Boeing Aero Magazine, Volume 12 - Angle of Attack
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Since the early days of flight, angle of attack (AOA) has been a key aeronauticalengineering parameter and is fundamental to understanding many aspects of airplane performance, stability, and control. Virtually any book on these subjects, as well as basic texts and instructional material written for flight crews, defines AOA and discusses its many attributes. AOA can be used for many indications on the flight deck to improve flight crew awareness of airplane state relative to performance limits. Dedicated AOA indicators have been used on military aircraft for many years, but this form of display has not been used often on commercial airplanes. On Boeing models currently in production, AOA is used to drive stall warning (stick shaker), stall margin information on airspeed indicators, and the pitch limit indicator (PLI) on the primary attitude displays. AOA information is combined with other data and displayed as an integral part of flight deck displays. Recent accidents and incidents have resulted in new flight crew training programs for upset recovery and terrain avoidance, and these in turn have heightened industry interest in AOA as a useful flight parameter for commercial aviation. The U.S. National Transportation Safety Board (NTSB) has recommended visual indication of AOA in commercial airplanes. This indication may take the form of a dedicated AOA indicator or other implementation, such as the PLI. A dedicated AOA indicator shown on the primary flight display (PFD) recently has been developed in cooperation with airline customers. The new indicator is offered as an option on the 737600/700/800/900, 767400, and 777 at this time. During the development of the new indicator, discussions with airlines, the NTSB, and U.S. Federal Aviation Administration (FAA) pilots and engineers provided a unique opportunity to examine potential uses of AOA and the many existing uses that have evolved in recent decades along with advances in display and indication technology. This article discusses the following:
description
Article by Boeing Commercial Airplanes on Angle of Attack sensors (AOA) and the use of this information in flight.
Transcript of Boeing Aero Magazine, Volume 12 - Angle of Attack
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Sincetheearlydaysofflight,angleofattack(AOA)hasbeenakeyaeronauticalengineeringparameterandisfundamentaltounderstandingmanyaspectsofairplaneperformance,stability,andcontrol.Virtuallyanybookonthesesubjects,aswellasbasictextsandinstructionalmaterialwrittenforflightcrews,definesAOAanddiscussesitsmanyattributes.
AOAcanbeusedformanyindicationsontheflightdecktoimproveflightcrewawarenessofairplanestaterelativetoperformancelimits.DedicatedAOAindicatorshavebeenusedonmilitaryaircraftformanyyears,butthisformofdisplayhasnotbeenusedoftenoncommercialairplanes.OnBoeingmodelscurrentlyinproduction,AOAisusedtodrivestallwarning(stickshaker),stallmargininformationonairspeedindicators,andthepitchlimitindicator(PLI)ontheprimaryattitudedisplays.AOAinformationiscombinedwithotherdataanddisplayedasanintegralpartofflightdeckdisplays.
Recentaccidentsandincidentshaveresultedinnewflightcrewtrainingprogramsforupsetrecoveryandterrainavoidance,andtheseinturnhaveheightenedindustryinterestinAOAasausefulflightparameterforcommercialaviation.
TheU.S.NationalTransportationSafetyBoard(NTSB)hasrecommendedvisualindicationofAOAincommercialairplanes.ThisindicationmaytaketheformofadedicatedAOAindicatororotherimplementation,suchasthePLI.
AdedicatedAOAindicatorshownontheprimaryflightdisplay(PFD)recentlyhasbeendevelopedincooperationwithairlinecustomers.Thenewindicatorisofferedasanoptiononthe737600/700/800/900,767400,and777atthistime.
Duringthedevelopmentofthenewindicator,discussionswithairlines,theNTSB,andU.S.FederalAviationAdministration(FAA)pilotsandengineersprovidedauniqueopportunitytoexaminepotentialusesofAOAandthemanyexistingusesthathaveevolvedinrecentdecadesalongwithadvancesindisplayandindicationtechnology.
Thisarticlediscussesthefollowing:
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1. BasicprinciplesofAOA.2. AirplaneperformanceandAOA.3. AOAmeasurement.4. AOAindicationsandflightcrewproceduresin
currentBoeingproductionmodels.5. DesignandusesofaseparateAOAindicator.
