OPERATIONAL USE OF ANGLE OF ATTACK ON MODERN COMMERCIAL JET AIRPLANES Angle of attack (AOA) is an aerodynamic parameter that is key to understanding the limits of airplane performance. Recent accidents and incidents have resulted in new flight crew training programs, which in turn have raised interest in AOA in commercial aviation. Awareness of AOA is vitally important as the airplane nears AERO stall. It is less useful to the flight crew in the normal operational range. On most FLIGHT OPERATIONS Boeing models currently in production, AOA information is presented in several 10 JOHN E. CASHMAN DIRECTOR BRIAN D. KELLY TECHNICAL FELLOW BRIAN N. NIELD MANAGER ways: stick shaker, airspeed tape, and pitch limit indicator. Boeing has also devel- FLIGHT OPERATIONS FLIGHT CREW OPERATIONS INTEGRATION AERODYNAMICS ENGINEERING oped a dedicated AOA indicator integral to the flight crew’s primary flight displays. BOEING COMMERCIAL AIRPLANES GROUP BOEING COMMERCIAL AIRPLANES GROUP BOEING COMMERCIAL AIRPLANES GROUP ince the early days of flight, The U.S. National Transportation 1 BASIC PRINCIPLES OF AOA angle of attack (AOA) has been Safety Board (NTSB) has recom- AOA is one of the most important a key aeronautical-engineering mended visual indication of AOA parameters for understanding parameter and is fundamental in commercial airplanes. This airplane performance and handling to understanding many aspects indication may take the form of (see “What Is Angle of Attack?” on of airplane performance, stability, a dedicated AOA indicator or other p. 13) because a typical wing has a and control. Virtually any book on implementation, such as the PLI. limited range of angles of attack over these subjects, as well as basic texts which it can function efficiently. In A dedicated AOA indicator shown and instructional material written its simplest form, lift is a function of on the primary flight display (PFD) for flight crews, defines AOA and speed, air density, wing area, and recently has been developed in discusses its many attributes. AOA. At a given airspeed, as the AOA cooperation with airline customers. of a wing is increased, lift also will AOA can be used for many indica- The new indicator is offered as an increase (fig. 1). Therefore, at the tions on the flight deck to improve option on the 737-600/-700/-800/ same airspeed, a heavy airplane of flight crew awareness of airplane -900, 767-400, and 777 at this time. the same configuration must fly at a state relative to performance limits. higher AOA than a light one. Conversely, During the development of the Dedicated AOA indicators have been as an airplane decelerates, the AOA new indicator, discussions with used on military aircraft for many must be increased to maintain the airlines, the NTSB, and U.S. Federal years, but this form of display has same lift. So, in the normal oper- Aviation Administration (FAA) pilots ational range, there is a relationship not been used often on commercial and engineers provided a unique among lift, speed, and AOA. airplanes. On Boeing models currently opportunity to examine potential uses in production, AOA is used to drive This relationship will change if the of AOA and the many existing stall warning (stick shaker), stall AOA gets too high (fig. 1). The air uses that have evolved in recent margin information on airspeed indi- flowing over the wing will separate decades along with advances in from the upper surface, resulting in cators, and the pitch limit indicator display and indication technology. a loss of lift, or a stall. It should be (PLI) on the primary attitude dis- plays. AOA information is combined This article discusses the following: noted that this stall condition could with other data and displayed as an occur at a wide range of speeds 1. Basic principles of AOA. (depending on the airplane weight or integral part of flight deck displays. 2. Airplane performance and AOA. load factor, or g loading) and at any Recent accidents and incidents 3. AOA measurement. attitude (depending on the flight have resulted in new flight crew path angle). What is important is the training programs for upset recovery 4. AOA indications and flight crew AOA. Therefore, it is imperative to and terrain avoidance, and these in procedures in current Boeing know when the wing is approaching turn have heightened industry production models. the stall AOA and to take steps to interest in AOA as a useful flight 5. Design and uses of a separate avoid it. parameter for commercial aviation. AOA indicator. However, many other parameters influence the lift that a wing produces. The most basic is the configuration 1 LIFT VERSUS AOA of the wing, specifically the position of FIGURE the trailing-edge