Boeing AERO Angle of Attack - Leonardo

estimated based on a measuring devicemounted somewhere on the airplane.Any such device has inherent errorsthat must be addressed.Wherever the device is located, itis measuring the flow angle in its ownlocal vicinity, not at the wing. Stallwarning devices have been mounted onthe wing, but most modern commercialjet airplanes have movable leading edgesthat would interfere with such an installation.Most have the sensor locatedon the fuselage, far ahead of the wing,reducing the effect of changes in liftand configuration. Nearer to the noseof the airplane, the airflow is relativelyclean and the boundary layer is thin,minimizing the required probe height.Even at the nose, many factors canaffect the relationship between thelocal AOA and true wing AOA (fig. 9).The angle of airflow around the noseis not the same as at the wing.Also, the sensitivity to changes inAOA is greater, so a 1-deg changein true wing AOA causes a local flowchange at the nose of 1.5 to 2 deg. Thetrailing-edge flap position has an influenceon a typical AOA sensor calibration,as has landing gear position (inparticular, that of the nose landing geardoors). Mach number affects the flowaround the nose and therefore changesthe sensor calibration.Pitching the airplane can cause erroneousreadings at the sensor. While thenose is pitching up (as in a turn), thelocal flow angle is reduced, causingthe reading to be too low. Although thesensors are placed to minimize theeffect of sideslip, it is not eliminatedand can be quite significant at sideslipangles that may occur on short finalapproaches or with an engine out.Even variations in the contour of theskin near the sensor can subtly affectthe local flow angle. Many of thesedesign challenges also affect pitot andstatic port installation and accuracy.The sensor itself has potentialfor error. The combination of installationerror, zero bias, and aerodynamicinaccuracy can total 0.5 deg or more.Contamination or damage can alsoaffect the sensor’s accuracy.9FIGUREAOA MEASUREMENT ERRORSMach numberMachnumber0.4 deg0.4 degAOA vane AOA vaneAOA body AOA bodyGear positionAOA vane AOA vane Gear down0.5 deg0.5 degGear upAOA body AOA bodyPitch rateAOA vane AOA vanePitch rateGroundeffect0.3 deg0.5 degFree airAOA body AOA bodyFor the most part, the effectsdiscussed above can be compensatedfor and, depending on the airplane,many have been. It should be noted,however, that each correction has itsown inherent uncertainty and can alsocause erroneous readings if the inputdata is incorrect.In the philosophy of “keep it simple,”the fewer dependencies on otherdata, the more robust the AOA system willbe. For example, Mach number affectsthe sensor calibration. While this relationshipcould be compensated for, thiswould make the sensor output dependenton good Mach information. If theairspeed data were inaccurate, thecalculated Mach number and thereforethe calibrated AOA reading would beincorrect. This would affect the usefulnessof AOA in the event of an airspeedsystem failure. Note that because theFlap positionFlappositionSideslipSideslipGround effectsensors are located near the nose andthe air data probes, certain conditions,such as radome damage or loss, maycause erroneous measurement of AOAas well as airspeed.AOA INDICATIONS AND FLIGHT4 CREW PROCEDURES IN CURRENTBOEING PRODUCTION MODELSAOA is most useful to the flight crewat high angles of attack to show themargin to stall or stall warning. Allindications driven by AOA—stick shaker,PLI, and speed tape indications—arerelated to this important information.Stick shaker. An artificial stallwarning system is required for airplanecertification if the natural prestallbuffet characteristics of the airplaneare insufficient to warn the flight crewof an impending stall. This warningAERO17

8FIGUREAngle ofattackAPPROACH AOA0.5 degIncreasing altitudeGross weightcan be seen that the optimal long-rangecruise Mach number does not varysignificantly as gross weight (hence,lift and AOA) changes. Superimposedon this chart are two lines of constantAOA. It is apparent that flying aconstant AOA will not yield optimalperformance. If a flight crew tried to flya target AOA and there was an error ofas little as 0.5 deg, the penalty in fuelmileage could be 3 percent or more.Wind is a more fundamental consideration.For best fuel mileage in a headwind,the airplane should be flownfaster than the speed for best range instill air; in a tailwind, it should beflown more slowly. Most modern Boeingairplanes have a flight managementcomputer (FMC) that accounts for airplane,engine, and wind characteristicsand can compute the optimal speedto be flown.Approach speed. Approach speed iscritical to landing performance and isestablished during the airplane certificationprocess. It is determined notonly by margin above stall speed butalso may be increased by considerationof minimum control speed andtail clearance at touchdown.Regulations require that the approachspeed be no smaller than a specificmultiple of the stall speed. Becausestall speed is a function of