedbyspringstogivethefullstrokepositionoftheplungers.Thepistonissubjecttoservopressureon the spring sideand onthe otherside topumpdeliverypressure, thus,variations in the pressure difference across the servo piston cause it to move withcorrespondingvariationsofthecamplateangleandthereforepumpstroke.

Withavariableflowpump,thefuelcontrolunitcanautomaticallyandaccuratelyregulatethepumppressureanddeliverytotheengine.

Figure1.2-7

Fuel pressure differential switch

Thedifferentialpressureswitchisusedinthefuelsystemtodetectthepresenceoficingonthefuelfilterandilluminatesacockpitwarninglightwhenthepressuredifferentialreachesasetamount.

Afuelpressuredifferentialswitchtakesapressurereadingfromnearthefuelflowtransmitterandfrombetweenthefuelfilterandhighpressurepumptogiveanindicationthatthefuelfilterisbecomingblockedbyiceorforeignmaterialinthefuelthusenablingthepilottoselectfuelheatingtoremoveicefromthefilter.



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Fuel Control Units

Thecontrolofpower(orthrust)inagasturbineengineisaffectedbyregulatingthequantityoffuelinjectedintothecombustionchamber.If toomuchfuelissuppliedtothecombustion

chamber,theturbinesectionmaybedamagedbyexcessheat,thecompressormaystallorsurgebecauseofbackpressurefromthecombustionchambersorarichblowoutmayoccur. A rich blowout occurs when the mixture is to rich too burn. If too little fuel enters thecombustionchambersaleandieoutoccurs. Aleandieoutoccurswhen themixtureis toleantoburn.

Theusualmethodofvaryingthefuelflowtothecombustionchamberisviaafuelcontrolunit.Fuel control units operate using either, hydropneumatic, hydromechanical,electro-hydromechanicalorelectroniccontrolprinciples.

Hydromechanical.

Formanyyears themajorityof fuel controlunits havebeenhydromechanicalinoperation.Thismeans theiroperation iscontrolledboth byhydraulic (fuel)andmechanicalmeans to

controlthefuelflowtotheengine.

Hydropneumatic.

Thesefuelcontrolunitsuseengineairpressuresandmechanicalforcestooperateitsfuelschedulingmechanisms.

Electro-hydromechanical.

Latermodelgasturbineenginesarecontrolledbyelectronicfuelcontrolsystems.Theseareknown as electro-hydromechanical fuel control units. These systems use computers thatsenseinputstosetthehydromechanicalsectionofthefuelcontrolunitthatlimitsthefuelflowtotheengine.

Electronic.

Many modern engines, now use a computer or electronic device that controls the fuelmanagementsystem.Withthesecontrolsit ispossibletopressthestartbutton,thenmovethe throttle to maximum power, the engine control then regulates the engine to achievemaximumpowerwithoutexceedingRPM,acceleration,temperatureandpressure limitsofthatengine.



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Hydropneumatic Fuel Control

AsimpleRPMcontrolsystem,showninFigure1.2-8,provides:

RPMcontrol.

Accelerationanddecelerationcontrol.

Minimumandmaximumflowcontrol.

Ithasinputsof:

RPMcommand.

ActualRPM.

Inlettemperature.

Compressoroutletpressure.

InregardtoFigure1.2-8,thefuelpumpsuppliesmorefuelthanisrequiredandthebypassvalve returns excess back to the pump inlet. The bypass valve incorporates a pressureregulator to ensure the pressure differential across the metering valve is unaffected bymovementof themeteringvalve. Therefore, fuel flowiscontrolledonlybymetering valveposition.

RPM is the primary control parameter, and compressor discharge pressure and inlet airtemperaturearesecondaryparameters.Togethertheycontrolthemeteringvalveviaaservobellowsassembly.

“On speed” RPM ismaintained by the governor in conjunction with the governor bellowspressurePy. The flyweightsof thegovernorrespond toanRPMchangebyincreasingordecreasingtheopeningofthegovernorvalve,whichinturnaltersPyandthustheextensionofthegovernorbellows.ThebellowsassemblyopensthemeteringvalveslightlywhenthereisafallinRPMandclosesitslightlywhenthereisariseinRPM.

POWERLEVER

IDLE MAX

INLET AIR TEMPERATURESENSOR

DECELERATION BELLOWS GOVERNOR BELLOWS

MAXIMUM FLOW STOP

METERING VALVE

OP E N

C L O S E D

FUEL TO ATOMISERS

BYPASS ANDPRESSUREREGULATINGVALVE

MINIMUM FLOWSTOP ACCELERATION BELLOWS

PUMP

FUEL IN

COMPRESSOR OUTLET PRESSURE Pc

AIRFLOW

BI METALLIC

DISCS

RPM GOVERNOR

FLYWEIGHTS

SPEEDER SPRING

GOVERNOR VALVE

Py

Px

Figure1.2-8



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Figure1.2-9



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Foracceleration,aninputforcefromthepowerleverincreasescompressiononthespeederspring.Thismovestheflyweightsofthegovernorinwardsandclosesthegovernorvalve.Withthegovernorvalveclosed,accelerationcontrolpressure(Px)andgovernorpressure(Py)bothincreasewithcompressorpressure(Pc),causingthebellowsassemblytogradually

open the metering valve. System design ensures that increasing fuel flow matchesincreasingairflowthroughtheengineandthataccelerationtakesplacewithoutriskofstallorsurge.WhenthedesiredRPMisreached,thegovernoragainmaintains“onspeed”RPM.

Duringdeceleration,thereversesequenceoccurs.Therateofdecelerationiscontrolledbythedecelerationbellows,whichensuressmoothdecelerationwithouttheriskofflameout.

The bi-metaldiscsare a typicalmeans ofsensing inletduct temperature. TheycontrolameteringdevicewhichaffectspressuresPxandPy.Thisreducestheaccelerationrateunderhotconditions,preventingexcessiveturbinetemperatureandtheriskofcompressorstallorsurge.

Hydro-mechanical Fuel Control System

Hydro-mechanicalFCU’sunitusefloworpressurecontroltoregulatetheflowoffuel.Flow Control

Flowcontrolunitsregulatethefuelsystembybypassingexcessunwantedfuelbacktotheinletsideofthefuelpump.

PriortothestartbeingactivatedtheFCUisinthefollowingconditions:

Fuelshutoffvalveclosed.

Powerleveratidle.

Governorspeederspringisinanexpandedcondition.

Governorflyweightsinanunderspeedcondition.

Burnerandinletpressurebellowsaresensingbarometricpressureandthemultiplyinglinkageisinthedecreaseposition.

Differentialpressureregulatingvalvewillbeclosed.

Meteringvalveisheldofftheminimumflowstopbythebalancedspringpressuresofthegovernorandmainmeteringvalve.

ReferFigure1.2-9:



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Figure1.2-10



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Starting

After thestartbutton ispressedtheenginebeginsto rotate, the flyweightsin thegovernorbegintoopen,overcominginitialspeederspringtensionmovingtherollercageupwardsthus

reducingthemeteringvalveopening.The fuel pump pressurises the fuel system until the relief valve pressure in the pump isreached.

WhentheenginehasacceleratedbythestartertoasetRPM,orafteracertainperiod,thefuelshutoffvalveisopenedcausing:

Fueltoflowtotheburnerscausingadifferentialpressureacrossthemeteringvalve,thereforethedifferentialpressureregulatorsensesthedifferenceandbeginstoregulatethefuelpressure.

Oncecombustioncommences,theenginebeginstoaccelerate,theburnerpressureincreasescausingtheburnerpressurebellowstomovethemultiplyinglinkageto

beginopeningthemainmeteringvalvethroughtherollercage. AstheengineacceleratestowardsidleRPM,thespeedgovernorandpressurebellowsbegintoregulatemeteringvalveopeningcommencinggovernedoperationatidlespeed.

ReferFigure1.2-10:



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Figure1.2-1.2



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Governed or Steady Operation

Duringgovernedorsteadyoperation,considerthatthepowerleverissetatacertainpositionandnotchanged.Aftertheenginespeedisset,theengineissubjecttocertainoperatingvariablessuchasaircraftspeedandaltitudetowhichitmustreact.

Ifanaircraft isincreasingspeedordescending itwill increases inletramair pressureandmassairflow.Alternativelyanaircraftthatisinaclimbandorslowingwilldecreaseinletairpressureandmassairflow.

Theinletandburnerpressurebellowssensethesechangesandmovesthemultiplyingleverinanappropriatedirectiontomaintainthefuelmixtureratio.Atthesametime,theenginespeedgovernorreactstoanyspeedvariations,movingthepilotservorodvalvetoreturntheenginetoasteadygovernedstate.

ReferFigure1.2-1.2:



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Figure1.2-12



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Acceleration

Movementofthepowerleverinanincreasedirection,causesthespringcaptoslidedownthepilotservovalverodandcompresstheflyweightspeederspring.

In doing so, the spring base pushes down and forces the flyweights in at the top to anunderspeedcondition,movingthepilotvalverodinadownwardsdirection.

Thepilotservovalvefunctionstoslowthemovementofthepilotservocontrolrodpreventingsuddenfuelratiochangesbyusingitsfluiddisplacedtoptobottomasarestrictor.

Whenthepilotvalverodmovesdown,therollerwillmovedowntheinclineplaneandtotheleft. As itmoves left, the roller will force themeteringvalve to the left against its spring,allowingincreasedfuelflowtotheengine.

Asfuelflowincreasesthedifferentialpressurevalvewill senseadecreaseddifferentialandclosetomaintainthedifferential.Withincreasedfuelflow,theenginewillspeedupanddrivethefuelcontrolshaft faster,as theenginespeed increases theburnerpressure increaseswhich expands the burner pressurebellows thatmoves the multiplying linkage to the left

furtherincreasingthefuelflow.

The new flyweight force will come to equilibrium with the speeder spring force as theflyweightsreturntowardanuprightposition.

Theyarenowinpositiontoactatthenextspeedchange.

ReferFigure1.2-12:



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Figure1.2-13



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Deceleration

Movementofthepowerleverinadecreasedirection,causesthespringcaptoslideupthepilotservovalverodandreleasepressureontheflyweightspeederspring. Indoingso,thespringbasemovesupandtheflyweightsmovetoanoverspeedcondition,movingthepilot

valverodinaupwardsdirection.

Thepilotservovalvefunctionstoslowthemovementofthepilotservocontrolrodpreventingsuddenfuelratiochangesbyusingitsfluiddisplacedtoptobottomasarestrictor.

Whenthepilotvalverodmovesup,themeteringvalvespringwillforcethemeteringvalveand the roller to the right as it moves up the inclineplane, allowing less fuel flow to theengine.

With decreased fuel flow, the differential pressure valve senses the increased differentialacross themeteringvalveandopens tomaintain the differential and the enginewill slowdownanddrivethefuelcontrolshaftslower,thisslowingoftheenginedecreasestheburnerpressure which through the bellows moves the multiplying linkage to the right further

decreasingthefuelflow. As the new flyweight force comes into equilibrium with the speeder spring force, theflyweightsreturntowardanuprightposition.

Theyarenowinpositiontoactatthenextspeedchange.

ReferFigure1.2-13:



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Figure1.2-14



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Shut Down

Priorto shutdowntheenginemustbeallowedtostabiliseatidleforaperiodtoensureagradual cooling of the turbine and scavenging of propeller control oil in turbo propellerengines.Onasimplifiedfuelcontrolunit,shutdowntakesthefollowingprocedure(Figure

1.2-14):

Withtheengineatidlegovernedspeed,thefuelshutoffvalveisclosed.