flaps, leading-edge flaps or slats, and spoilers. As the Maximum lift trailing-edge flaps are extended, the curvature (or camber) and area of Lifting range Stalled the wing are increased, and the wing will produce more lift at the same Lift AOA (fig. 2). Note that although the maximum lift is increased, the AOA at which stall occurs is actually less because the wing cannot sustain the higher lift levels up to the same AOA. The airflow separates earlier. Wing-mounted speed brakes or spoilers have the opposite effect. Angle of attack They reduce the lift at a given AOA; AERO 12 EFFECT OF TRAILING-EDGE FLAP that the wing must provide enough AIRPLANE PERFORMANCE is usually slower than that for best 2 AND SPEED BRAKE POSITION 3 EFFECT OF LEADING-EDGE POSITION lift to compensate for the download 2 AND AOA lift-to-drag (L/D) ratio or angle of climb. FIGURE FIGURE on the tail in addition to the weight of However, if the nose were to be kept Takeoff. During rotation, pitch angle the airplane. Note that the AOA of stall is the critical parameter that ensures down and the airplane accelerated Cruise Landing is not changed, but the lift required to higher speeds, short-term climb Increasing Takeoff tail clearance. Once the airplane is flap deflection of the wing is greater, and therefore airborne and at a sufficient altitude performance would be sacrificed and Landing ng the stall speed is increased. where ground effect and crosswinds do a close-in obstacle may not be cleared. wi Takeoff e not affect the sensor reading, AOA uis Lift Thrust also can affect lift in three Cr ways. First, the component of thrust will provide valid information. Cruise. Range is a function of both the Cruise Lift Contaminated that acts in the lift direction offsets During takeoff climb, there is no aerodynamics of the airplane and the Increasing speed brakes some of the lift required of the single target AOA to fly that will guar- fuel-flow characteristics of the engines. wing (fig. 6). Therefore, as thrust is antee certified takeoff performance. Aerodynamically, the minimum drag increased, the AOA for trimmed flight Takeoff-climb AOA will vary with such point occurs at the point where the L/D is reduced and the maximum lift is factors as airplane gross weight, thrust, is a maximum. But this value depends increased. Second, thrust changes the altitude, flap setting, and CG. Takeoff- on both AOA and Mach number, so the Angle of attack climb speeds (hence, AOA) are limited optimal AOA will vary as Mach number Angle of attack airflow around the wing and flaps, by stall speed, tail clearance, and mini- is changed. which does not usually have a large mum control speeds. The higher speed effect on jet transport airplanes. Third, The fuel-flow characteristics of the they also reduce the maximum lift On most transport category airplanes, the resultant increase in Mach number and greater thrust of an all-engine thrust affects airplane trim, usually takeoff reduce the AOA significantly engines are not affected by AOA, but achievable but, surprisingly, increase the lift that the wing produces is also will cause a stall at a higher speed and the AOA at which stall occurs. a function of Mach number, particularly lower AOA. This is true even at takeoff by reducing the download on the tail relative to an engine-out takeoff at the they do depend on the thrust required as the airplane approaches transonic and landing speeds with the flaps down. (see previous paragraph on CG). engine-out climb speed (V2). (drag), Mach number, and temperature. Leading-edge devices, such as Krueger flaps and slats, permit the wing to speeds typical of cruise flight (fig. 4). The center of gravity (CG) also affects The examples cited above show that The key to optimal takeoff perform- Combining the wing and engine operate at a higher AOA before it Of course, lift at a given AOA will the lift that the wing must produce. many parameters affect the relationship ance is to “fly the speeds.” The takeoff characteristics yields the fuel mileage of stalls by delaying the flow separation. increase with speed, but even at the As the CG moves forward, the nose-down of lift and AOA. For AOA information flight path that guarantees clearance of the airplane, so fuel mileage is a strong Figure 3 illustrates this and the effect same airspeed, as Mach number is moment increases because of the airplane to be useful to a flight crew, these all obstacles ahead is calculated based function of Mach number. Figure 7 of contamination, such as ice or dents, increased (the speed of sound changes parameters must be considered and on flight at these speeds. Following rota- shows the fuel mileage of a 757-200 at weight and wing lift (fig. 