Mach number,stall-limited approach speed willoccur at a different AOA at differentgross weights and altitudes (fig. 8).Those airplanesthat do not accountfor the variationof stall speed withMach number setthe approach speedat the most conservativealtitude.The speeds alsoallow for the mostadverse CG (forward)that requires themost lift out of thewing, resulting inthe highest stallspeed and, therefore,the highestapproach speed.In addition, the approach speedcannot be smaller than a multipleof the minimum control speed in thelanding configuration (V mcl ). Thisspeed is not significantly influencedby movement of the CG. So, duringan approach atthe aft CG, if theflight crewreduces speed tofly at the sameAOA as requiredfor the forwardCG, an approachspeed below theCruiseminimum controlspeed may result.A further considerationis theclearance of theaft body from theground as theairplane lands.Some airplanes,particularly thosewith stretchedfuselages, haveincreased approachspeeds to reducethe AOA andhence the pitchangle on touchdown.This providesadequateclearance betweenthe body and theground at themost critical CG.However, in revenue service, CG is rarelyat the forward limit. So, if the approacheswere flown on a daily basis by referenceto a fixed-approach AOA based ona margin above stall, at any CG aft ofthe forward limit, the probability of tailstrike would be greater than the currentpractice of using approach airspeeds.In addition, variations in thrust will affectthe approach AOA-speed relationship.From the discussion above, it can beseen that approach speed may belimited by many different requirementsand that no single AOA can be targetedto ensure proper speed or landingattitude margins.AOA MEASUREMENTThe previous section dealt with therelationship between the aerodynamicsof the airplane and the true AOA of thewing. In practice, the true AOA ofthe wing is not known. It only can beAOA AND ERRORS IN SPEED AND GROSS WEIGHTAirspeedStall0.5 deg10 kt3RELATIONSHIP BETWEEN AIRSPEED AND AOA0.5 degAngle of attack1 ktAn error in AOA corresponds to a much higher errorin airspeed or gross weight at high speeds than at lowspeeds. For a mid-sized airplane, such as the 757-200,a 0.5-deg error in AOA results in the following errors:Flight regime Speed error Gross weight errorHigh speed 10 kt 30,000 lb*Takeoff/landing 2 kt 6,000 lbStall warning

10FIGURESTALL WARNING SCHEDULEAngle ofattackPerformanceimprovementTerminaloperationsmust be in a form other than visualto be effective, even if the flight crewis not looking at the instrument panel.Beginning with early commercial jetliners,standard practice has been toequip these airplanes with a stick shakeras a means of stall warning. Someairplanes also have employed sticknudgers or stick pushers to improvestall avoidance and stall characteristics.All these indications have been drivenby an AOA threshold, which is usuallya function of flap configuration, landinggear configuration, or both.Because of the effect of Mach numberon stall AOA, the stall warning AOA typicallywas set at a conservative level toaccommodate gross weight and altitudevariations expected in the terminal area.The early stall warning systemthresholds were not set to be effectiveat cruise altitudes and speeds becausethey did not correct for Mach number(fig. 10). This kept the system simple.The stick shaker was set at an AOAeffective for low altitudes but at toohigh a value for cruise. Natural stallbuffet was found to give satisfactorywarning at higher Mach numbers.Later stall warning systems usedMach number from the pitot or staticair data system to adjust the stallwarning AOA threshold down as Machnumber increased. This provided theflight crew with a stall warning relatedto the actual available performance.However, it also made the stall warningMach-independent scheduleCruiseMach numberMach-dependent scheduleInitial buffetboundarysystem dependent on good pitot andstatic data, a factor that will beconsidered in the next section on thededicated AOA indicator.It should be noted from figure 10that the stall warning schedule doesnot follow the buffet boundary at veryhigh Mach numbers. The buffet here iscaused by Mach buffet, or too high aspeed. Setting the stall warning systemto activate at this point may lead theflight crew to believe the airplane isnear stall and increase, rather thandecrease, speed.11FIGUREAirspeed tapelowerbarber polePitch limit indicator. The PLI originallywas developed as part of an industryeffort to address windshear escapetraining. Because stall warning isprimarily a function of AOA, the PLIshows AOA margin to stall warning,even though it is part of the pitchattitude display (fig. 11). The distancefrom the airplane symbol to the PLI iscalculated from the difference betweenthe AOA of the airplane and the AOAat which stall warning will occur. Thisprovides the flight crew with goodsituational awareness, enabling themto monitor airplane attitude in pitchand roll relative to the horizon, whilesimultaneously showing whether theairplane is approaching its maximumAOA. In general, when the airplanesymbol and the amber PLI bars meet,the stall warning system will activate.However, the PLI also is limited to30 deg of pitch attitude, regardless ofAOA. If AOA or AOA margin to stickshaker were to be used as the first andprimary focus of the flight crew duringwindshear escape or terrain avoidanceprocedures, extremely high pitchattitudes could be reached before stallwarning if the maneuver is enteredwith sufficient speed. Therefore, thePLI shows the lesser of either marginto stick shaker, or 30 deg of pitch.PRIMARY FLIGHT DISPLAY STALL WARNING MARGIN INDICATIONSBased onAOA ss – AOA currentBoeing-designed,series-700modelsPitch limitindicator (PLI)AOA ss – AOA currentAERO18