Whentheshutoffvalveisclosed,therewillbenofuelflowtogiveadifferentialfuelpressure,thusclosingthedifferentialpressureregulatingvalvecausingthefuelpumppressurereliefvalvetocontrolmaximumfuelpressure.

Oncecombustionceases,theenginespeedwillbegintodecreasesendingthegovernorintoanunderspeedcondition,atthesametimetheburnerpressurewilldecreasemovingthemultiplyinglinkagetoclosethemeteringvalve.

ReferFigure1.2-14:



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Figure1.2-15



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Hydropneumatic Fuel Control

Hydro-pneumatic fuel control units rely heavily on compressor discharge pressures tomaintainthecorrectairfuelratio.AcommonsystemisshownatFigure1.2-15.

Fuelissuppliedtothefuelcontrolunitatpumppressure(P1)whichisappliedtotheentrancetothemeteringvalve.Themeteringvalve,inconjunctionwiththemeteringheadregulatorvalvesystem,servestoestablishfuelflow.

Thefuelpressureimmediatelydownstreamofthemeteringheadbecomes(P2).Thebypassvalve maintains a constant fuel pressure differential (P1-P2) across the metering valveassuringthatfuelflowisafunctionofthemeteringheadorificeonly.

Operation of Control

Unmeteredfuelpressure(P1)issuppliedtotheFCUbythefuelpump

Thedifferentialmeteringheadregulatormaintainsaconstantpressuredropacrossthemeteringhead(P2).Ensuringconstantflow.

Fuelbypassedbacktopumpinletbecomes(Po)

Theairsectionisoperatedbycompressordischargeair(Pc).

Whenmodifiedthisairbecomes(Px&Py)whichacttopositionthemeteringvalve.

Tt2 Sensor

TheTt2 sensor acts tovaryPxbleed inlinewithvaryingair densityatidlepositionsthuspreventingidlestallproblemsthroughoverorunderfuelling. Thiscircuitlosesit’sauthorityabovetheidleposition.

When the Power Lever is Advanced

Theflyweightsdroopin,thespeederspringforcebeinggreaterthantheflyweightforce.

ThegovernorvalveclosesoffthePybleed.

Theenrichmentvalvemovestowardsclosed,reducingPcairflow(notasmuchairpressureisrequiredwhenPybleedsareclosed).

Px&Pypressuresequaliseonthesurfaceofthegovernor.

Pxaircontractstheaccelerationbellowsandthegovernorbellowsrodisforceddownward.Thediaphragmallowsthismovement.

Thetorquetuberotatescounterclockwiseandthemainmeteringvalvemovesto

open. TheflyweightsmoveoutwardsasenginespeedincreasesandthegovernorvalveopenstobleedPyair.

The enrichment valve re-opens and Px air increases over the Py value

ReducedPyvalueallowsthegovernorbellowsandrodtomoveuptoanewstabilisedposition.

Themeteringvalveresumesanewpositionthroughtheactionofthetorquerodassembly.



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When the Power Lever is Retarded

Theflyweightsmoveoutwards-speederspringforcebeinglessthanflyweightforceduetohighengineRPM

ThegovernorvalveopensdumpingPyair.Thebackupvalveisalsodepressed,dumpingadditionalPyair

Theenrichmentvalveopens,allowingincreasedPxairflow

Pxairexpandsthegovernoranddecelerationbellowstoit’sstop

Thegovernorrodalsomovesupandthemainmeteringvalvemovestowardsclose.

Pxairdecreaseswithenginespeeddecreasebuttheaccelerationbellowsholdsthegovernorrodup.

Asenginespeedslows,theflyweightsmovebackin,closingthePybleedatthegovernorvalveandthebackupvalve

TheenrichmentvalvemovestowardsclosedandPyairincreasesinrelationtothePxvalue

Thedecelerationbellowsmovesdownward.Themeteringvalvemovesslightlyopentoproduceastabilisedfuelflow



Part 66 Subject



Electro-hydromechanical Control System Operation

Electro-hydromechanical fuelcontrol systemsaresometimes referred to aselectronic fuelcontrolsbecausethemajorityofthesystemismadeupofelectroniccircuits.Becauseofthe

needtopreciselycontrolmanyfunctionsintheoperationofmodernhighbypassturbofanengines, electronicenginecontrolsystemshavebeendeveloped. Thesesystemsprolongengine life, save fuel, improve reliability, reduce crew workload and reduce maintenancecosts.Twotypesofelectronicenginecontrol(EEC)systemsinuseare:

SupervisoryElectronicEngineControl.

FullAuthorityElectronicEngineControl.

Supervisory Electronic Engine Control System

Essentially the supervisory electronic engine control system is a electronic device whichreceives information from various engine parameters and then limits the fuel flow to thehydromechanicalfuelcontrolandengine.

As can be seen in Figure 1.2-16 the control amplifier receives a signal from turbine gastemperature(TGT)andtwocompressorspeedsignals(N1andN2).

Thiscontrol,worksasahydromechanicalunituntilnearfullpower,whentheelectroniccircuitstarts to function as a fuel limiting device to control maximum TGT and, N1 and N2compressorspeeds.

Thepressureregulatorinthisinstallation,regulatesthefuelpressureatthefuelpumpratherthanthefuelcontrolunit.Nearfullpower,whenpredeterminedTGTandcompressorspeedvaluesarereached,thepressureregulatorreducesfuelflowtothespraynozzlesbyreturningincreasingamountsoffueltothefuelpumpinlet.

Thefuelflowregulator in thiscontrolactsasahydromechanical control, receiving signals

from high speed compressor (N3), gas path pressure (P1, P2 and P4) and power leverpositiontoregulatefuelflowtotheengine.

Figure1.2-16



Part 66 Subject



Full Authority Electronic Engine Control System

Fullauthorityelectronicfuelcontrolunitsuseanelectronicdevicethatsensesvariousinputsfromtheengineandpilottodeterminehowmuchfuelshouldbedeliveredtothefuelnozzles.

The full authority electronic engine control system performs all functions necessary tooperateaturbofanengineefficientlyandsafelyduringalloperatingconditionsfromstartuptoshutdown.

Benefitsofusingelectronicenginecontrolarereducedcrewworkload, increasedreliability,improvedreliability,andreducedfuelconsumption.

Flight crewworkloadis decreasedbecause thepilotutilisestheEPRgaugeto set enginethrustcorrectly.TheEECwillautomaticallyaccelerateordeceleratetheengineto theEPRlevelwithout thepilothaving tomonitor theenginegauges. Reducedfuel consumption isattainedbecausetheEECcontrolstheengineoperatingparameterssothatmaximumthrustisobtainedfortheamountoffuelconsumed.

Engine trimming iseliminatedby the use of full authorityEEC, as the engine fuel control

systemhasfaultsensing,selftestingandcorrectingfeaturesdesignedintotheEECgreatlyincrease the reliability and maintainability of the system. The only adjustments that arecarriedoutbythemaintainerisspecificgravityandidleRPM.

TheEECisprovidedwithfeedbackviavalvesandactuatorsfittedwithdualsensors.

Theelectroniccomputermayhavemanyinputsandoutputsincluding:

N1 Fanspeed.

N2 Intermediatepressurecompressorspeed.

N3 Highpressurecompressorspeed.

Tt2 Inlettotaltemperature.

Tt8 Highpressureturbineinlettemperature.

Pt2 Inlettotalpressure.

28V DC Inletpower.

PMG PermanentmagnetACpower.

PLA Powerleverangle.

IGV AInletguidevaneangle.

Ps6 Highpressurecompressordischargestaticpressure.

Wf Fuelflow. ACC Activeclearancecontrol(compressorandturbineblade.Coolingair suppliedbyfanair).

EPR Enginepressureratio.



Part 66 Subject



Toprovideahighdegreeofreliability,FADECsystemsaredesignedwithseveralredundantanddedicatedsubsystems.AnEECconsistsoftworedundantchannels(AandBchannels)that send and receive data. Each channel consists of its own processor, power supply,memory,sensors,andactuators.Inaddition,anyonechannelcantakeinformationfromthe

otherchannel.Thisway,theEECcanstilloperateevenifseveralfaultsexist.Asasecondbackupshouldbothchannelsfail,theactuatorsarespringloadedtoafailsafepositionsothefuel flow will go to minimum. If both channels are serviceable, the Active channel willalternatewitheachenginestart.TheotherchannelisinStandbymode.Powermanagementcontrolstheenginethrustlevelsbymeansof throttleleverinputs.Itusesfanspeed(N1)asthethrustsettingparameter.

As shown in Figure 1.2-17, the full authority electronic engine control receives data fromvarious areas, then analyses the data and sends commands to position the Inlet GuideVanesandschedulefuelflowthroughthehydro-mechanicalsectionofthefuelcontrolunit.

Figure1.2-17



Part 66 Subject



Fuel System Maintenance

Properandregularinspectionofaircraftfuelsystemsiscriticaltothesafeoperationoftheaircraft. The failure ofa componentmay result inanengine failure due to insufficient orexcessfuelbeingthedeliveredtotheengineorcouldresultinafireinoraroundtheengine.

While the following text providesgeneral informationconcerning engine fuel systems, themanufacturers specific guidelinesmust be followed when performingmaintenanceof gasturbineenginefuelsystems.

Routine maintenance of gas turbine engine fuel systems include the following genericinspectionsandoperations:

Allfuellinesforleaks,chafedorfrayedwallsandcontactwithothercomponents.

Cleaningandinspectingfuelfilters.

FCUcontrolsforserviceabilityandrigging.

Lowandhighpressureshutoffvalvesforoperationandsealing.

Drainvalvesforoperationandsealing.

Pressurisinganddumpvalvesforoperationandsealing.

Fuelheatersforleaksandoperation.

Oilcoolersforleaksandoperation.

Pressuresensinglinesforrestrictions.

OnenginesfittedwithEEC,performselftestandanalysisofthecomputersystem

Bleedingofanytrappedairinsystemsthatarenotselfbleedingafterdisturbinganyfuelsystemcomponent.

Regularengineperformancechecks.

Anymaintenancethatmaybedeemednecessaryfollowinginspectionoftheenginefuelorrelatedsystems.

Fuel System Faults Table 1)

Fuel systems, Fuel Pumps and Fuel Control Units can cause a wide variety of enginemalfunctions someofwhichmay be difficult toanalyse. A thorough understandingof thesystemanditscomponentsisessentialif thetechnicianhopestoresolvetheproblemsofaparticularsystemeffectively.

The following chart lists some common problems encountered with fuel systems andsuggestsgenericremedies.

Technicians should analyse the type of system on which they are working and becomefamiliarwiththeoperationofthefuelcontrolandothercomponentsusedinthatsystem.



Part 66 Subject



INDICATION POSSIBLE CAUSE REMEDY

Enginemotorsoverbut

doesnotstart.

Improper rigging of shut off

valve.

Checkandre-rigairframeand

enginelinkages. Cloggedoricedfuelfilters. Clean.

Malfunctioningfuelpump. Checkandorreplace.

Malfunctioningfuelcontrol. Replacecontrol.

Pressurising and dump valvestuckopen.

Replacevalve.

Enginestarts,butwillnotacceleratetocorrect

speed.

Insufficientfuelsupplytocontrolunit.

Check fuel system to ensureall valves are open andpumpsareoperative.

Fuel control main meteringvalvesticking.

Flush system. Replacecontrol.

Fuel control bypass valvestickingopen.


Drainvalvestuckopen. Replacedrainvalve.