1.BASICPRINCIPLESOFAOA
AOAisoneofthemostimportantparametersforunderstandingairplaneperformanceandhandling(see"WhatIsAngleofAttack?")becauseatypicalwinghasalimitedrangeofanglesofattackoverwhichitcanfunctionefficiently.Initssimplestform,liftisafunctionofspeed,airdensity,wingarea,andAOA.Atagivenairspeed,astheAOAofawingisincreased,liftalsowillincrease(fig.1).Therefore,atthesameairspeed,aheavyairplaneofthesameconfigurationmustflyatahigherAOAthanalightone.Conversely,asanairplanedecelerates,theAOAmustbeincreasedtomaintainthesamelift.So,inthenormaloperationalrange,thereisarelationshipamonglift,speed,andAOA.
ThisrelationshipwillchangeiftheAOAgetstoohigh(fig.1).Theairflowingoverthewingwillseparatefromtheuppersurface,resultinginalossoflift,orastall.Itshouldbenotedthatthisstallconditioncouldoccuratawiderangeofspeeds(dependingontheairplaneweightorloadfactor,orgloading)andatanyattitude(dependingontheflightpathangle).WhatisimportantistheAOA.Therefore,itisimperativetoknowwhenthewingisapproachingthestallAOAandtotakestepstoavoidit.
However,manyotherparametersinfluencetheliftthatawingproduces.Themostbasicistheconfigurationofthewing,specificallythepositionofthetrailingedgeflaps,leadingedgeflapsorslats,andspoilers.Asthetrailingedgeflapsareextended,thecurvature(orcamber)andareaofthewingareincreased,andthewingwillproducemoreliftatthesameAOA(fig.2).Notethatalthoughthemaximumliftisincreased,theAOAatwhichstalloccursisactuallylessbecausethewingcannotsustainthehigherliftlevelsuptothesameAOA.Theairflowseparatesearlier.
Wingmountedspeedbrakesorspoilershavetheoppositeeffect.TheyreducetheliftatagivenAOAtheyalsoreducethemaximumliftachievablebut,
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surprisingly,increasetheAOAatwhichstalloccurs.
Leadingedgedevices,suchasKruegerflapsandslats,permitthewingtooperateatahigherAOAbeforeitstallsbydelayingtheflowseparation.Figure3illustratesthisandtheeffectofcontamination,suchasiceordents,ontheleadingedge.ContaminationcancausetheairflowtoseparateatalowerAOA,causingthewingtostallatalowerAOAthanexpected.Whiletheseeffectsareaccountedforintheairplanedesignandmaintenanceprogram,itisimportanttorememberthispotentialvariabilityinstallAOA(see"WinterOperationsKeepItClean,"Airliner,Oct.Dec.1983).
Onmosttransportcategoryairplanes,theliftthatthewingproducesisalsoafunctionofMachnumber,particularlyastheairplaneapproachestransonicspeedstypicalofcruiseflight(fig.4).Ofcourse,liftatagivenAOAwillincreasewithspeed,butevenatthesameairspeed,asMachnumberisincreased(thespeedofsoundchangeswithtemperature),liftwillincrease.
However,higherMachreducesthemaximumliftthewingcanattainandtheAOAatwhichstalloccurs.Thismeansthatasgrossweight,altitude,orloadfactorisincreased,theresultantincreaseinMachnumberwillcauseastallatahigherspeedandlowerAOA.Thisistrueevenattakeoffandlandingspeedswiththeflapsdown.
Thecenterofgravity(CG)alsoaffectstheliftthatthewingmustproduce.AstheCGmovesforward,thenosedownmomentincreasesbecauseoftheairplaneweightandwinglift(fig.5).Therefore,thedownforceonthehorizontaltailrequiredtotrimisincreased.Thismeansthatthewingmustprovideenoughlifttocompensateforthedownloadonthetailinadditiontotheweightoftheairplane.NotethattheAOAofstallisnotchanged,buttheliftrequiredofthewingisgreater,andthereforethestallspeedisincreased.
Thrustalsocanaffectliftinthreeways.First,thecomponentofthrustthatactsintheliftdirectionoffsetssomeoftheliftrequiredofthewing(fig.6).Therefore,asthrustisincreased,theAOAfortrimmedflightisreducedandthemaximumliftisincreased.Second,thrustchangestheairflowaroundthewingandflaps,whichdoesnotusuallyhavealargeeffectonjettransportairplanes.Third,thrustaffectsairplanetrim,usuallybyreducingthedownloadonthetail(see
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previousparagraphonCG).
TheexamplescitedaboveshowthatmanyparametersaffecttherelationshipofliftandAOA.ForAOAinformationtobeusefultoaflightcrew,theseparametersmustbeconsideredandaccountedforintheindicationsandassociatedcrewprocedures.