5). Therefore, on the leading edge. Contamination with temperature), lift will increase. accounted for in the indications and tion at VR, V2 is the resulting engine- an altitude of 35,000 ft as a function the downforce on the horizontal tail can cause the airflow to separate at However, higher Mach reduces the required to trim is increased. This means associated crew procedures. out speed at an altitude of 35 ft and of gross weight and Mach number. It a lower AOA, causing the wing to stall maximum lift the at a lower AOA than expected. While wing can attain these effects are accounted for in the and the AOA at 5EFFECT OF CENTER OF GRAVITY (CG) 6 EFFECT OF THRUST 7 CRUISE PERFORMANCE airplane design and maintenance which stall occurs. FIGURE FIGURE FIGURE Long-range cruise versus program, it is important to remember This means that constant angle of attack: 757/35,000 ft this potential variability in stall AOA as gross weight, Aft limit Climb (see “Winter Operations—Keep It Clean,” altitude, or load Forward limit Airliner, Oct.–Dec. 1983). factor is increased, Idle LRC 2 deg 2 to 3 deg AOA = 2.0 deg Lift Lift 4 EFFECT OF MACH NUMBER AOA = 2.5 deg FIGURE 180,000 lb Fuel mileage, 200,000 lb Angle of attack mile/ Angle of attack pound of fuel Lift Wing lift 220,000 lb Gross thrust 1 to 2 deg Increasing Mach number Lift Wing pitching moment Tail lift V(Flight path vector) s As CG moves forward, tail lift (down) for trim is greater. A component of thrust s Wing lift must increase to compensate. acts to increase airplane lift. Angle of attack s Airplane must fly at higher angle of attack to maintain overall lift. Mach number AERO AERO 14 15 Those airplanes However, in revenue service, CG is rarely estimated based on a measuring device 8 APPROACH AOA that do not account at the forward limit. So, if the approach- mounted somewhere on the airplane. 9 AOA MEASUREMENT ERRORS FIGURE for the variation es were flown on a daily basis by refer- Any such device has inherent errors FIGURE Mach number Flap position of stall speed with ence to a fixed-approach AOA based on that must be addressed. Mach number set a margin above stall, at any CG aft of Flap Wherever the device is located, it Mach position Increasing altitude the approach speed the forward limit, the probability of tail is measuring the flow angle in its own number at the most con- strike would be greater than the current AOA vane AOA vane local vicinity, not at the wing. Stall servative altitude. practice of using approach airspeeds. warning devices have been mounted on 0.4 deg Angle of 0.5 deg The speeds also In addition, variations in thrust will affect the wing, but most modern commercial 0.4 deg attack allow for the most jet airplanes have movable leading edges the approach AOA-speed relationship. adverse CG (forward) that would interfere with such an instal- that requires the From the discussion above, it can be AOA body AOA body lation. Most have the sensor located most lift out of the seen that approach speed may be on the fuselage, far ahead of the wing, wing, resulting in limited by many different requirements reducing the effect of changes in lift Gear position Sideslip the highest stall and that no single AOA can be target- and configuration. Nearer to the nose speed and, there- ed to ensure proper speed or landing of the airplane, the airflow is relatively Sideslip Gross weight fore, the highest attitude margins. clean and the boundary layer is thin, AOA vane Gear down AOA vane approach speed. minimizing the required probe height. 0.5 deg 0.5 deg can be seen that the optimal long-range In addition, the approach speed 3 AOA MEASUREMENT Even at the nose, many factors can cruise Mach number does not vary cannot be smaller than a multiple The previous section dealt with the affect the relationship between the Gear up significantly as gross weight (hence, of the minimum control speed in the relationship between the aerodynamics local AOA and true wing AOA (fig. 9). lift and AOA) changes. Superimposed landing configuration (Vmcl). This of the airplane and the true AOA of the The angle of airflow around the nose AOA body AOA body on this chart are two lines of constant speed is not significantly influenced wing. In practice, the true AOA of is not the same as at the wing. AOA. It is apparent that flying a by movement of the CG. So, during the wing is not known. It only can be Also, the sensitivity to changes in Pitch rate Ground effect constant AOA will not yield optimal an approach at AOA is greater, so a 1-deg change performance. If a flight crew tried to fly the aft CG, if the AOA AND ERRORS IN SPEED AND GROSS WEIGHT in true wing AOA causes a local flow Pitch rate a target AOA and there was an