Starting fuel enrichmentpressure switch setting toohigh.

Replacepressureswitch.

Control has entrapped airpreventingproperoperation.

Bleed control as permaintenancemanual.

EGTtoolowduringstart.

Accelerationcam in fuelcontrolincorrectlyadjusted.

Re-trimasrequired.

EGTtoohighduringstart.

Fuel control bypass valvestickingclosed.


Fuel control acceleration camincorrectlyadjusted.

Replacecontrol.

Defectivefuelnozzle. Replacenozzlewith a knownserviceableitem.

Fuelcontrolthermostatfailure. Replacecontrol.

Pressurisation and dump valvewitheithervalvepartiallyopen.

Replace pressurisation anddumpvalve.

EnginehashighEGTattargetenginepressureratiofortakeoff.

Engineoutoftrim. Re-trimasrequired.

Enginerumblesduringstartandatlowpowercruiseconditions.

Pressurising and drain valvemalfunction.

Replace pressurising anddrainvalve.

Fuelcontrolmalfunction. Replacefuelcontrol.



Part 66 Subject



INDICATION POSSIBLE CAUSE REMEDY

EngineRPM“hangs-up”

duringstart.

Lowambienttemperatures. If hang-up is due to low

ambient temperature, engineusually can be started byturning on fuel boosterpumporbypositioningstartlevertorun earlier in the startingcycle.

Engineunabletoobtaintakeoffpower.

Incorrectcontrolrigging. Check or re-rig engine andairframe.

Partiallycloggedfuelfilters. Cleanfilters.

Incorrectfuelpumppressure. adjust pressure or replacepump.

Incorrect control outputpressure.

Re-rigorreplacecontrol.

Highfueltemperature. Valvestuckopeninfuelheater. Replacefuelheater.

Highfuelconsumption. Fuelsystemleak. Repairasrequired.

Dump valve stuck partiallyopen.

Replace pressurisation anddumpvalve.

Lackofthrottleresponsefrommaximumcontinuous.

Fuelcontrolunitinternalfailure. Replacecontrolunit.



Part 66 Subject



Factors Controlling FCU Performance

TheFCUmust sense the variousoperating and environmental parameters toenable it tosupplytheenginefuelinthecorrectquantities.ParametersthatdirectlyeffecttheFCUare:

Powerleverangle.

RPM.

Airtemperature.

Airpressure.

Burnerpressure.

Fueldensity.

Power Lever Angle

The power leverangle is the pilotsmain controlover the engine. The power leverangle

schedules the fuel required to the enginewithout taking into account the otheroperatingparameters.

RPM

Tobeabletoproducethevaryingpowersrequired,theenginemustbeabletooperateatdifferentspeeds.RPMissensedsothattheFCUcanprovidetheappropriatefuelflowfortheRPMatwhichtheengineisoperating.

Air Pressure

Asexplainedin Fundamentals,air pressurehasadirectrelationshipwith theairdensityormassie.ifweweretotakeasealedballoonofairfromsealevel,toabout16500,feettheballoonwouldhaveexpandedtotwiceitssize,wewouldthenhavehalveditspressure.

For a turbineengine, an increase inaltitude / a decrease inair pressure, will reduce theweightofthetotalairmassthatwillflowthroughtheengineatagivenRPM.

Air Temperature

As explained in Trade Fundamentals, air temperature has a direct relationship with airdensity, ie. an increase in temperaturewill give an increase in volume. Therefore for aturbineengine,anincreaseinairtemperaturewillreducetheweightofthetotalairmassthatwill flow throughthe engineatagivenRPM, requiring the FCU toreduce the fuel flow tomaintainthecombustionprocess.AscanbeseeninFigure1.2-18,temperaturehasaneffectontheengineperformance.

Figure1.2-18



Part 66 Subject



Burner Pressure

Staticpressureinthecombustionchamberisausefulmeasureofmassairflow.Ifthemassairflowisknown,theair/fuelratiocanbemorecarefullycontrolled.Asaircraftusebleedairfrom the engine compressor to provide various services, it is imperative that the burner

pressureisknowntoprovideanaccuratefuelregulationwhentheseservicesarebeingused.

Combustion chamber pressure and inlet pressure acting through bellows and leverassembliescangiveaccuratecontrolofthefuelbeingintroducedintotheenginetocontroltheair/fuelmixture.

Fuel Density

As the different types of fuels that may be used in gas turbine engines have differentdensitiesorspecificgravities,thefuelcontrolunitneedsamethodofbeingabletoadjustforthevariousflowsthatoccurifdifferentfuelsareused.

Variationofthefueldifferentialpressurevalvespringtensioncanbeusedtochangethefuelflowtoaccommodatefordifferentfuel’sspecificgravity.

Specificgravityadjustment,shownatFigure1.2-19,isameansofresettingthetensiononthedifferentialpressureregulatorvalvespringwithinthefuelcontrolwhenanalternatefuelisused.

Figure1.2-19

Fuel control unit components

Toadjusttovaryingconditionsandthrustrequirements,anFCUhasdifferentcomponents

fittedthatreacttoensurecombustioniskeptwithinallowablelimits.Thesecomponentsare:

Speedgovernors.

Differentialpressureregulator.

Accelerationgovernors.

Pressuresensors.



Part 66 Subject



Speed governors

The FCU’sspeedgovernor(s) senseengine speedand act tomaintain the desiredRPM.Whendifferentloadsareapplied,flyweightsandaspeederspringoperate leversorbleed

controlstoadjustthemeteringvalveopeningallowingtheenginetomaintainthesetRPM.Differential Pressure Regulator

Thepressure regulatingvalvediaphragm isexposedononeside topumpoutlet pressureandontheothersidetothecombinedeffectofthrottlevalvedischargepressureandaspringforcepresettomaintainthedesiredpressuredropacrossthethrottlevalve.Withaconstantpressuredropacrossthethrottlevalve,flowthroughthethrottlevalvewillbeproportionaltoitsorificearea.Onflowcontrolsystems,anyexcessfuelabovethatrequiredtomaintaintheset pressuredifferential isbypassedback tothe inletsideofthe fuelpump. Onpressurecontrol systems, the pressure differential regulator controls the piston pump swash plateangle,thereforecontrollingpressureandflow.

Acceleration limiters

Precisecontroloffuelflowisnecessaryforgoodaccelerationresponsewithoutriskofturbineovertemperatureandcompressorstallorsurge.

Thefuelcontrolunitmustalsopreventoverrichmixturesduringacceleration,andoverleanmixturesduringdeceleration,asbothcancauseflameout.Theformerisrarelyaproblembecausethemaximumturbinetemperatureoccursbeforetherichlimitisreached.

Largeengineswiththeirhighinertiarotatingpartsaremoredifficulttoaccelerateandcontrolthansmallerengines.Theyusuallyhavecomplexaccelerationcontrolsystemswhichreactto RPM, inlet temperature, inlet pressure and compressor discharge pressure. Theseparameterscontrolthepositionoftheaccelerationcam,whichinturncontrolsthefuelflowtoallow the maximum acceleration rate (the rate varies with temperature, RPM, and

compressorpressureratio).Simpler acceleration control systems can be used on smaller engines, because theseengines have low inertia rotating parts, which naturally gives them a good accelerationresponse.

Pressure Sensors

ThepressuresensorsofanFCUaresubjecttoinletandcompressoroutletairpressuresandacttoeffectthemeteringvalveopeningthereforecontrollingfuelflow.



Part 66 Subject B2-14 Propulsion

B:January2008 Revisi 1

AA Form TO-19

B2-14.1.2EngineFuelSystemsIssue on Page34of34




Part 66 Subject


B2-14.2IndicatingSystemsIssueB:January2008 Revision1 Page1of26

TOPIC14.2:ENGINEINDICATIONSYSTEMS

Althoughengine installationsmaydiffer,dependingonthetypeofbothaircraftandengine,gasturbineengineoperationisusuallycontrolledbyobservingsomeoralloftheinstruments.

Engineindicationsaredividedintothreegroups.Theseare:

Performanceinstruments.

Conditioninstruments.

Warningsystems.

Performance Instruments

Performance instruments allow the operator, at a glance, to monitor the output orperformanceoftheengine.Thisisdonebycheckingthethrustonturbojetenginesorthe

horsepowerforturboprops.Thetwomainperformanceinstrumentsare:

EnginePressureRatio(EPR).

Torque.

Engine Pressure Ratio EPR)

Enginepressureratioisameasureofthethrustbeingdevelopedbytheengine.WhenEPRis measured the ratio is usually that of turbine discharge pressure to compressor inletpressure,however,onafanenginetheratiomaybethatofturbinedischargepressureandfanoutletpressuretocompressorinletpressure.

Suitablypositionedpitottubessensethepressuresappropriatetothetypeofindicationbeingtakenfromtheengine.ThesepitottubesareeitherdirectlyconnectedtotheindicatorortoapressuretransmitterwhichsendsanelectricalsignaltotheindicatorasshowninFigure2.1.

Figure2-1.



Part 66 Subject



EPR reflects the difference between the compressor inletpressureand turbine dischargepressure ie. the amount of work the engine is doing on the air. For example, an EPRindication of 2.4 means that the turbine discharge pressure is 2.4 times greater thancompressorinletpressure.

An example of how, when planning a flight, pilots use ambient temperature and apredetermined “Takeoff Thrust SettingCurve” tocalculate the EPR required for takeoff isillustratedinFigure2.2.

Figure2-2.

Withtheambientairtemperatureoftheairfield20°CandusingthegraphabovethepilotwillcalculatetheE.P.R.requiredfortakeofftobe2.6.

Thisfigureiswhattheengineshoulddevelopwhenoperatingat,ornear,fullthrottle.



Part 66 Subject



Torque

Enginetorqueisusedto indicatethepowerdevelopedbyaturbo-propellerengine,andthe

indicatorisknownasatorquemeterasdepictedinFigure2.3.

Figure2-3.

Theenginetorque,orturningmoment,istransmittedthroughthereductiongearboxtothepropeller.Thetorquemetersystemistheprimaryperformanceinstrumentforturbo-propellerengines.

An explanation of two different types of torquemeter systems is covered in the followingparagraphs.Theseare:

Hydraulictorqueindicatorsystem.

Torqueshaftindicationsystem.

Hydraulic Torque Indicator System

TheHydraulic Torque Indication system indicates torquebymeasuring hydraulic pressurecreatedbyatorquemetersystem.Thetorquemetersystemformspartofareductiongearassembly between theenginedriveshaftand thepropellershaft. The construction ofthesystemdependsonthetypeofengine,butallarebasedonthesameprincipleofoperation.

Thedriveshaftfromtheenginesuppliesatorquetothereductiongearassembly.Thisdrivestheplanetgearsaroundinthesamedirectionbutatafractionoftheenginespeed.Astheplanetgearsrotate,thepropellerrotatesaswell.

Thepropellerconvertsthisrotationforceintothrust.Todothistherotationofthepropellerisresistedduetoaerodynamicforces.Thisresistancecausestheplanetgearstotransferaportionofthetorquetothestationaryringgear.Figure2.4showshowthisoccurs.



Part 66 Subject



Figure2-4.

AsshowninFigure2.5theringgearmovementis resistedbypistonsworkinginhydrauliccylinderssecuredtothegearboxcasing.Oilissuppliedtothecylindersfromaspecialpumpandisallowedtodrainviaacalibratedbleedline.

Theoilissubjectedtoapressurewhichisproportionaltothetorqueorloadwhichisappliedtothepropellershaft.Thisoilpressureissensedbyabourdontubewhichiscoupledtoasynchrotransmitter.