2.AIRPLANEPERFORMANCEANDAOA
Takeoff.Duringrotation,pitchangleisthecriticalparameterthatensurestailclearance.Oncetheairplaneisairborneandatasufficientaltitudewheregroundeffectandcrosswindsdonotaffectthesensorreading,AOAwillprovidevalidinformation.
Duringtakeoffclimb,thereisnosingletargetAOAtoflythatwillguaranteecertifiedtakeoffperformance.TakeoffclimbAOAwillvarywithsuchfactorsasairplanegrossweight,thrust,altitude,flapsetting,andCG.Takeoffclimbspeeds(hence,AOA)arelimitedbystallspeed,tailclearance,andminimumcontrolspeeds.ThehigherspeedandgreaterthrustofanallenginetakeoffreducetheAOAsignificantlyrelativetoanengineouttakeoffattheengineoutclimbspeed(V2).
Thekeytooptimaltakeoffperformanceisto"flythespeeds."Thetakeoffflightpaththatguaranteesclearanceofallobstaclesaheadiscalculatedbasedonflightatthesespeeds.FollowingrotationatVR,V2istheresultingengineoutspeedatanaltitudeof35ftandisusuallyslowerthanthatforbestlifttodrag(L/D)ratioorangleofclimb.However,ifthenoseweretobekeptdownandtheairplaneacceleratedtohigherspeeds,shorttermclimbperformancewouldbesacrificedandacloseinobstaclemaynotbecleared.
Cruise.Rangeisafunctionofboththeaerodynamicsoftheairplaneandthefuelflowcharacteristicsoftheengines.Aerodynamically,theminimumdragpointoccursatthepointwheretheL/Disamaximum.ButthisvaluedependsonbothAOAandMachnumber,sotheoptimalAOAwillvaryasMachnumberischanged.
ThefuelflowcharacteristicsoftheenginesarenotaffectedbyAOA,buttheydodependonthethrustrequired(drag),Machnumber,andtemperature.
Combiningthewingandenginecharacteristicsyieldsthefuelmileageoftheairplane,sofuelmileageisastrongfunctionofMachnumber.Figure7showsthe
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fuelmileageofa757200atanaltitudeof35,000ftasafunctionofgrossweightandMachnumber.ItcanbeseenthattheoptimallongrangecruiseMachnumberdoesnotvarysignificantlyasgrossweight(hence,liftandAOA)changes.SuperimposedonthischartaretwolinesofconstantAOA.ItisapparentthatflyingaconstantAOAwillnotyieldoptimalperformance.IfaflightcrewtriedtoflyatargetAOAandtherewasanerrorofaslittleas0.5deg,thepenaltyinfuelmileagecouldbe3percentormore.
Windisamorefundamentalconsideration.Forbestfuelmileageinaheadwind,theairplaneshouldbeflownfasterthanthespeedforbestrangeinstillairinatailwind,itshouldbeflownmoreslowly.MostmodernBoeingairplaneshaveaflightmanagementcomputer(FMC)thataccountsforairplane,engine,andwindcharacteristicsandcancomputetheoptimalspeedtobeflown.
Approachspeed.Approachspeediscriticaltolandingperformanceandisestablishedduringtheairplanecertificationprocess.Itisdeterminednotonlybymarginabovestallspeedbutalsomaybeincreasedbyconsiderationofminimumcontrolspeedandtailclearanceattouchdown.
Regulationsrequirethattheapproachspeedbenosmallerthanaspecificmultipleofthestallspeed.BecausestallspeedisafunctionofMachnumber,stalllimitedapproachspeedwilloccuratadifferentAOAatdifferentgrossweightsandaltitudes(fig.8).ThoseairplanesthatdonotaccountforthevariationofstallspeedwithMachnumbersettheapproachspeedatthemostconservativealtitude.ThespeedsalsoallowforthemostadverseCG(forward)thatrequiresthemostliftoutofthewing,resultinginthehigheststallspeedand,therefore,thehighestapproachspeed.
Inaddition,theapproachspeedcannotbesmallerthanamultipleoftheminimumcontrolspeedinthelandingconfiguration(Vmcl).ThisspeedisnotsignificantlyinfluencedbymovementoftheCG.So,duringanapproachattheaftCG,iftheflightcrewreducesspeedtoflyatthesameAOAasrequiredfortheforwardCG,anapproachspeedbelowtheminimumcontrolspeedmayresult.