error of flight crew Ground reduces speed to change at the nose of 1.5 to 2 deg. The as little as 0.5 deg, the penalty in fuel RELATIONSHIP BETWEEN AIRSPEED AND AOA trailing-edge flap position has an influ- effect fly at the same AOA vane AOA vane mileage could be 3 percent or more. ence on a typical AOA sensor calibra- 0.5 deg AOA as required 0.3 deg Wind is a more fundamental consid- for the forward tion, as has landing gear position (in Free air eration. For best fuel mileage in a head- CG, an approach particular, that of the nose landing gear wind, the airplane should be flown speed below the 10 kt doors). Mach number affects the flow faster than the speed for best range in minimum control Cruise around the nose and therefore changes AOA body AOA body still air; in a tailwind, it should be speed may result. the sensor calibration. flown more slowly. Most modern Boeing Airspeed Pitching the airplane can cause erro- For the most part, the effects sensors are located near the nose and airplanes have a flight management A further con- 0.5 deg sideration is the neous readings at the sensor. While the discussed above can be compensated the air data probes, certain conditions, computer (FMC) that accounts for air- for and, depending on the airplane, such as radome damage or loss, may nose is pitching up (as in a turn), the plane, engine, and wind characteristics clearance of the many have been. It should be noted, cause erroneous measurement of AOA aft body from the 1 kt local flow angle is reduced, causing and can compute the optimal speed however, that each correction has its as well as airspeed. ground as the the reading to be too low. Although the to be flown. own inherent uncertainty and can also airplane lands. Stall 0.5 deg sensors are placed to minimize the effect of sideslip, it is not eliminated cause erroneous readings if the input AOA INDICATIONS AND FLIGHT Some airplanes, Approach speed. Approach speed is critical to landing performance and is particularly those Angle of attack and can be quite significant at sideslip data is incorrect. 4 CREW PROCEDURES IN CURRENT with stretched angles that may occur on short final In the philosophy of “keep it simple,” BOEING PRODUCTION MODELS established during the airplane certifi- fuselages, have An error in AOA corresponds to a much higher error approaches or with an engine out. the fewer dependencies on other AOA is most useful to the flight crew cation process. It is determined not increased approach in airspeed or gross weight at high speeds than at low data, the more robust the AOA system will at high angles of attack to show the only by margin above stall speed but Even variations in the contour of the speeds to reduce speeds. For a mid-sized airplane, such as the 757-200, be. For example, Mach number affects margin to stall or stall warning. All also may be increased by consideration skin near the sensor can subtly affect the AOA and a 0.5-deg error in AOA results in the following errors: the sensor calibration. While this rela- indications driven by AOA —stick shaker, of minimum control speed and the local flow angle. Many of these tail clearance at touchdown. hence the pitch tionship could be compensated for, this PLI, and speed tape indications —are design challenges also affect pitot and Flight regime Speed error Gross weight error would make the sensor output depend- related to this important information. Regulations require that the approach angle on touch- static port installation and accuracy. down. This pro- High speed 10 kt 30,000 lb* ent on good Mach information. If the speed be no smaller than a specific The sensor itself has potential airspeed data were inaccurate, the Stick shaker. An artificial stall multiple of the stall speed. Because vides adequate Takeoff/landing 2 kt 6,000 lb for error. The combination of installa- calculated Mach number and therefore warning system is required for airplane stall speed is a function of Mach num- clearance between Stall warning <1 kt <4,000 lb tion error, zero bias, and aerodynamic the calibrated AOA reading would be certification if the natural prestall ber, stall-limited approach speed will the body and the inaccuracy can total 0.5 deg or more. incorrect. This would affect the useful- buffet characteristics of the airplane occur at a different AOA at different ground at the *14 percent of maximum takeoff weight. Contamination or damage can also ness of AOA in the event of an airspeed are insufficient to warn the flight crew gross weights and altitudes (fig. 8). most critical CG. affect the sensor’s accuracy. system failure. Note that because the of an impending stall. This warning AERO AERO 16 17 Pitch limit indicator. The PLI