Asimplesynchroindicatorinthecockpitdisplaysthetorqueinformation.

Figure2-5.



Part 66 Subject



Torque Shaft Indication System

Anothermethodofobtaininganindicationof torqueisbymeasuringtheamountoftwistinashaftcalledatorqueshaft.

The torque shaftconnects the engine to the propeller reduction gearbox. Ahollow shaft,called a reference shaft, ismounted so that it formsa sleeve around the torque shaftasshowninthecutawaydiagraminFigure2.6.

Figure2-6.

Figure6.6showsthatthetorqueshaftisconnectedtoboththeengineandthegearbox.Theenginerotatesandthepropellerisdraggedthroughtheair.Thepropellerwilllagslightly,thiscausesthetorqueshafttotwistslightly.

Thereferenceshaftisnotsubjectedtoanytorqueasitisonlyconnectedtotheengine.

Ontheendofbothshaftsisagear,calledanexciterwheel.Amagneticpick-upassemblyismounteddirectlyaboveeachexciterwheelasshowninFigure2.7.

Figure2-7.



Part 66 Subject



Astheshaftsrotate,theteethoftheexciterwheelspassbythemagneticpick-upassemblies.Eachtoothcausesapulsetobegeneratedbyitspick-upassembly.

When the engine isnot delivering anypowerto the gearbox, the teethonthe torque andreferenceshaftswillbealignedasshowninFigure2.8.

Figure2-8.

Whentheengineisdeliveringpowertothegearbox,thetorqueshaftwillbesubjectedtoatorquethatwillcauseittotwistslightly.Thisresultsintheteethonthetorqueshaftbecomingmisaligned with the teeth on the reference shaft (remember the reference shaft is notconnectedtoanythingandthereforewillnottwist)asshowninFigure2.9.

Figure2-9.



Part 66 Subject



Thissystemhas two othercomponents,aphase detector, anda torque indicator. Figure2.10showsacompletesystem.

Figure2-10.

Thesignalsfrombothpick-upassembliesarefedtothephasedetector.

The phase detector calculates the difference between the two signals and generates anoutputthatrepresentsthetorquethatisbeingmeasured.

Theoutputfromthephasedetectorisusedtodriveapointerinthetorqueindicator.

Condition Instruments

Condition instruments show the operator how hard the engine isworking to produce the

powerseenontheperformanceindicators.

Engineconditioninstrumentsinclude:

Gastemperature.

Fuelflow.

Compressorspeed.

Oiltemperature.

Oilpressure.

Inletairtemperature.

Enginevibration.

For free turbine engines, engine rpm is broken down into free turbine rpm (Nf) and gasgeneratorrpm(Ng).Forturbojetengines,enginerpmisbrokendownintolowpressurespoolrpm(N1),andhighpressurespoolrpm(N2).

Therelationshipbetweeninstrumentindicationsisaveryimportantguidetoenginecondition,efficiencyandperformance.Forinstance,iftorqueoilpressureorenginepressureratioislower than normal for a particular combination of turbine temperature, fuel flow, rpm, airtemperature, aircraft altitude and airspeed, then a loss of engine performance can besuspected.

Byanalysing instrument indications, flight crews andmaintenancepersonnel can forecast

troubleandtakepreventativeactionbeforeamajormalfunctiondevelops.Thisisknownas"trendmonitoring".



Part 66 Subject



Pressure Sensors

Figure2.11showsthecommonbasicdevicesusedforsensingpressure.Theyareusedtoactuatefluidicvalves,indicatorpointers,switchesandelectricalsignaltransmittersincontrol

andinstrumentapplications.

Theymaybedesigned,calibratedandconnectedtosense

Absolutepressure-thepressureabovethezeroofacompletevacuum.

Gaugepressure-thepressureaboveorbelowthe'ambient'(surrounding.atmosphereor,

Differentialpressure-thedifferencebetweentwopressures.

A flexible 'diaphragm'separating two chambers,as in (a), is sensitive to the difference inpressureeachsideofit.Thediaphragmdeflectsintothechamberwiththelowerpressure.Itisusually corrugated to increase its movement. If the chamber onone side isvented to

atmosphere,diaphragmdeflectiondependsonthegaugepressureintheotherchamber.Insomeapplicationsthepressuresinbothchambersmaydifferfromatmosphericpressureandfromeachother.

The'capsules'in(b)and(c)aremadefrompairsofdiaphragmsjoinedattheiredges.Apairofdiaphragmsformedintoacapsuleismoresensitivethanasinglediaphragmofthesamearea,thicknessandmaterial.

Sensitivitycanbefurtherincreasedbystackingcapsulesasin(c).Theamountacapsuleexpandsorcontractsdependsonthedifferenceinpressurebetweentheinsideandoutsidesurfaces. In(b) and (c) the capsulescouldbe 'plumbed' todifferential pressureor gaugepressure. Evacuating the capsules and then sealing them makes them sensitive to theabsolute pressure on their outside surfaces. They are then called 'aneroid' (without air)

capsules.'Bellows'liketheonein(d)arecylinderswithcorrugatedsidesthatallowthemtobereadilylengthenedwheninsidepressureishigherthanoutsidepressure,ortoshortenwhenoutsidepressureishigherthaninsidepressure.Theymaybeplumbedtosensegaugeordifferentialpressuresortheycanbeevacuatedandsealedtomakethemsensitivetoabsolutepressureontheirexternalsurface.

'Bourdontubes'arecurvedandhaveanovalcross-sectionasshownin(e).Pressureappliedtotheinsideofthetubetendstochangethecross-sectionfromovaltoround.Thiscausesthe tube tostraighten resulting inanoutwardsmovementof its freeend. Thereare also'helical'and'spiraltubes'asin(f)and(g)thatgivegreateroutputmovement.



Part 66 Subject



Thepressuresensorsillustratedandvariantsofthemareusedinmanyaircraftsystemsandcomponentsincluding

Enginefuelmeteringsystems.

Engineairsystems.

Indicatinginstrumentsthatmeasurealtitude,airspeed,MachNo.,verticalspeed;oilfuelandgaspressures;andtemperature.

Figure2-11.



Part 66 Subject



Pressure Indicator

A pressure capsule similar to (B) at Figure 2.11 is shown at Figure 2.12 installed in aninstrumentcase.Throughport"B"pressureissuppliedtotheinsideofthecapsule.Throughport "A" the case of' the indicator is vented to atmosphere or "ambient" pressure. Aspressure increasesaboveambient,thecapsuleexpandsandthroughtheleverandrockingshaft,thesectorgearismoved.Thepiniongearnowrotatesthepointeragainstthetensionofahairspring.Theindicatorwillreadinpoundstothesquareinchorthemetricequivalent.Thisreadingwillbe"gaugepressure”andwillvaryduetopressurechangesinsideoroutsidethecapsule.

Considerhowthisindicatorcouldbeadaptedtoread(a)airspeed,(b)altitude.

Figure2-12.



Part 66 Subject



Oil Pressure Warning

General

Theoilpressurewarningsystemprovidesindication inthe flightcompartmentwhenengine

oilpressureisbelowapredeterminedsetting,orscavengefilterdifferentialpressureisaboveapredeterminedsettingasshownatFigure2.13.

Oil Pressure Light

Theoilpressurewarninglightprovidesanindicationwhenengineoilpressuredropsbelowspecifiedlimits,orscavengefilterdifferentialishigh.Fourlights,oneforeachengine,arelocatedonthePilot'sCentreInstrumentPanel.

Low Oil Pressure Warning Switch

Thelowoilpressurewarningswitchismountedonanadapterassemblyontherearfaceofthehighspeedexternalgearbox.Theswitchconsistsbasicallyofametalbodythathousesanelectricalswitchandconnector,andapressuresensingbellowstowhichoilissupplied

throughtwoholesinthemountingbase.Feedoilissuppliedtotheinsideofthebellowsandreturnoilissuppliedtothechambersurroundingthebellows.Expansionofthebellowsisopposedbyasnapactionspring,whichpreventsthebellowsfromactuatingtheswitchuntilapredeterminedoilpressuredifferentialisreached.

Adecreaseinfeedoilpressureoranincreaseinreturnoilpressurewillcontractthebellowsand,atthepredetermineddifferentialpressure,actuatetheswitchtocompletethecircuit tothewarninglight.Thisdifferentialpressureissetat19-23PSIDforincreasingpressuresand20-16P510fordecreasingpressures.

Filter Pressure Differential Switch

The Filter Switch is mounted on the same assembly as the low oil pressure switch, itspurposeistoprovideawarninglightearthifthefilterisblockedbeyondacceptablelimits.Its

twopipelinesareconnectedonetothefilterinletandonetothefilteroutlet.If thepressuredifferencebetweenthesetwopointsexceedsnormalvaluestheswitchclosesandcompletesthecircuitforthewarninglight.



Part 66 Subject



Figure2-13.

Oil Pressure Indicating System

General

Theoilpressure indicatingsystemprovidesvisual indicationon the flight engineer's lowerinstrument panel of oil supply pressure and main filter differential pressure. Systemcomponents for each engine consist of a supply pressure transmitter, a main oil filterpressuredifferentialtransmitter,andadualindicatingoilpressure/differentialindicator.

Oil Pressure Transmitter

The oil supply pressure transmitter basically consists of a cylindrical case, housing, twoidenticalstatorwindingssurroundanarmaturecarried-onacentralspindle,whichlocatestoacapsulestack.Twoholesinabaseplatealignwithholesinthemountingfacesothatone

connectstomain feedoilpressure,which is routed toachambersurroundingthe capsulestack,andonetoreturnoilpressure,whichisroutedtotheinsideofthecapsulestack.

Variation in the differential oil pressure causes the capsule stack to expand or contract,impartinglinearmovementtothespindleandarmature.Theresultantchangeininductanceofthestatorwindingsandthereforetheratioofcurrenttotheindicatorcircuitisshownasanincreasedordecreasedindicatorreading.

Oil Filter Pressure Differential Transmitter

Theoilpressurefilterinlettransmitterismountedontheoilpressurefilteratthefrontfaceofthehigh-speedexternalgearbox. Itissimilartotheoilpressuretransmitter,butitscapsulestacksensesoilpressureintotheoilfilterandoutoftheoilfilter.



Part 66 Subject



Temperature Indicating Capillary

Temperature Sensors

Thisbasictypeof"temperaturesensor"reliesontheexpansionandcontractionofliquidsandgases.

A 'Capillary' (small bore tube) type consisting of a temperature sensing bulb, a movingelement suchasa bourdon tube,andaconnectingcapillary tubeall completely filledwithmercury or alcohol. Changes of temperature vary the volume of the liquid. This in turncauses the bourdon tube to straighten with increasing temperature, or to curl more withdecreasingtemperature.

Thiscouldalsobea 'vapourpressuretemperaturesensor'. It issimilar to the expandingliquidtypedescribedaboveexceptthatatnormaldaytemperaturethebulbispartlyfilledwithavolatileliquid,andtherestofthesystemisfilledwithvapourfromthatliquid.Theamountof vaporisation and hence the pressure and bourdon tube movement varies with the

temperatureatthebulb.Thistypeoftemperaturesensorissuitedtoaircraftapplicationsbecausethesensingbulbcanberemotelylocatedfromtheindicator.It isusedasanengineoiltemperatureindicatoronmanylightaircraft.

Figure2-14.