Afurtherconsiderationistheclearanceoftheaftbodyfromthegroundastheairplanelands.Someairplanes,particularlythosewithstretchedfuselages,have
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increasedapproachspeedstoreducetheAOAandhencethepitchangleontouchdown.ThisprovidesadequateclearancebetweenthebodyandthegroundatthemostcriticalCG.However,inrevenueservice,CGisrarelyattheforwardlimit.So,iftheapproacheswereflownonadailybasisbyreferencetoafixedapproachAOAbasedonamarginabovestall,atanyCGaftoftheforwardlimit,theprobabilityoftailstrikewouldbegreaterthanthecurrentpracticeofusingapproachairspeeds.
Inaddition,variationsinthrustwillaffecttheapproachAOAspeedrelationship.
Fromthediscussionabove,itcanbeseenthatapproachspeedmaybelimitedbymanydifferentrequirementsandthatnosingleAOAcanbetargetedtoensureproperspeedorlandingattitudemargins.
3.AOAMEASUREMENT
TheprevioussectiondealtwiththerelationshipbetweentheaerodynamicsoftheairplaneandthetrueAOAofthewing.Inpractice,thetrueAOAofthewingisnotknown.Itonlycanbeestimatedbasedonameasuringdevicemountedsomewhereontheairplane.Anysuchdevicehasinherenterrorsthatmustbeaddressed.
Whereverthedeviceislocated,itismeasuringtheflowangleinitsownlocalvicinity,notatthewing.Stallwarningdeviceshavebeenmountedonthewing,butmostmoderncommercialjetairplaneshavemovableleadingedgesthatwouldinterferewithsuchaninstallation.Mosthavethesensorlocatedonthefuselage,faraheadofthewing,reducingtheeffectofchangesinliftandconfiguration.Nearertothenoseoftheairplane,theairflowisrelativelycleanandtheboundarylayeristhin,minimizingtherequiredprobeheight.
Evenatthenose,manyfactorscanaffecttherelationshipbetweenthelocalAOAandtruewingAOA(fig.9).Theangleofairflowaroundthenoseisnotthesameasatthewing.
Also,thesensitivitytochangesinAOAisgreater,soa1degchangeintruewingAOAcausesalocalflowchangeatthenoseof1.5to2deg.ThetrailingedgeflappositionhasaninfluenceonatypicalAOAsensorcalibration,ashaslandinggearposition(inparticular,
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thatofthenoselandinggeardoors).Machnumberaffectstheflowaroundthenoseandthereforechangesthesensorcalibration.
Pitchingtheairplanecancauseerroneousreadingsatthesensor.Whilethenoseispitchingup(asinaturn),thelocalflowangleisreduced,causingthereadingtobetoolow.Althoughthesensorsareplacedtominimizetheeffectofsideslip,itisnoteliminatedandcanbequitesignificantatsideslipanglesthatmayoccuronshortfinalapproachesorwithanengineout.
Evenvariationsinthecontouroftheskinnearthesensorcansubtlyaffectthelocalflowangle.Manyofthesedesignchallengesalsoaffectpitotandstaticportinstallationandaccuracy.
Thesensoritselfhaspotentialforerror.Thecombinationofinstallationerror,zerobias,andaerodynamicinaccuracycantotal0.5degormore.
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Contaminationordamagecanalsoaffectthesensorsaccuracy.
Forthemostpart,theeffectsdiscussedabovecanbecompensatedforand,dependingontheairplane,manyhavebeen.Itshouldbenoted,however,thateachcorrectionhasitsowninherentuncertaintyandcanalsocauseerroneousreadingsiftheinputdataisincorrect.
Inthephilosophyof"keepitsimple,"thefewerdependenciesonotherdata,themorerobusttheAOAsystemwillbe.Forexample,Machnumberaffectsthesensorcalibration.Whilethisrelationshipcouldbecompensatedfor,thiswouldmakethesensoroutputdependentongoodMachinformation.Iftheairspeeddatawereinaccurate,thecalculatedMachnumberandthereforethecalibratedAOAreadingwouldbeincorrect.ThiswouldaffecttheusefulnessofAOAintheeventofanairspeedsystemfailure.Notethatbecausethesensorsarelocatednearthenoseandtheairdataprobes,certainconditions,suchasradomedamageorloss,maycauseerroneousmeasurementofAOAaswellasairspeed.