Because stall AOA is a function ofMach number, a PLI on airplanes withfixed stall warning schedules woulddisplay an excessively large marginat typical cruise Mach and altitude.To avoid this misleading display, PLIwas available only with flaps extendedwhen it was introduced in the mid-1980s.Later airplanes have employed stallwarning schedules that adjust thestall warning threshold as a functionof Mach number. The design of the777, 717, and 767-400ER has takenadvantage of this and will display thePLI full time when flaps are down,as well as when flaps are up if speedor load factor causes stall marginto decrease to an AOA within 1.3 gof stall warning.Work is currently under way tointroduce this type of PLI indicationon other models. Recent changes tothe 757 and 767 enable the PLI to bedisplayed with flaps up.Speed tape indications. Soon afterthe introduction of the PLI, a verticalscale airspeed indicator was developedand added to electronic flight displays.This offered the opportunity tocalculate and place airspeed-relateddata such as maximum, minimum,maneuvering, and reference speeds onthe airspeed instrument (fig. 11). AllBoeing models currently in productionhave this capability.Of particular interest are the minimumspeed amber and red bands, or barberpole. At low speeds on Boeing-designedairplanes currently in production, theseindications are based on sensed AOAand the AOA margin to stick shaker.At higher Mach numbers, most airplaneswith fixed AOA stall warning schedulesshow margins to stick shaker ormargin to initial buffet, whichevercorresponds to the highest speed. Onthese airplanes, the margin to buffetat higher Mach numbers is calculatedby the FMC.On newer models, such as the777 and 767-400, the amber and redbands show margin to stall warningat all times because the stall warningschedule generally follows the initialbuffet boundary at higher speeds up12FIGUREOPTIONAL AOA GAUGE FOR 737-600/-700/-800/-900, 767-400, 777AnalogpointerDigitalreadout6.0to cruise. The position of the amberand red bands is always a function ofAOA margin to stall warning.The speed tape is designed to providethe flight crew with situationalawareness of the flight envelope.It shows the crew where the airplanespeed is relative to the limits (i.e.,maximum placard speeds or minimumstall warning speed, as well as themaneuvering capability available).DESIGN AND USES OF A5 SEPARATE AOA INDICATORBoeing and several operators workedtogether to develop the displayformat for an optional AOA indicator(fig. 12). The upper right locationwas chosen as one that can be accomplishedwithout significant rearrangementof the existing PFD or electronicflight display formats. The indicatoritself consists of an analog scale andpointer, and digital representationsimilar to displays of many otherAOA gauge on primary flight displayStall warning (stick shaker) AOAApproach reference bandparameters throughout the flight deck.Stall warning AOA is shown witha red tick mark, which will changeposition as a function of Mach numberfor those airplanes with Machdependentstall warning schedules.A green approach reference band isshown whenever landing flaps areselected. The range of the approachreference band accounts for normallyexpected variations in CG, thrust,sideslip, and other considerations.Many AOA indicators used in the pasthave been of the “normalized” type,where AOA is shown in arbitrary unitsand scaled so that zero load factor isshown as an AOA of zero and stallis shown as an AOA of one. NormalizedAOA on a commercial jetliner wouldrequire that Mach number be introducedinto the calculation of AOAbecause stall AOA and buffet marginsare a function of Mach number.The indicator developed shows bodyAOA in degrees and is not normalized,AERO19