Part 66 Subject



BI-Metal Temperature Sensor

Figure2.15illustratestheactionof'stripand'disc'typesof'bimetallictemperaturesensor'.Twometalsofhigh(brass)andlow(invar)temperaturecoefficientsarebondedtogether.Atsomedatumtemperaturethestripat (a)isstraight. Ifthestripisheatedthebrassexpands

morethantheinvartocauseittocurlasat(b).Ifthestripiscooledthebrasscontractsmorethantheinvartocauseittocurltheoppositewayasat(c).

Discshapedbimetallicsensorsarecommoninapplicationsrequiringasnapaction.Whenheated,aslightly domedbimetallicdiscwillsuddenlysnapacross tobeingdomedon theoppositeside.See(d)and(c).

Bimetallictemperaturesensorsareused:

Intemperatureindicators.

Astemperaturecompensatorsandcorrectorsinvariousinstrumentsandmechanisms.

Tooperateswitchcontactsincircuitbreakers,firedetectors,thermostatsandtimers.

Figure2-15.

Resistance Bulb Temperature Sensors

Theresistancewire,whichistheessentialfeatureoftheresistancebulb,restsinthespiralgroovesofaninsulatingmaterialandiscoveredwithametalshield,whichconductsheattoand from it very quickly. (See below) This metal shield must be able to withstand thecorroding influenceofengineoilsathigh temperatures, thehigh flash temperatures in thecarburettorofabackfiringengine,andthedeterioratinginfluenceoftheatmosphere.Eventhough the resistance bulb is covered with a metal shell and substantial insulation, itrespondstochangesintemperatureveryrapidly.Thissensitivityisimportantbecausethemembersof the flight crewarenot interestedinpasttemperatures; theywant toknowthesituationattheexactsecondthattheinstrumentisread.

Theactionofaresistancebulbmaybeunderstoodbystudyingthegraphbelow.Itwillbe

notedthattheincreaseinresistanceofatemperaturebulbisalmostlinearwithrespecttotemperaturechanges.



Part 66 Subject



A typical application of the resistance bulb temperature sensor is the engine oil tempindicatingsystem.Whenthebulbisconnectedtoasuitablecircuit,suchasaratiometerorwheatstonebridge,itwillindicateariseinmeterreadinginalinearfashionasitheatsup.The bulbis immersed intheengineoil and anelectricalconnectorplugconnects it tothe

indicator,whichissuppliedwith28vd.c.Becauseofthepositivetemperatureco-efficientofresistance,anopencircuitwillcauseafullscalehighreadingandashortedbulbwillreadfullscalelow.

Ratiometer Temperature Indicator

Aschematiccircuit illustratinghowa resistancebulb is connectedina ratiometercircuit isshownbelowatFigure2.16.Notethatthevoltagefurnishedbyabatteryisdividedbetweenthecircuitsofthetwocoilsby the fixedresistor inonesideandtheresistancebulb intheother. The series and shunt resistancesshown are for the purposeofcompensationandadjustment. Itisobviousfromthecircuitthatthecurrentthroughthetwosidesofthecircuitwillbeequalonlywhentheresistanceofthetemperaturebulbisequaltotheresistanceofthe fixed resistor. At this point themoving coils assume positions in fields of equal flux

density,asshown.Anychangeintheresistanceoftheresistancebulbwillcausetheratioofthecurrentstochangeandthecoilstoshifttoanotherposition.

Ratiometerthermometersmaybeusedforavarietyoftemperatureindications,amongwhicharethoseofinletairinajetengine,freeair,andengineoil.

Becausethepointerismovedbytheratioofcurrentinthetwocoilsthesystemdoesnothaveerrorsduetovariationsinsupplyvoltage.

Figure2-16.



Part 66 Subject



Thermocouple Instrument Systems

Oeofthemaincharacteristicsandadvantagesofthermocouple-typetemperaturemeasuringinstruments is their complete independence of the electrical system of the aircraft.Thermocouple-type instruments are used to measure cylinder head temperature (CHT),

turbineinlettemperature(TIT),andexhaustgastemperature(EGT)onreciprocatingengine-poweredaircraft.Onturbineengine-poweredaircrafttheyareusedtomeasuretheexhaustgastemperature(EGT),turbineinlettemperature(TIT),orintermediateturbinetemperature(ITT). Regardlessof the parameter theymeasure, these instruments work on the sameprinciple.

AsshownatFigure2.17,whenthejunctionofwiresmadeoftwodissimilarmetalsisheated,currentwillflowfromthejunctionthroughoneofthewires,throughthecoilofthemeasuringinstrument,andbacktothejunction.Theamountofthiscurrentisdeterminedbytwofactors:by the resistance of the circuit and by the temperature difference between the hot, ormeasuringjunction,andthecold,orreferencejunction.

Figure2-17.



Part 66 Subject



Exhaust Gas Temperature

The complete EGT system for a turbine engine consists of: the probes that sense thetemperatureof theexhaustgas,theharnessthatsurroundstheenginetailpipeandservesasconnectionforalloftheprobes,extensionwiresthatcarrythecurrentfromtheprobesinto

thecockpit,resistorstoadjusttheresistanceof thethermocouplestothevaluerequiredforthesystem,andthe indicating instrument in theaircraft instrumentpanel. Theprobesaremounted in the tail pipeandare connected inparallelso that theiroutput isaveragedasshownatFigure2.18.

Some EGT systems use as their indicator a special form of direct current measuringD’Arsonvalmetermovement very similar to theonesued inreciprocatingenginesystems.ButsomeoftheothersystemsfeedtheoutputofthethermocouplesintoanelectroniccircuitwheretheDCvoltagefromthethermocouplesisconvertedintopulsatingDCwhichisfedintoaservo-typeinstrument.Thistypeofindicatorcangivethepiloteitherananalogoradigitalreadoutand,inmanyinstances,bothtypes.

Figure2-18.

EGT Thermocouple Probes and Harness

Probes

Eachoftheeightprobescontainstwochromel-alumelthermocouplejunctionsencasedinaswaged stainless steel housing insulated with magnesium oxide. The junctions are atdifferentimmersiondepthswithaprotectivesleevedrilledtoprovidepositivegascirculation.

Theprobesareinstalledusingatwo-boltmountingflangeattachedtoamountingbossontheengine. The probes are permanently connected into pairs using a steel tube that alsoencasestheelectricalleads.

TheeightthermocoupleprobesareconnectedinparallelandtheindicatorreadstheaverageoftheE.M.F.generatedineach.



Part 66 Subject



Tachometers RPM)

Non-electrical tachometers

Almostallofthesmallgeneralaviationaircraftusenon-electricalmagnetic-dragtachometers.ThemechanismintheseinstrumentsisthesameasthatusedinanautomobilespeedometerandisshownatFigure2.19.

Analuminiumcupfitscloseoverthespinningmagnetbutitdoesnottouchit.Asthemagnetspins,itslinesoffluxcutacrossthealuminiumcupandinducesavoltageinit.Thisvoltagecauses current (eddycurrent) toflow inthealuminium,and thiseddycurrentproducesitsownmagneticfieldthatopposesthefieldthatcausedit.Thetwofieldsproduceatorquethatrotatesthedragcupagainsttherestraintofacalibratedhairspring.Thefasterthemagnetspins,thegreatertheeddycurrentandthegreateritsmagneticfield,andthemoredragcupwillberotated.Thedragcupissupported,inabrassbushingbyasteelshaftWhentheengine is not running, the restraining hairspring holds the drag cup over so the pointerindicateszeroRPMonthedial

Figure2-19.



Part 66 Subject



Electrical RPM Indication

Adirect-drivea.c.generatorisatFigure2.20,therotorbeingeithertwo-poleortwelve-pole,anddrivenviaasquare-endedshaft.Thetwopolegeneratorisutilisedinconjunctionwithathree-phase synchronous motor type of indicator, while the twelve-pole generator, which

produces a single-phase output at a high frequency is utilised in conjunction withcounter/pointerindicators,andalsoforsupplyingsignalstoenginecontrolunits.

Atypicalindicator,shown in(B)consistsof twointerconnectedelements:a drivingelementandaneddy-current-dragspeed-indicatingelement.

Letusconsiderfirstthedrivingelement.Thisis,infact,asynchronousmotorhavingastar-connectedthree-phasestatorwindingandarotorrevolvingontwoballbearings.Therotorisofcompositeconstruction,embodyinginonepartsoft-ironlaminations,andintheotherpartalaminatedtwo-polepermanentmagnet.

Analuminiumdiscseparates the twoparts,anda seriesof longitudinalcopperbarspassthrough the rotor forming a squirrel-cage. The purpose of constructing the rotor in this

manner is tocombine' theself-startingandhigh torquepropertiesofasquirrel-cagemotorwiththeself-synchronouspropertiesassociatedwithapermanent-magnettypeofmotor.

Thespeed-indicatingelementconsistsofacylindricalpermanent-magnetrotorinsertedintoadrumsothatasmallairgapisleftbetweentheperipheryofthemagnetanddrum.Ametalcup,calledadragcup,ismountedonashaftandissupportedinjewelledbearingssoastoreducefrictionalforcesinsuchawaythatitfitsoverthemagnetrotortoreducetheairgaptoaminimum.

Acalibratedhairspringisattachedatoneendofthedrag-cupshaft,andat theotherendtothemechanismframe.Atthefrontendofthedrag-cupshaftageartrainiscoupledtotwoconcentricallymountedpointers;asmalloneindicatinghundredsandalargeoneindicatingthousandsofrev./min.

Figure2-20.a



Part 66 Subject



Sectionalview(Figure2.20b)ofatypicalsynchronousmotortypetachometerindicator:

Figure2-20b

1.

CantileverShaft

2.

TerminalBlockAssembly3. RearballBearing

4. MagneticCupAssembly

5. DragElementAssembly

6. SmallPointSpindleandGear

7.

OuterSpindlebearing

8.

BearingLockingTag

9.

IntermediateGear

10.

bearingplate11. HairspringAnchorTag

12. InnerSpindleBearing

13. Frontballbearing

14. Rotorand

15.

Stator



Part 66 Subject



System Operation

As the generator rotor is driven round inside its stator, the poles sweeppast each statorwindinginsuccessionsothatthreewavesorphasesofalternatinge.m.f.aregenerated,the

wavesbeing120ºCapart(seebelow).Themagnitudeofthee.m.f.inducedbythemagnetdependsonthestrengthofthemagnetandthenumberofturnsonthephasecoilsasshownatFigure2.21.

Furthermore,aseachcoilispassedbyapairofrotorpoles,theinducede.m.f.completesonecycleatafrequencydeterminedbytherotationalspeedoftherotor.'Therefore,rotorspeedandfrequencyaredirectlyproportional,andsincetherotorisdrivenbytheengineatsomefixedratiothenthefrequencyofinducede.m.f.isameasureoftheenginespeed.

Thegeneratore.m.f'saresuppliedtothecorrespondingphasecoilsoftheindicatorstatortoproducecurrentsofamagnitudeanddirectiondependentonthee.m.f.'s.Thedistributionofstatorcurrentsproducesaresultantmagneticfieldwhichrotatesataspeeddependentonthegeneratorfrequency.

As the field rotates it cuts through the copper bars of the squirrel-cage rotor, inducing acurrent inthemwhich, inturnsetsup amagnetic fieldaroundeachbar. The reaction ofthesefieldswiththemainrotatingfieldproducesa torqueontherotorcausingit torotateinthesamedirectionasthemainfieldandatthesamespeed.