4.AOAINDICATIONSANDFLIGHTCREWPROCEDURESINCURRENTBOEINGPRODUCTIONMODELS
AOAismostusefultotheflightcrewathighanglesofattacktoshowthemargintostallorstallwarning.AllindicationsdrivenbyAOAstickshaker,PLI,andspeedtapeindicationsarerelatedtothisimportantinformation.
Stickshaker.Anartificialstallwarningsystemisrequiredforairplanecertificationifthenaturalprestallbuffetcharacteristicsoftheairplaneareinsufficienttowarntheflightcrewofanimpendingstall.Thiswarningmustbeinaformotherthanvisualtobeeffective,eveniftheflightcrewisnotlookingattheinstrumentpanel.Beginningwithearlycommercialjetliners,standardpracticehasbeentoequiptheseairplaneswithastickshakerasameansofstallwarning.Someairplanesalsohaveemployedsticknudgersorstickpusherstoimprovestallavoidanceandstallcharacteristics.AlltheseindicationshavebeendrivenbyanAOAthreshold,whichisusuallyafunctionofflapconfiguration,landinggearconfiguration,orboth.
BecauseoftheeffectofMachnumberonstallAOA,thestallwarningAOAtypicallywassetataconservative
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leveltoaccommodategrossweightandaltitudevariationsexpectedintheterminalarea.
TheearlystallwarningsystemthresholdswerenotsettobeeffectiveatcruisealtitudesandspeedsbecausetheydidnotcorrectforMachnumber(fig.10).Thiskeptthesystemsimple.ThestickshakerwassetatanAOAeffectiveforlowaltitudesbutattoohighavalueforcruise.NaturalstallbuffetwasfoundtogivesatisfactorywarningathigherMachnumbers.
LaterstallwarningsystemsusedMachnumberfromthepitotorstaticairdatasystemtoadjustthestallwarningAOAthresholddownasMachnumberincreased.Thisprovidedtheflightcrewwithastallwarningrelatedtotheactualavailableperformance.However,italsomadethestallwarningsystemdependentongoodpitotandstaticdata,afactorthatwillbeconsideredinthenextsectiononthededicatedAOAindicator.
Itshouldbenotedfromfigure10thatthestallwarningscheduledoesnotfollowthebuffetboundaryatveryhighMachnumbers.ThebuffethereiscausedbyMachbuffet,ortoohighaspeed.Settingthestallwarningsystemtoactivateatthispointmayleadtheflightcrewtobelievetheairplaneisnearstallandincrease,ratherthandecrease,speed.
Pitchlimitindicator.ThePLIoriginallywasdevelopedaspartofanindustryefforttoaddresswindshearescapetraining.BecausestallwarningisprimarilyafunctionofAOA,thePLIshowsAOAmargintostallwarning,eventhoughitispartofthepitchattitudedisplay(fig.11).ThedistancefromtheairplanesymboltothePLIiscalculatedfromthedifferencebetweentheAOAoftheairplaneandtheAOAatwhichstallwarningwilloccur.Thisprovidestheflightcrewwithgoodsituationalawareness,enablingthemtomonitorairplaneattitudeinpitchandrollrelativetothehorizon,whilesimultaneouslyshowingwhethertheairplaneisapproachingitsmaximumAOA.Ingeneral,whentheairplanesymbolandtheamberPLIbarsmeet,thestallwarningsystemwillactivate.
However,thePLIalsoislimitedto30degofpitchattitude,regardlessofAOA.IfAOAorAOAmargintostickshakerweretobeusedasthefirstandprimaryfocusoftheflightcrewduringwindshearescapeorterrainavoidanceprocedures,extremelyhighpitchattitudescouldbereachedbeforestallwarningifthe
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maneuverisenteredwithsufficientspeed.Therefore,thePLIshowsthelesserofeithermargintostickshaker,or30degofpitch.
BecausestallAOAisafunctionofMachnumber,aPLIonairplaneswithfixedstallwarningscheduleswoulddisplayanexcessivelylargemarginattypicalcruiseMachandaltitude.Toavoidthismisleadingdisplay,PLIwasavailableonlywithflapsextendedwhenitwasintroducedinthemid1980s.LaterairplaneshaveemployedstallwarningschedulesthatadjustthestallwarningthresholdasafunctionofMachnumber.Thedesignofthe777,717,and767400ERhastakenadvantageofthisandwilldisplaythePLIfulltimewhenflapsaredown,aswellaswhenflapsareupifspeedorloadfactorcausesstallmargintodecreasetoanAOAwithin1.3gofstallwarning.