which is related to the second objectiveabove, that the indicator be usefulwhen pitot or static data, and thereforeMach calculations, are unreliable becauseof blockage or a fault in the system.The pointer of a normalized indicator inthis condition would behave erratically,making the indicator unusable.With the nonnormalized design, theposition of the needle is a functiononly of sensed AOA. The red tick markfor stall warning may behave erraticallyin a pitot or static failure state, as maystick shaker, PLI, and speed tape amberand red bands. However, the AOAneedle and digits will remain stable,and the indicator itself still will beuseful as a backup for unreliableairspeed, provided the AOAvanes are undamaged.A variety of potential uses forAOA were examined during thedesign of the new AOA indicator:■ Improved situationalawareness and flight crewtraining.■ AOA backup indicationfollowing pitot or staticsystem failures.■ Reference during upsetrecovery, windshearescape, and terrainavoidance maneuvers.■ Indication of maximumL/D or range, detectionof weight errors, and acheck of fuel consumptionduring cruise.■ Cross-check to detectweight or configurationerrors on approach toreduce the probability oftail strikes on landing.AOA can be used for some of thesepurposes, but it does not work aswell for others. From the standpointof flight operations, some of the goalscan be met with certain caveats thattake into account the principles andlimitations of AOA measurementand aerodynamic performance of moderncommercial jet airplanes.■■■■Improved situational awareness andflight crew training. There is a desireto use AOA information to increasethe flight crew’s understanding of thephysics of flight and their generalawareness of the state of the wing duringnormal and nonnormal conditions.Within certain limitations, the displayprovides this indication in a clear,unambiguous format. The degree towhich AOA can be used to increaseknowledge and airmanship depends, ofcourse, on the approach taken by theairline in training its flight crews andthe use of the indicator in training scenariosfor nonnormal procedures. Someof the limitations are discussed below.KEY POINTS TO EMPHASIZE IN TRAININGAOA is most useful in high-AOA, low-speedparts of the envelope; it is less useful at mostnormal speeds.Airspeed and Mach are still the primary sourcesfor performance data for reasons of precision,regulatory basis, system redundancy, and integrity.Therefore, if the AOA indicator is used, flight crewsshould cross-check with other instruments, justas they would with airspeed.The AOA approach reference green band may beused as a cross-check for configuration errors,reference speed calculation errors, or very largeerrors in gross weight. Normal variations in AOAmeasurement dictate the width of the green band.Also, because approach speed in some cases canbe determined by issues not related to or sensedby AOA, increasing or decreasing approach speed bytargeting the center of the green band can resultin inappropriate approach speeds.Pulling to stick shaker AOA from a high-speed conditionwithout reference to pitch attitude can leadto excessive pitch attitudes and a higher probabilityof stall as a result of high deceleration rate.AOA backup indication followingpitot or static system failures. TheAOA instrument described in this articleis useful as a backup for unreliableairspeed indication caused by pitot orstatic source blockage because the calculationof indicated AOA is not greatlyaffected by pitot or static pressureinputs for its calibration, and the displayedvalue has not been normalized.Pitot or static system failurerequires the flight crew to take severalfundamental steps to resolve theproblem (see “Erroneous FlightInstrument Information,” Aero no. 8,Oct. 1999):■ Recognize an unusual or suspectindication.■ Keep control of the airplanewith basic pitch and power skills.■ Take inventory of reliableinformation.■ Find or maintain favorableflying conditions.■ Get assistance from others.■ Use checklists.Recognition of a problemwill be accomplished byinstrument scanning andcross-check practices or crewalerts, depending on thedesign of the system in theairplane. In this respect, AOAinstruments can be useful asan additional cross-check.Present procedures for unreliableairspeed call for flyingthe airplane by reference topitch attitudes, and referthe pilots to reference tablesshowing pitch attitudes forvarious configurations,weights, and altitudes thatwill result in safe angles ofattack and speeds. AOA couldbe useful if the relevant datais included in the pitch andpower tables that alreadyexist in the nonnormalchecklist procedures. AOAwould be most useful in flyingthe airplane in multiplefailure conditions where allpitot or static sources areaffected, making all airspeed indicatorsunreliable.Care should be taken when flyingthe airplane by reference to AOA in lieuof airspeed. Control should be made byreference to pitch attitude, using AOAas a cross-check to ensure that thepitch attitude results in the desiredspeed or AOA. Attempting to followAOA or speed indications too closelyAERO20