Astherotorrotatesitdrivesthepermanentmagnetofthespeed-indicatingunit,andbecauseofrelativemotionbetweenthemagnet and thedrag-cupeddycurrents are induced inthelatter. Thesecurrentscreate amagnetic fieldwhich reacts with the permanentmagneticfield,andsincethereisalwaysatendencytoopposethecreationofinducedcurrents(Lenz'slaw),thetorquereactionofthefieldscausesthedrag-cuptobecontinuouslyrotatedinthesamedirectionasthemagnet.

However, this rotationof the drag-cup is restricted by the calibrated hairspring in such a

mannerthatthecupwillmovetoapositionatwhichtheeddy-current-dragtorqueisbalancedbythetensionofthespring.Theresultingmovementofthedrag-cupshaftandgeartrainthuspositionsthepointersoverthedialtoindicatetheenginespeedprevailingatthatinstant.

Figure2-21.



Part 66 Subject



Autosyn Instruments

An Autosyn system activates indicators in the cockpit without using excessively longmechanicallinkagesortubing.Theindicationispickedupbythetransmitterneartheengine,oratsomeotherremotepoint,andissentbyelectricalmeanstotheindicatorinthecockpit.

AnAutosynsynchrohastheappearanceofasmallsynchronousmotor.Forthisreason,theword"synchro"hasbecomesynonymouswiththisandothersimilarsystems.IntheAutosynsystem,onesynchroisemployedasatransmitterandanotherasanindicator.

A schematic diagramof an Autosyn system is shownbelow. The system is basicallyanadaptationof theself-synchronousmotor principle,whereby twowidely, separatedmotorsoperateinexactsynchronism;thatis,therotorofonemotorspinsatthesamespeedastherotorof theother.WhenthisprincipleisappliedtotheAutosynsystem,however,therotorsneitherspinnorproducepower.InsteadtherotorsofthetwoconnectedAutosynunitscomeinto coincidencewhen theyareenergisedbyanalternatingelectriccurrent,andthereaftertherotorofthefirstAutosynmovesonlythedistancenecessarytomatchanymovementoftherotorofasecondautosyn,nomatterhowslightthatmovementmaybe.

ItmustbeunderstoodthatthetransmitterandindicatorofAutosynunitsareessentiallyalike,bothinelectricalcharacteristicsandinconstruction.Eachhasarotorandastator.Whena-cpowerisappliedandarotorisenergised,thetransformeractionbetweentherotorandstatorcausesthreedistinctvoltagestobeinducedintherotorrelativetothestator.Foreachtinychange in the position of the rotor, a new and completely different combination of threevoltagesininduced.

When two Autosynsare connected asshown atFigure 2.22,and the rotors ofboth unitsoccupyexactlythesamepositions relativeto their respectivestators,bothsetsof inducedvoltagesare equaland opposite. For this reason, nocurrent flows in the interconnectedleads,withthe result thatbothrotorsremainstationary. Ontheotherhand,whenthetworotorsdonotcoincideinposition,thecombinationofvoltagesofonestatorisnotlikethatof

theother,androtationtakesplace,continuinguntiltherotorsareinidenticalpositions.Theinducedvoltagesarethenequalandopposite,andsothereisnocurrentflowinanyofthethreeconductors;hencetherotorswillbeinstationaryandidenticalpositions.

AnAutosynsystemmaybeused forawidevarietyof indicationsonanairplane. Amongthesearemanifoldpressure,oilpressure,rpm(tachometer),remotecompassindication,percentofpower,andfuelpressure.

Figure2

Figure2-22.



Part 66 Subject



Flow Sensors and Indicators

Themain application on aircraft is to sense the rate of fuel flow to the engines. ThoseillustratedatFigure2.23(a)to(e)andvariationsofthemareusedforthispurpose.

Withthe'taperedtube'typeat(a)thefloatiscarriedtoaheightintheverticaltubewhereitsweight equals the upward forceon it caused by the flowing fluid. Because the float isarestrictioninthetubeadifferentialpressureiscreatedacrossit.Foranygivenrateofflowthedifferentialpressure,andthereforetheupwardsforceonthefloat,willvarywiththecross-sectionalareaoftherestrictedpatharoundthefloat.

Therestrictionisgreatestatthebottomwherethetubeisnarrowest.Asthefloatisforcedupthewidening tubethereis lessrestriction, sotheupwardforceon the floatreducesuntilitequalstheweightofthefloat.Thisequalityoccurshigherorlowerinthetubedependingonwhetherthe{lowisincreasedordecreased.

Figure2-23.



Part 66 Subject



Vibration Analysis

Intheearlyseventies,aLockheedTri-StarwithRolls-RoyceRB211enginescrashedduetodisintegrationofoneoftheengines,killingmanypeople.Theaccidentinvestigationshowedthattheenginedisintegratedafteralubricationproblemwiththenumber1bearing,whichled

toseparation of the fan from the engine. Engine parts then flew away and damaged thefuselage.

Theinvestigationalsoshowedthattheaccidentcouldhavebeenavoidedifthepilothadhadanenginevibration indication.Thiswouldhaveshownthe vibration increasingdue to thelubrication problembuildingup. So the pilotcouldhave shut down the engine beforeanydamagecouldhappen.

Followingthis,theUSAFAAdeclaredenginevibrationmonitoringsystemsmandatoryfortheTristarandlaterforallaircraftwithenginesbiggerthanacertaindiameter.

Asexplainedbefore, the first functionofanenginevibrationmonitoringsystem(EVM)istogivethepilotacontinuousindicationofthevibrationleveloftheenginestoallowhimtotake

appropriatemeasuresifthevibrationreachesadangerouslevel.Forthisreason,everyenginevibration-monitoringunitconditionssomecombinationofrotorout-of-balance vibrationdata forcockpitdisplay.According to theaircraftandengine type,thesedataareselectedandconditioneddifferently.Typicaldisplaysmayinclude:

FanvibrationandLowPressureTurbine(LPT)vibration.

Fanvibration,LPTvibrationandoverallenginevibrationlevel(olderconcept)

Onevibrationindicationonly,computedasthemaximumlevelmeasured,eitherFanorLPTvibrations.

Togiveanunmistakablewarningtothepilotincaseofproblems,theEVMusuallymonitorsthevibrationlevelsforexceedingacertainalertthresholdandactivatesacockpitwarningin

caseofexceedance.

Apartfromcatastrophicevents,theout-of-balancevibrationlevelofanengineusuallyshowsamoreorlesssteadyincreaseovertimeduetomechanicalwear(birdstrikes,friction,etc...).Sincethetendencyofthevibrationevolutionovertimeissteady,itisquiteeasytopredictthetimewhenthevibrationwillreachacertainvibrationlevel,e.g.themaintenancealertlevel.This allowsmaintenancepersonnel toanticipatemaintenanceactionsandtoplan them inadvance.

For this reason, the vibrationdatafromtheEVMareusuallysent totheAircraftConditionMonitoringSystem(ACMS),orsimilarequipment,fromwheretheyareused,alongwithotherengineparameters,asinputtotheEngineConditionMonitoring(ECM)system.

Figure2-24.(Vibrationevolutionduetowearandcatastrophicevent.)



Part 66 Subject



TheairborneEVMsystemutilizepiezo-electrictransducers(accelerometers)tosenseenginevibration.Thechargesignalgeneratedby theaccelerometersis thenwiredthroughspeciallow-noisecablingtotheEVMU.Thislow-noisecablingisindispensableduetotheextremelysmallamplitudeofthechargesignal,leadingtoahighsusceptibilitytonoise.

ThesignalprocessingisprovidedbytheEVMU,whichextractstherelevantinformationfromthetotalvibrationsignalprovidedbytheaccelerometers.

Piezoelectricaccelerometersaremountedatrightanglestotheturbineshaft.Thecrystalwilloscillatewithapredeterminedelectricalinput.Thisoscillationismonitored.

Whenenginevibrationoccurs,thisalterstheoscillationfrequencyproducedbythecrystal,astheincreaseinvibrationwillcompressthecrystal.Asthevibrationincreases,sodoesthecompressionofthecrystal.Thisproducesachangeinthesignaloutputfromthecrystalanditisthischangeinfrequencythatisdetectedbythesignalconditionerandsenttotheindicator.

Thepiezo-electricaccelerometersproduceachargeoutputofverysmallamplitudewhichisdirectlyproportionaltotheaccelerationofthevibration,appliedtothem.Theirsensitivityis

expressedthereforeintermsofpico-Coulombs.Thesesensitivitiesarelimitedbythepiezo-electricmaterialssuitableforuseinthehostileengineenvironment.

Themaincharacteristicsoftheaccelerometersare:

Verylinearresponsebetweenapprox5Hzandatleast3kHz.

Veryhighreliabilityduetonomovingparts.

Resonanceandthereforehighamplificationofthevibrationat10to20kHz.




B:January2008 Revisi 1

AA Form TO-19

B2-14.2IndicatingSystemsIssue on Page26of26

Table2.1showstypicalareasmonitoredforvibration.

Component Frequency Area Description

StructuralVibration

1toabout15Hz Erraticvibrationorconstantcausedbytheaircraftstructure(wings,fuselage.)

AerodynamicVibration

5toabout40Hz Erraticvibrationcausedbytransientaerodynamicphenomena(turbulences,shockwaves...)intheengineinlet,betweentheengineandfuselage,etc.

RotorImbalance 10to250Hz Vibrationcausedbytheimbalanceoftheenginerotors(highpressure,lowpressureshaft).Showsinthespectrumassteadyvibrationpeaks.

AccessoryVibration

80to500Hz Vibrationcausedbyrotatingaccessories(pumps,etc...)drivenbyN2.Showsinthespectrumassteadyvibrationpeaks.

BladePassingVibration

300to10’000Hzandmore

Erraticvibrationcausedbytheperiodicmechanicalloadvariationsontherotorbladesinducedbytheir

passinginfrontofthestatorblades.

1/FNoise 0toabout20Hz Noisetypicalofwornelectricalcontacts(e.g.oxidized)leadingtoinstablecontactresistance.

BroadBandNoise Anyfrequency Noisetypicalofcontactproblems(e.g.looseconnections)leadingtobrutalinterruptionsofcontact.

Table2.1



Part 66 Subject

B2-14 Propulsion

B2-14.15.13StartingandIgnitionSystemsIssueB:January2008 Revision1 Page1of18

TOPIC15.13:ENGINESTARTINGANDIGNITIONSYSTEMS

Starting Systems

Electric Starters

Electricstartersarenotinverycommonuseonaircraftenginesbecauseoftheirexcessiveweight,althoughwhenusedasacombinationstarter-generator,theyprovideaweightsavingthat makes them feasible for use on small engines. Electric starters are, however, incommonuseonauxiliaryandgroundpowerunits.

Operation

Atypicalstartermotor,showninFigure15.13-1,isa12or24voltseries-woundmotor,whichdevelopshighstartingtorque.Thetorqueofthemotoristransmittedthroughreductiongearstotheclutch.Thisactionactuatesahelicallysplinedshaft,movingthestarterjawoutwardtoengage the engine cranking jaw before the starter jaw begins to rotate. After the enginereachesapredeterminedspeed,thestartermotorwillautomaticallydisengage.

Figure15.13-1.

Othertypesofelectricstartersnormallycontainanautomaticreleaseclutchmechanismtodisengage the starter drive from the engine drive when the engine has reached selfsustainingspeed,asdepictedinFigure15.13-2,adetailedbreakdownoftheclutchanditsoperationiscoveredintheensuingtextandFigure15.13-3.

Figure15.13-2.