WorkiscurrentlyunderwaytointroducethistypeofPLIindicationonothermodels.Recentchangestothe757and767enablethePLItobedisplayedwithflapsup.
Speedtapeindications.SoonaftertheintroductionofthePLI,averticalscaleairspeedindicatorwasdevelopedandaddedtoelectronicflightdisplays.Thisofferedtheopportunitytocalculateandplaceairspeedrelateddatasuchasmaximum,minimum,maneuvering,andreferencespeedsontheairspeedinstrument(fig.11).AllBoeingmodelscurrentlyinproductionhavethiscapability.
Ofparticularinterestaretheminimumspeedamberandredbands,orbarberpole.AtlowspeedsonBoeingdesignedairplanescurrentlyinproduction,theseindicationsarebasedonsensedAOAandtheAOAmargintostickshaker.AthigherMachnumbers,mostairplaneswithfixedAOAstallwarningschedulesshowmarginstostickshakerormargintoinitialbuffet,whichevercorrespondstothehighestspeed.Ontheseairplanes,themargintobuffetathigherMachnumbersiscalculatedbytheFMC.
Onnewermodels,suchasthe777and767400,theamberandredbandsshowmargintostallwarningatalltimesbecausethestallwarningschedulegenerallyfollowstheinitialbuffetboundaryathigherspeedsuptocruise.ThepositionoftheamberandredbandsisalwaysafunctionofAOAmargintostallwarning.
Thespeedtapeisdesignedtoprovidetheflightcrew
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withsituationalawarenessoftheflightenvelope.Itshowsthecrewwheretheairplanespeedisrelativetothelimits(i.e.,maximumplacardspeedsorminimumstallwarningspeed,aswellasthemaneuveringcapabilityavailable).
5.DESIGNANDUSESOFASEPARATEAOAINDICATOR
BoeingandseveraloperatorsworkedtogethertodevelopthedisplayformatforanoptionalAOAindicator(fig.12).TheupperrightlocationwaschosenasonethatcanbeaccomplishedwithoutsignificantrearrangementoftheexistingPFDorelectronicflightdisplayformats.Theindicatoritselfconsistsofananalogscaleandpointer,anddigitalrepresentationsimilartodisplaysofmanyotherparametersthroughouttheflightdeck.
StallwarningAOAisshownwitharedtickmark,whichwillchangepositionasafunctionofMachnumberforthoseairplaneswithMachdependentstallwarningschedules.Agreenapproachreferencebandisshownwheneverlandingflapsareselected.TherangeoftheapproachreferencebandaccountsfornormallyexpectedvariationsinCG,thrust,sideslip,andotherconsiderations.
ManyAOAindicatorsusedinthepasthavebeenofthe"normalized"type,whereAOAisshowninarbitraryunitsandscaledsothatzeroloadfactorisshownasanAOAofzeroandstallisshownasanAOAofone.NormalizedAOAonacommercialjetlinerwouldrequirethatMachnumberbeintroducedintothecalculationofAOAbecausestallAOAandbuffetmarginsareafunctionofMachnumber.
TheindicatordevelopedshowsbodyAOAindegreesandisnotnormalized,whichisrelatedtothesecondobjectiveabove,thattheindicatorbeusefulwhenpitotorstaticdata,andthereforeMachcalculations,areunreliablebecauseofblockageorafaultinthesystem.Thepointerofanormalizedindicatorinthisconditionwouldbehaveerratically,makingtheindicatorunusable.Withthenonnormalizeddesign,thepositionoftheneedleisafunctiononlyofsensedAOA.Theredtickmarkforstallwarningmaybehaveerraticallyinapitotorstaticfailurestate,asmaystickshaker,PLI,andspeedtapeamberandredbands.However,theAOAneedleanddigitswillremainstable,andtheindicatoritselfstillwillbeusefulasabackupfor
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unreliableairspeed,providedtheAOAvanesareundamaged.
AvarietyofpotentialusesforAOAwereexaminedduringthedesignofthenewAOAindicator:
Improvedsituationalawarenessandflightcrewtraining.AOAbackupindicationfollowingpitotorstaticsystemfailures.Referenceduringupsetrecovery,windshearescape,andterrainavoidancemaneuvers.IndicationofmaximumL/Dorrange,detectionofweighterrors,andacheckoffuelconsumptionduringcruise.Crosschecktodetectweightorconfigurationerrorsonapproachtoreducetheprobabilityoftailstrikesonlanding.