without stabilizing the airplane in pitchcan lead to an oscillatory flight path.Reference during upset recovery,windshear escape, and terrainavoidance maneuvers. Windshearescape and terrain avoidance maneuversrequire immediate change inpitch attitude and thrust, followedby monitoring of the situation andfurther increases in pitch attitude ifneeded, while avoiding stick shakeractivation. The PLI was developedprimarily with these purposes in mindand works well. On all current productionmodels, PLI is shown when flapsare down. At this time, PLI is availablewith flaps retracted on the 717,767-400, 777, and MD-11. Work isunder way to make this capabilityavailable on other Boeing-designedmodels currently in production.The first steps in windshear escapeand terrain avoidance proceduresinvolve applying maximum certifiedthrust and control of airplane pitchattitude to an initial target, whilehonoring stall warning. AOA margin tostick shaker, whether shown with thePLI or the AOA display, is a secondaryreference during this part of themaneuver, not the primary target. Asmentioned in the section on PLI, pitchingup by sole reference to AOA-basedindications can result in excessivelyhigh pitch attitudes if the maneuveris entered at sufficiently high speeds.Because the AOA display is separatefrom the pitch attitude display, it doesnot provide protection against highpitch attitudes if the indicator is usedas the flight crew’s primary focus ortarget during such maneuvers.For upset recovery, either the PLI orthe red stall warning mark on the AOAindicator may be used to assess themargin to stall warning.Indication of maximum L/D orrange, detection of weight errors,and a check of fuel consumptionduring cruise. As shown in the sectionon airplane performance, AOA is not theappropriate parameter for optimizingcruise flight, because of the stronginfluence of Mach number on airplaneperformance. Because AOA is not verysensitive to speed or weight changesat cruise speeds, even large grossweight errors may not be detectable.A 0.5-deg error in AOA is equivalentto 30,000 lb on a 757-200, or approximately14 percent of the maximumtakeoff weight.Cross-check to detect weight orconfiguration errors on approachto reduce the probability of tailstrikes on landing. AOA can be usedduring approach as an extra cross-checkfor errors in configuration, weight, orreference speed calculation. Proximityof the barber pole to the referencespeed on the airspeed tape can beused in a similar manner because it isbased on AOA margin to stick shaker.However, for either method, the errorsmust be large enough that they are notmasked by other factors.Normal variations in AOA as a result ofthe regulatory requirements on approachspeed, as well as those caused bydifferences in thrust, CG, sideslip, andthe installed accuracy of the AOAmeasurement system, may act togetherto mask all but large errors in weightor configuration. These factors are takeninto account in determining the sizeof the green approach reference band.To keep the size of the green bandfrom becoming too large, these variationswere root-sum-squared becauseof the low probability that they wouldall add in the same direction at anyone time. The resulting green band isabout 2 deg wide for the 777 and 3 degfor the 737. The band is centered at anAOA equivalent to V ref +5 kt, assuminga nominal gross weight, mid-CG, nosideslip, a stabilized 3-deg glideslopethrust level, and no system error.A 20,000-lb weight error on a 757,corresponding to approximately10 percent of maximum landing grossweight or about a 40 percent error inpayload, yields a change in AOA of1.7 deg. So, it can be seen that evenrelatively large weight errors may notbe enough to move the needle out ofthe green band. Conversely, it is alsopossible that flying at the proper speedand configuration may yield an AOAthat is outside the reference band.Figure 13 illustrates how errors can be13AOA GAUGE UNDER VARIOUS CONDITIONS ON APPROACH TO LANDINGFIGURE4.7 6.9 7.5 6.7 6.2Weight error023,000(14%)0016,000(9%)23,000(14%)0CGmidmidmidfwdmidmidaftSpeedV ref + 5V ref + 5V ref + 5V refV ref + 5V ref + 5V ref + 8Speed brakedowndown40%downdowndown40%AERO21

masked or canceled out by variationin the other parameters.For these and other reasons, the AOAindicator can be used as an additionalmeans to check for large errors in weightor configuration, but it should not beused as a substitute for current proceduresto establish approach speeds andverify configurations. To determine theapproach speed based solely on placingAOA in the green band can cause situationsof excessively high or low approachspeeds, depending on a variety ofcircumstances.SUMMARYAOA is a long-standing subject thatis broadly known but one for whichthe details are not broadly understood.While AOA is a very useful andimportant parameter in some instances,it is not useful and is potentiallymisleading in others.■ The relationship between AOAand airplane lift and performanceis complex, depending on manyfactors, such as airplane configuration,Mach number, thrust, and CG.■ AOA information is most importantwhen approaching stall.■ AOA is not accurate enoughto be used to optimize cruiseperformance. Mach number is thecritical parameter.■ AOA information currently isdisplayed on Boeing flight decks.The information is used to drivethe PLI and speed tape displays.■ An independent AOA indicator isbeing offered as an option for the737, 767-400, and 777 airplanes.The AOA indicator can be used toassist with unreliable airspeedindications as a result of blockedpitot or static ports and mayprovide additional situation andconfiguration awareness to theflight crew.