origi- Because stall AOA is a function of 10 STALL WARNING SCHEDULE nally was developed as part of an industry Mach number, a PLI on airplanes with 12 OPTIONAL AOA GAUGE FOR 737-600/-700/-800/-900, 767-400, 777 FIGURE effort to address windshear escape fixed stall warning schedules would FIGURE training. Because stall warning is display an excessively large margin primarily a function of AOA, the PLI at typical cruise Mach and altitude. Mach-independent schedule shows AOA margin to stall warning, To avoid this misleading display, PLI Performance even though it is part of the pitch was available only with flaps extended improvement attitude display (fig. 11). The distance when it was introduced in the mid-1980s. Mach-dependent schedule from the airplane symbol to the PLI is Later airplanes have employed stall calculated from the difference between warning schedules that adjust the Angle of attack the AOA of the airplane and the AOA stall warning threshold as a function Initial buffet at which stall warning will occur. This of Mach number. The design of the AOA gauge on primary flight display boundary provides the flight crew with good 777, 717, and 767-400ER has taken situational awareness, enabling them advantage of this and will display the Terminal Cruise to monitor airplane attitude in pitch PLI full time when flaps are down, operations as well as when flaps are up if speed and roll relative to the horizon, while simultaneously showing whether the or load factor causes stall margin Mach number to decrease to an AOA within 1.3 g airplane is approaching its maximum Stall warning (stick shaker) AOA AOA. In general, when the airplane of stall warning. must be in a form other than visual system dependent on good pitot and symbol and the amber PLI bars meet, Work is currently under way to to be effective, even if the flight crew static data, a factor that will be the stall warning system will activate. introduce this type of PLI indication Analog is not looking at the instrument panel. considered in the next section on the on other models. Recent changes to pointer However, the PLI also is limited to Beginning with early commercial jet- dedicated AOA indicator. the 757 and 767 enable the PLI to be Approach reference band 30 deg of pitch attitude, regardless of liners, standard practice has been to It should be noted from figure 10 displayed with flaps up. AOA. If AOA or AOA margin to stick equip these airplanes with a stick shaker that the stall warning schedule does shaker were to be used as the first and 6.0 as a means of stall warning. Some not follow the buffet boundary at very Speed tape indications. Soon after primary focus of the flight crew during airplanes also have employed stick high Mach numbers. The buffet here is the introduction of the PLI, a vertical windshear escape or terrain avoidance Digital nudgers or stick pushers to improve scale airspeed indicator was developed caused by Mach buffet, or too high a procedures, extremely high pitch readout stall avoidance and stall characteristics. and added to electronic flight displays. speed. Setting the stall warning system attitudes could be reached before stall All these indications have been driven This offered the opportunity to to activate at this point may lead the warning if the maneuver is entered by an AOA threshold, which is usually calculate and place airspeed-related flight crew to believe the airplane is with sufficient speed. Therefore, the a function of flap configuration, landing data such as maximum, minimum, to cruise. The position of the amber parameters throughout the flight deck. near stall and increase, rather than PLI shows the lesser of either margin gear configuration, or both. maneuvering, and reference speeds on and red bands is always a function of decrease, speed. to stick shaker, or 30 deg of pitch. Stall warning AOA is shown with Because of the effect of Mach number the airspeed instrument (fig. 11). All AOA margin to stall warning. a red tick mark, which will change on stall AOA, the stall warning AOA typi- Boeing models currently in production The speed tape is designed to pro- position as a function of Mach number cally was set at a conservative level to 11 PRIMARY FLIGHT DISPLAY STALL WARNING MARGIN INDICATIONS have this capability. vide the flight crew with situational for those airplanes with Mach- accommodate gross weight and altitude FIGURE Of particular interest are the minimum awareness of the flight envelope. dependent stall warning schedules. variations expected in the terminal area. speed amber and red bands, or barber It shows the crew where the airplane A green approach reference band is The early stall warning system pole. At low speeds on