Part 66 Subject

B2-14 Propulsion


Theclutchmechanismalsoprovidesanover-torqueprotectiontoprotecttheenginedrive.Atapproximately130inlboftorque,smallclutchplatesinsidetheclutchslip

andactasafrictionclutch.Thissettingisadjustable.

Duringstarting,thefrictionclutchisdesignedtoslipuntilengineandstarterspeedincreasetodeveloplessthan the slip torque setting. It is important that the slip torque tensionbecorrectlysettoavoiddamagetotheenginedriveratchet,orslowandhot(hung)starts.

Anotherfunctionoftheclutchassemblyistoprovidean“overrunning”clutch.Thisconsistsofa pawl and ratchet assembly that contains three pawls that are spring loaded into thedisengageposition.

When the starter isenergised, inertia causes the pawls tomove inwardsandengage theratchetgearonthestarterdriveshaftasillustratedinFigure15.13-3.

Theinertiausedispresentbecausethepawlcageassembly,whichfloatsintheoverrunningclutchhousing,triestoremainstationarywhenthestarterarmaturetriestodrivetheclutchhousingaround.

The overrunning clutch housing overcomes the disengage springs and forces the pawlsinward.

Whentheengineacceleratesuptoapproximatelyselfsustainingspeed,itisturningfasterthanthestartermotorandthepawlsslipoutofthetaperedslotsoftheenginedrivegear,anddisengageundertheinfluenceofthedisengagesprings.

Thisoverrunningfeaturepreventstheenginefromdrivingthestartertoselfdestructspeed.Typically,startercircuitsdonotcontainfusesorcircuitbreakers.Thereasonisthatinitialmotorcurrent(serieswoundDCmotor)canbeexcessive.

Figure15.13-3.



Part 66 Subject

B2-14 Propulsion


Starter-Generators

Starter-generators, illustrated in Figure 15.13-4, are most commonly found on private tomedium sized jets.Thesestartingsystemsuse astartermotor todrive the engine duringstarting. After the engine has reached a self-sustaining speed, it then operates as a

generatortosupplytheelectricalsystempower.Thestartergeneratorsimplyhasasheardrivesplinethatispermanentlyengagedintheengine.

Starter-generator units are desirable from an economical standpoint, because one unitperformsthefunctionsofbothstarterandgenerator.Alsothetotalweightofstartingsystemcomponentsisreducedandfewerpartsarerequired.

Figure15.13-4.

Pneumatic Starters

Pneumaticstartingisthemethodmostcommonlyusedoncommercialandmilitaryjetenginepoweredaircraft.Ithasmanyadvantagesoverothersystemsinthatitislightweight,simple

andeconomicaltooperate.Apneumaticstartermaytransmititspowerthroughareductiongearandclutchtothestarteroutputshaftwhichisconnectedtotheengine.AtypicalairstarterisshowninFigure15.13-5.

Figure15.13-5.



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B2-14 Propulsion


Thestarterturbineisrotatedbyhighvolumelowpressureairtakenfromanexternalgroundsupply,anauxiliarypowerunit(APU)orbleedairfromarunningengine.Theairsupplytothestarteriscontrolledbyanelectricallyoperated,controlandpressureregulatingvalveasshowninFigure 15.13-6. This valve isoperatedwhenan engine start isselectedand is

automaticallyclosedatapredeterminedstarterspeed.

Figure15.13-6.

Thestarterclutchalsoautomaticallydisengagesastheengineacceleratesuptoidlespeed,andtherotationofthestarterceases.AtypicalairstartingsystemisshowninFigure15.13-7.

C R O S S F E E D F R O MR U N N I N G E N G I N E

A IR F R A M E P Y L O N

G R O U N DS T A R T S U P P L

A U X IL IA R YP O W E R U N I T (A

A IR C O N T R O L VA LV E

E N G I N E A I R S T A R T E R E X T E R N A L G E A R B O XE X H A U S T A IR

H I G H V O L U M EL O W P R E S S U R E

Figure15.13-7.



Part 66 Subject

B2-14 Propulsion


Operation

Thepressureregulating/shutoffvalve,showninFigure15.13-8,consistsoftwosub-assemblies:

Thepressure-regulatingvalve,whichcontainsabutterfly-typevalve. Thepressure-regulatingvalvecontrol,whichcontainsasolenoidthatisusedtostoptheactionofthecontrolcrankinthe“off”position.

Theoperationoftheairstarter(Figure15.13-8)proceedsasfollows:

Turnonthestarterswitch.Thisenergisestheregulatingvalvesolenoidwhichretractsandallowsthecontrolcranktorotatetothe“open”position.

Thecontrolcrankisrotatedbythecontrolrodspringmovingthecontrolrodagainsttheclosedendofthebellows.

Sincetheregulatingvalveisclosedanddownstreampressureisnegligible,thebellowscanbefullyextendedbythebellowsspring.

Asthecrankrotatestotheopenposition,itcausesthepilotvalverodtoopenthepilotvalveallowingupstreamairtoflowintotheservopistonchamber.

Thedrainsideofthepilotvalve,whichbleedstheservochambertoatmosphere,isnowclosedbythepilotvalverodandtheservopistonmovestowardspositionB.

Thislinearmotionoftheservopistonittranslatedtorotarymotionofthevalveshaft.

Thisinturnopenstheregulatingvalve.

Astheregulatingvalveopens,downstreampressureincreasesandisbledbacktothebellowsthroughthepressure-sensingline.Thiscompressesthebellows.

Thecompressionofthebellowsmovesthecontrolrod. Thisturnsthecontrolcrankandmovesthepilotrodgraduallyawayfromtheservochambertoventtheairtoatmosphere.

Figure15.13-8.



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B2-14 Propulsion


Whendownstreampressurereachesapresetvalue,theamountofairflowingintotheservochamberequalstheamountofairbeingbledtoatmosphereandthesystemisinastateofequilibrium.

Whentheregulatingvalveisopen,theregulatedairpassingthroughtheinlethousingofthe

starterimpingesontheturbineinthestartermotor,showninFigure15.13-5.

Astheturbineturns,thegeartrainisactivatedandtheinboardclutchgear,whichisthreadedonto a helical screw, moves forward as it rotates and its jaw teeth engage those of theoutboardclutchgeartodrivetheoutputshaftofthestarter.

When engine startingspeed is reached, a set of flyweights ina centrifugal cutout switchactuatesaplungerwhichbreaks the ground circuit of the regulating valvesolenoid. Thiscutoutswitchislocatedintheexternalgearbox.

Whenthegroundcircuitisbrokenandthesolenoidisde-energised,thepilotvalveis forcedback tothe "off"positionopening the servochamber toatmosphere(seeFigure 15.13-9).Thisactionallowstheactuatorspringtomovetheregulatingvalvetothe"closed"position.

Tokeepleakagetoaminimuminthe"off"position,thepilotvalveincorporatesaninnercapwhichsealsofftheupstreampressuretotheservoandtheservochamberbleedpassage.

Figure15.13-9.

Somegasturbineenginesarenotfittedwithstartermotors,butuseanairimpingementontotheturbinebladesasameansofrotatingtheengineasdepictedinFigure15.13-10.Theairforthissystemissuppliedfromanexternalsource,orfromanenginethatisoperating.Theairisdirectedthroughnon-returnvalvesandnozzlesontotheturbineblades.

Figure15.13-10.



Part 66 Subject

B2-14 Propulsion


Hydraulic Starters

Hydraulicstartersareusedforstartingsomesmalljetengines.Inmostapplications,oneoftheenginemountedhydraulicpumpsisutilisedandiscalledapump/starter,althoughotherapplications may use a separate hydraulic motor. Methods of transmitting the torque

producedtotheenginemayvary,butatypicalsystemwouldincludeareductiongearandclutchassembly.

Operation

Powertorotatethestarterisprovidedbyhydraulicpressurefromagroundsupplyunit,oranaircraftaccumulator,andistransmittedtotheenginethroughthereductiongearandclutch.Thestartingsystemiscontrolledbyanelectriccircuitthatalso,insomeinstances,operateshydraulicvalvessothatoncompletionofthestartingcyclethepumpfunctionsasanormalhydraulicpump.Ahydraulicstarterissimilartoahydraulicmotorwiththefluiddrivingthegearinthestarter.

Starting Sequence

Two separate systems are required to ensure that a gas turbine engine will startsatisfactorily:

Rotationofthecompressor.

Ignitionofthefuel/airmix.

Tohelpensurethattheenginecomesonspeedquicklyandwithoutdamage,itisnecessarytocontrolthesequenceofeventsduringagasturbineenginestartingcycle.

Theexactsequenceofthestartingprocedureisimportant,becausetheremustbesufficientairflowthroughtheenginetosupportcombustionatthetimethefuel/airmixtureisignited.The fuel ratewill not besufficient toaccelerateuntilafterself sustainingspeedhas beenattainedandafailuretocorrectlysequencethestartingeventswillpreventtheenginefrom

reachingthisspeed.

Theusualsequenceofeventsduringanenginestartare:

Selectstart(ignitionon).

Highpressurefuelon.

Lightup.

SelfsustainingRPM.

Startercircuitcancelled.

IdleRPMstabilised.



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Illustrated in Figure 15.13-11 is a graphical representation of RPM and TGT during acorrectlysequencedstart.

Figure15.13-11.

For easeofmaintenance itmustbepossible tomotorover theenginewithout the ignitionsequenceinitiating,andoperatetheignitionsystemwithoutrotatingthestartermotorforinflightrelightingoftheengineintheeventofaflameout.



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IGNITION SYSTEMS

System Types

Theretwocommonclassificationsofjetengineignitionsystems.Theseare:

Lowtension(DCvoltage).

Hightension(ACvoltage).

Both low and high tension systems are in general use on todays aircraft. Low tensionsystemsaredesignedtousedirectcurrent(DC)andhightensionsystemsaredesignedtousealternatingcurrent(AC)asinputpower.DCoperatedsystemsreceivetheirpowerfromthe battery bus, and AC systems are powered from the aircraft AC bus. Although theoperatingvoltagesofthesystemsaredifferent,bothsystemscontainsimilarcomponentsasillustratedbyFigure15.13-12.

Figure15.13-12.



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Allignitionsystemscanbegroupedintooneoftwotypesofsystems.Theseare:

Intermittentdutycycle.

Continuousdutycycle.

Thetermdutycyclereferstothetimelimitplacedontheoperationoftheignitionsystembythemanufacturertopreventdamagetoitscomponents.Intermittent duty cycletypesdrawsufficiently high amounts of current to causeoverheatingwithin their units if operated forextendedperiods.Forthisreasontheyhavearestricteddutycyclebasedonoperatingtime,followedbyacoolingoffperiod.Forexample,twominutes“on”,threeminutes“off”(cooling).

Continuous dutytypeshavelongdutycyclesorinsomecasesnolimitsatall.Thatistheycanbeincontinuousoperation.

Intermittent Duty Cycle

Intermittentdutycycleignitionsystemscanonlybeusedforshortperiodsandonlyusuallyduring ground starting. Once the engine has reached self sustaining RPM, the ignition

systemisturnedoff.Someaircraftprovideforadditionaluseoftheleftorrightplugfromthemainsystematfulltransformercapacity(fullpower)asrequiredbut forlimitedperiodsonly,eg. take off. These time periodsarescheduledby thepilots andcan select ignition onwhenevertheywish.

Onotherintermittentdutycycletypeignitionsystems,alowtension,continuousdutycircuitisincorporatedwithin one of the transformerunits. Thisallows low powerdischarge tooneigniterplug(whichagaincanbeselectedbythepilot).ThissystemcanbeoperatedforaslongasthereisaneedforselfrelightcapabilityinflightasshowninFigure15.13-13

Figure15.13-13.