AOAcanbeusedforsomeofthesepurposes,butitdoesnotworkaswellforothers.Fromthestandpointofflightoperations,someofthegoalscanbemetwithcertaincaveatsthattakeintoaccounttheprinciplesandlimitationsofAOAmeasurementandaerodynamicperformanceofmoderncommercialjetairplanes.
Improvedsituationalawarenessandflightcrewtraining.ThereisadesiretouseAOAinformationtoincreasetheflightcrewsunderstandingofthephysicsofflightandtheirgeneralawarenessofthestateofthewingduringnormalandnonnormalconditions.Withincertainlimitations,thedisplayprovidesthisindicationinaclear,unambiguousformat.ThedegreetowhichAOAcanbeusedtoincreaseknowledgeandairmanshipdepends,ofcourse,ontheapproachtakenbytheairlineintrainingitsflightcrewsandtheuseoftheindicatorintrainingscenariosfornonnormalprocedures.Someofthelimitationsarediscussedbelow.
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AOAbackupindicationfollowingpitotorstaticsystemfailures.TheAOAinstrumentdescribedinthisarticleisusefulasabackupforunreliableairspeedindicationcausedbypitotorstaticsourceblockagebecausethecalculationofindicatedAOAisnotgreatlyaffectedbypitotorstaticpressureinputsforitscalibration,andthedisplayedvaluehasnotbeennormalized.
Pitotorstaticsystemfailurerequirestheflightcrewtotakeseveralfundamentalstepstoresolvetheproblem(see"ErroneousFlightInstrumentInformation,"Aerono.8,Oct.1999):
Recognizeanunusualorsuspectindication.Keepcontroloftheairplanewithbasicpitchandpowerskills.Takeinventoryofreliableinformation.Findormaintainfavorableflyingconditions.Getassistancefromothers.Usechecklists.
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Recognitionofaproblemwillbeaccomplishedbyinstrumentscanningandcrosscheckpracticesorcrewalerts,dependingonthedesignofthesystemintheairplane.Inthisrespect,AOAinstrumentscanbeusefulasanadditionalcrosscheck.
Presentproceduresforunreliableairspeedcallforflyingtheairplanebyreferencetopitchattitudes,andreferthepilotstoreferencetablesshowingpitchattitudesforvariousconfigurations,weights,andaltitudesthatwillresultinsafeanglesofattackandspeeds.AOAcouldbeusefuliftherelevantdataisincludedinthepitchandpowertablesthatalreadyexistinthenonnormalchecklistprocedures.AOAwouldbemostusefulinflyingtheairplaneinmultiplefailureconditionswhereallpitotorstaticsourcesareaffected,makingallairspeedindicatorsunreliable.
CareshouldbetakenwhenflyingtheairplanebyreferencetoAOAinlieuofairspeed.Controlshouldbemadebyreferencetopitchattitude,usingAOAasacrosschecktoensurethatthepitchattituderesultsinthedesiredspeedorAOA.AttemptingtofollowAOAorspeedindicationstoocloselywithoutstabilizingtheairplaneinpitchcanleadtoanoscillatoryflightpath.
Referenceduringupsetrecovery,windshearescape,andterrainavoidancemaneuvers.Windshearescapeandterrainavoidancemaneuversrequireimmediatechangeinpitchattitudeandthrust,followedbymonitoringofthesituationandfurtherincreasesinpitchattitudeifneeded,whileavoidingstickshakeractivation.ThePLIwasdevelopedprimarilywiththesepurposesinmindandworkswell.Onallcurrentproductionmodels,PLIisshownwhenflapsaredown.Atthistime,PLIisavailablewithflapsretractedonthe717,767400,777,andMD11.WorkisunderwaytomakethiscapabilityavailableonotherBoeingdesignedmodelscurrentlyinproduction.Thefirststepsinwindshearescapeandterrainavoidanceproceduresinvolveapplyingmaximumcertifiedthrustandcontrolofairplanepitchattitudetoaninitialtarget,whilehonoringstallwarning.AOAmargintostickshaker,whethershownwiththePLIortheAOAdisplay,isasecondaryreferenceduringthispartofthemaneuver,nottheprimarytarget.AsmentionedinthesectiononPLI,pitchingupbysolereferencetoAOAbasedindicationscanresultinexcessivelyhighpitchattitudesifthemaneuverisenteredatsufficientlyhighspeeds.BecausetheAOAdisplayisseparatefromthepitchattitudedisplay,itdoesnotprovideprotection
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againsthighpitchattitudesiftheindicatorisusedastheflightcrewsprimaryfocusortargetduringsuchmaneuvers.Forupsetrecovery,eitherthePLIortheredstallwarningmarkontheAOAindicatormaybeusedtoassessthemargintostallwarning.
IndicationofmaximumL/Dorrange,detectionofweighterrors,andacheckoffuelconsumptionduringcruise.Asshowninthesectiononairplaneperformance,AOAisnottheappropriateparameterforoptimizingcruiseflight,becauseofthestronginfluenceofMachnumberonairplaneperformance.BecauseAOAisnotverysensitivetospeedorweightchangesatcruisespeeds,evenlargegrossweighterrorsmaynotbedetectable.A0.5degerrorinAOAisequivalentto30,000lbona757200,orapproximately14percentofthemaximumtakeoffweight.
Crosschecktodetectweightorconfigurationerrorsonapproachtoreducetheprobabilityoftailstrikesonlanding.AOAcanbeusedduringapproachasanextracrosscheckforerrorsinconfiguration,weight,orreferencespeedcalculation.ProximityofthebarberpoletothereferencespeedontheairspeedtapecanbeusedinasimilarmannerbecauseitisbasedonAOAmargintostickshaker.However,foreithermethod,theerrorsmustbelargeenoughthattheyarenotmaskedbyotherfactors.
NormalvariationsinAOAasaresultoftheregulatoryrequirementsonapproachspeed,aswellasthosecausedbydifferencesinthrust,CG,sideslip,andtheinstalledaccuracyoftheAOAmeasurementsystem,mayacttogethertomaskallbutlargeerrorsinweightorconfiguration.Thesefactorsaretakenintoaccountindeterminingthesizeofthegreenapproachreferenceband.Tokeepthesizeofthegreenbandfrombecomingtoolarge,thesevariationswererootsumsquaredbecauseofthelowprobabilitythattheywouldalladdinthesamedirectionatanyonetime.Theresultinggreenbandisabout2degwideforthe777and3degforthe737.ThebandiscenteredatanAOAequivalenttoVref+5kt,assuminganominalgrossweight,midCG,nosideslip,astabilized3degglideslopethrustlevel,andnosystemerror.
A20,000lbweighterrorona757,correspondingtoapproximately10percentofmaximumlandinggrossweightorabouta40percenterrorinpayload,yieldsachangeinAOAof1.7deg.So,itcanbeseenthatevenrelativelylargeweighterrorsmaynotbeenoughto
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movetheneedleoutofthegreenband.Conversely,itisalsopossiblethatflyingattheproperspeedandconfigurationmayyieldanAOAthatisoutsidethereferenceband.Figure13illustrateshowerrorscanbemaskedorcanceledoutbyvariationintheotherparameters.
Fortheseandotherreasons,theAOAindicatorcanbeusedasanadditionalmeanstocheckforlargeerrorsinweightorconfiguration,butitshouldnotbeusedasasubstituteforcurrentprocedurestoestablishapproachspeedsandverifyconfigurations.TodeterminetheapproachspeedbasedsolelyonplacingAOAinthegreenbandcancausesituationsofexcessivelyhighorlowapproachspeeds,dependingonavarietyofcircumstances.
SUMMARYAOAisalongstandingsubjectthatisbroadlyknownbutoneforwhichthedetailsarenotbroadlyunderstood.WhileAOAisaveryusefulandimportantparameterinsomeinstances,itisnotusefulandispotentiallymisleadinginothers.
TherelationshipbetweenAOAandairplaneliftandperformanceiscomplex,dependingonmanyfactors,suchasairplaneconfiguration,Machnumber,thrust,andCG.AOAinformationismostimportantwhenapproachingstall.AOAisnotaccurateenoughtobeusedtooptimizecruiseperformance.Machnumberisthecriticalparameter.AOAinformationcurrentlyisdisplayedonBoeingflightdecks.TheinformationisusedtodrivethePLIandspeedtapedisplays.AnindependentAOAindicatorisbeingofferedasanoptionforthe737,767400,and777airplanes.TheAOAindicatorcanbeusedtoassistwithunreliableairspeedindicationsasaresultofblockedpitotorstaticportsandmayprovideadditionalsituationandconfigurationawarenesstotheflightcrew.
MilitaryApplicationsofAOA
JOHNE.CASHMANDIRECTORFLIGHTOPERATIONS
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