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Continuous Duty Cycle

Ifacontinuousdutycyclemainignition system is installed, full ignitioncanbeselected tobothoreitherplugatthepilotsdiscretion.Duringcriticalflightmanoeuvres(eg.takeoffandlanding),thepilotmayselectbothigniterplugstogiveinstantaneousrelight.Atnormalhigherlevel flight, one igniter plug is selected as a short delay in relighting the engine will notendangertheaircraftorcrew.

Ignition System Components

Gas turbineenginesare typically equippedwith adual highenergy ignitionsystem. TheprinciplecomponentsofadualsystemareshowninFigure15.13-14anddescribedonthefollowingpages.

Figure15.13-14.

Ignition and Relight Switches

The ignition and relight switches are located in the aircraft cabin, usually close to thethrottles. They connect bus voltage to the ignition relayand HEIUs (highenergy ignitionunits).

Ignition Relay

Whenenergised,theignitionrelaysupplieselectricalpowertothehighenergyignitionunits.Itiscontainedinacontrolboxwhichisusuallylocatedinanequipmentcompartmentinthe

enginenacelle.



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High Tension Ignition Leads

Thehightension(HT)ignitionleadsarelocatedontheaircraftengine,connectedbetweentheHEIUsandtheigniterplugs.TheyconductthehighvoltagefromHEIUstotheigniters.

High Energy Ignition Units HEIUs)

TheHEIUsdevelopthehighvoltagenecessaryforengineignition.Inadualignitionsystemtherearealwaystwounitsfittedtoeachengine.AnigniterplugisconnectedtoeachHEIU.

Types

TheignitionsystemcanbesuppliedwitheitherACorDCvoltage,dependingonthetypeofHEIUsfitted.ADCtypeHEIUcontainsatremblermechanism(coveredlaterinthistopic)oratransistorcircuit,whileanACtypeHEIUcontainsa transformer. Inanycase, thebasicoperationissimilarforeachofthesetypes.

HEIUsareratedin ‘joules’(one jouleequalsonewattper second). Theyaredesigned toproduceoutputswhichmayvaryaccordingtorequirementsandaregenerallyclassifiedaseither:

Highjoule(twelvejoule).

Lowjoule(threetosixjoule.

Althoughmany engines are fitted with high joule HEIUs, low joule units are sufficient fornormal starting requirements. The high joule units are requiredwhere it is necessary torelighttheengineathighaltitudes.

Undernormalflightconditions,theHEIUsareturnedOFFaftertheengineshavestarted.Butduring take-off where ice, heavy rain or snow exists, the HEIUs may be operatedcontinuouslytogiveanimmediaterelightshouldanengineflame-outoccur.Thiscontinuous

operationisusuallyperformedby lowjouleHEIUs,aspersistentoperationofthehighjouleunitsmayreducethelifeoftheigniterplugs.

Tosuitallengineoperatingconditions,acombinedsystemhasbeendevelopedwhereoneHEIUemitsahighoutputtooneigniterplug,andthesecondunitsuppliesalowvalueoutputtothesecondigniter.



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Construction

Asmentionedearlier,thebasicoperationofthedifferenttypesofHEIUsissimilar,sowewilllimitourdiscussiontotheDCtrembleroperatedHEIUshowninFigure15.13-15.Itcontainsthefollowingcomponents:

Induction Coil: consists of primary and secondary windings.

Trembler Mechanism: consistsofa capacitor and a set of contactswhich vibrate rapidly,openingandclosingtheprimarycircuitoftheinductioncoil.

Reservoir Capacitor:chargesup,thendischarges,supplyingtheHEIU’shighvoltageoutput.

Glass Sealed Discharge Gap:comprisestwometalliccontacts,separatedbyanairgap,allencapsulatedwithinasealedglasstube.

High Voltage Rectifier: convertstheoutputoftheinductioncoiltoDCtochargethereservoircapacitor.

Choke: aninductorwhichextendsthetimetakenforthereservoircapacitortodischarge.

Figure15.13-15.



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Operation

Figure15.13-16showsasimplifiedschematicdiagramofaDCtrembleroperatedHEIU.

Thisunitoperateswhen28VDCissuppliedtotheprimarywindingoftheinductioncoilandthetremblercontacts.Thetremblercontactsvibraterapidly,openingandclosingtheprimarycircuit,inducingavoltageintothesecondarywinding.Theresultinghighvoltageoutputisthenrectifiedbythehighvoltagerectifierandsuppliedtochargethereservoircapacitor.

Thereservoircapacitorisrepeatedlychargedinthiswayuntilitsstoredvoltageisequaltothe breakdown value of the sealed discharge gap. The reservoir capacitor will thendischargeacrossthegap,throughthechokeandsupplytheigniterplugwiththehightensionvoltagerequiredtoignitetheair/fuelmixtureintheenginecombustionchamber.Thechoke(inductor)extendsthedurationofthedischarge.

Thenormalsparkrateofatypicaljetengineignitionsystemisbetween60and100sparksperminute.

Figure15.13-16.

Theenergystoredinthereservoircapacitorispotentiallylethal. Forthisreason,dischargeresistorsareconnectedacrossthecapacitortoensurethatanychargeonthecapacitorisdissipatedwithinapproximatelyoneminuteofthesystembeingswitchedoff.

Thesafetyresistorsenabletheunittooperatewithoutdamagetotheunitifthehightensionleadisdisconnectedandisolated.

Igniter Plugs

Duetothemuchhigherintensityspark,igniterplugsforjetenginesdifferconsiderablyfromspark plugs used in reciprocating engines. They are normally constructed fromnickel-chromiumalloywiththethreadsbeingsilverplatedtopreventseizing.Thehotendofthe igniter plug is generally air cooled to keep it between 500-600o F cooler than thesurroundinggastemperatures.Coolingairispulledinwardthroughthecoolingholesintheflametube,andovertheendoftheigniter,bythepressuredifferentialbetweentheprimaryandsecondarycombustorairflow.



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Low Tension Igniters

Igniterplugsforlowtensionsystemsarereferredtoastheselfionisingorshuntedgaptype.Thefiringendcontainsasemi-conductivematerialwhichinitiallyprovidesapathbetweenthecentreelectrodeandthegroundelectrode.Astheinitialcurrentflows,thesemi-conductorreachesanincandescentstate(glowswhitehot).Thisheatingissufficienttoionisetheairgapandthemaincurrentflowtakesthispathtothegroundelectrode.AtypicallowtensionigniterisillustratedinFigure15.13-17.

Figure15.13-17.

High Tension Igniters

Hightensionigniterplugs(orannulargapplugs)operatewithasimilarprincipletoanormalsparkplug. Thehigh tensioncurrentpassingthroughtheplug initiallycauses theair gapbetweentheelectrodestobeionised.Thisionisationoftheairgapallowsthehighintensitysparktoflowbetweenthecentreandgroundelectrodes,(ReferFigure15.13-18).

Figure15.13-18.



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Many types of igniter plugs are available, as shown in Figure 15.13-19. Only one willnormally suit the needs of a particular engine. Care must be taken to ensure themanufacturersrecommendedigniterplugisused.

Figure15.13-19.

Ignition System Operation

Aschematicdiagramofabasic jetengineignitionsystem is illustratedinFigure15.13-20.

Forsimplicity,onlyoneHEIUandoneigniterplugisshown.

Figure15.13-20.



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Whenstartingtheengine,oncethestartermotorhasbeenengagedandtheengine’srotatingassemblybeginstoincreaseinspeed,theaircrewwillclosetheignitionswitch.28VDCwillnowbesuppliedtotheignitionrelay.Onceenergised,theignitionrelaywillsupplyvoltagetothe‘ignitionon’lightandtotheinputoftheHEIU.Thehighlevel,pulsatingDCoutputvoltage

fromtheHEIUwillbeconducted,viathehightensionignitionlead,tosupplytheigniterplug. Atthisstage,ifthestartermotorhasincreasedtheengine’sspeedsufficientlytocorrectlymixtheairandfuelsuppliedtotheengine,ignitionwilloccur.Oncetheair/fuelmixturehasbeenignited, the flame spreads rapidly through the engine combustion chambers; thus thecombustionisselfsustaining,andtheignitionsystemcanbeswitchedoff.Onmanyaircraft,a timer relay is employed to automatically shut down the ignition system after apredeterminedtime.

Engine Relight

Ifaflameoutoccurswhilsttheaircraftisinflight,theenginewillcontinuetorotateduetotheflowofairthroughthecompressor.Tore-ignitetheair/fuelmixtureintheenginecombustionchamber,only asource of ignition isnecessary. This isachieved by the selection of the

‘relight’switch.Withthisswitchclosed,28VDCwillbesupplieddirectlytotheHEIU.

In some cases however, a low joule HEIU is fitted and operated continuously, providingautomaticrelight.

Testing, Inspection and Maintenance

Maintenanceof the turbineengine ignition systemconsistsprimarilyof inspection, testing,troubleshooting,removalandinstallation.Thefollowinginstructionsaretypicalexamplesofinspectionproceduresthatyoumayberequiredtoperform.

IMPORTANT

Prior to performing maintenance on an ignition system, always consult the relevant technical

publication for all applicable safety precautions, maintenance procedures and specifications.

Igniter Plugs

The igniter plugsare inspected visually for burning and erosion of the electrode or shell,crackingoftheceramicinsulator,anddamagetothethreads,orflange.Ifdamageisvisible,theignitershouldbediscarded.

HT Ignition Leads

Theignitionleadsarecleanedwithanapprovedsolventandinspectedforwornorburned

areas,deepcuts,frayingandgeneraldeterioration.

Theignitionleadsconnectorsarevisuallyinspectedfordamagedthreads,corrosion,crackedinsulators,andbentorbrokenconnectorpins.

Thecontinuityoftheleadsischeckedwithamultimeterandinsulationpropertiescheckedwith a meggar in accordance with specifications laid down in the relevant technicalpublication.




Operational Test

SomeaircraftservicingmayrequireanoperationaltestoftheignitionsystemtochecktheserviceabilityoftheHEIUs,HTleadsandigniterplugs. Inthistest,theenginestarter-motorisdisabledsotheenginewillnotrotate,preventingenginestart.

Whenthe‘battery’andengine‘relight’switchesareclosed,sparkingfromtheigniterplugswill be clearly audible. This enables assessment of the ignition system’s serviceability. Anothermethodistosimplystarttheengine.

Safety Precautions

Theterm“HIGHENERGY”infersthatalethalchargeispresentandturbineengineignitionsystems require special maintenance and handling. The manufacturers instructions andenginemaintenancemanualsshouldbe fully understoodandfollowedwhen handlinganycomponentofajetengineignitionsystem.

Sometypicalprecautionsareasfollows:

WARNING

Ensure that the ignition switch is turned off before performing any maintenance on the

system.

To remove an igniter plug, disconnect the transformer input lead, wait the time

prescribe by the manufacturer usually 1-5 mins), then disconnect the igniter lead and

ground the centre electrode to the engine. The igniter plug is now safe to remove.

Exercise great caution in handling damaged transformer units. Some contain

radioactive material, eg. cesium-barium 137).

Unserviceable igniter plugs containing aluminium oxide and beryllium oxide, a toxic

insulating material, should be disposed of properly.

Before a firing test of igniters is performed, the fitter must ensure that the combustion

chamber is not fuel wetted, as a fire or explosion could occur.

Do not energise the system for troubleshooting if the igniter plugs are removed.

Serious overheating of the transformers can result.

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