Boeing-designed speed is relative to the limits (i.e., shown whenever landing flaps are Pitch limit airplanes currently in production, these maximum placard speeds or minimum selected. The range of the approach thresholds were not set to be effective indicator (PLI) reference band accounts for normally at cruise altitudes and speeds because indications are based on sensed AOA stall warning speed, as well as the they did not correct for Mach number and the AOA margin to stick shaker. maneuvering capability available). expected variations in CG, thrust, (fig. 10). This kept the system simple. Airspeed tape At higher Mach numbers, most airplanes sideslip, and other considerations. lower DESIGN AND USES OF A The stick shaker was set at an AOA with fixed AOA stall warning sched- Many AOA indicators used in the past effective for low altitudes but at too barber pole ules show margins to stick shaker or 5 SEPARATE AOA INDICATOR have been of the “normalized” type, high a value for cruise. Natural stall AOAss – AOAcurrent margin to initial buffet, whichever Boeing and several operators worked where AOA is shown in arbitrary units buffet was found to give satisfactory corresponds to the highest speed. On together to develop the display and scaled so that zero load factor is warning at higher Mach numbers. these airplanes, the margin to buffet format for an optional AOA indicator shown as an AOA of zero and stall at higher Mach numbers is calculated (fig. 12). The upper right location is shown as an AOA of one. Normalized Later stall warning systems used Based on by the FMC. was chosen as one that can be accom- AOA on a commercial jetliner would Mach number from the pitot or static AOAss – AOAcurrent On newer models, such as the plished without significant rearrange- require that Mach number be intro- air data system to adjust the stall Boeing-designed, ment of the existing PFD or electronic duced into the calculation of AOA warning AOA threshold down as Mach series-700 777 and 767-400, the amber and red models flight display formats. The indicator because stall AOA and buffet margins number increased. This provided the bands show margin to stall warning itself consists of an analog scale and are a function of Mach number. flight crew with a stall warning related at all times because the stall warning to the actual available performance. schedule generally follows the initial pointer, and digital representation The indicator developed shows body However, it also made the stall warning buffet boundary at higher speeds up similar to displays of many other AOA in degrees and is not normalized, AERO AERO 18 19 which is related to the second objec- Improved situational awareness and Pitot or static system failure without stabilizing the airplane in pitch from the pitch attitude display, it does However, for either method, the errors tive above, that the indicator be useful flight crew training. There is a desire requires the flight crew to take several can lead to an oscillatory flight path. not provide protection against high must be large enough that they are not when pitot or static data, and therefore to use AOA information to increase fundamental steps to resolve the pitch attitudes if the indicator is used masked by other factors. Mach calculations, are unreliable because the flight crew’s understanding of the problem (see “Erroneous Flight as the flight crew’s primary focus or Instrument Information,” Aero no. 8, Reference during upset recovery, Normal variations in AOA as a result of of blockage or a fault in the system. physics of flight and their general target during such maneuvers. Oct. 1999): windshear escape, and terrain the regulatory requirements on approach The pointer of a normalized indicator in awareness of the state of the wing dur- avoidance maneuvers. Windshear For upset recovery, either the PLI or speed, as well as those caused by this condition would behave erratically, ing normal and nonnormal conditions. s Recognize an unusual or suspect escape and terrain avoidance maneu- the red stall warning mark on the AOA differences in thrust, CG, sideslip, and making the indicator unusable. Within certain limitations, the display indication. vers require immediate change in indicator may be used to assess the the installed accuracy of the AOA With the nonnormalized design, the provides this indication in a clear, s Keep control of the airplane pitch attitude and thrust, followed margin to stall warning. measurement system, may act together position of the needle is a function unambiguous format. The degree to with basic pitch and power skills. by monitoring of the situation and to mask all but large errors in weight only of sensed AOA. The red tick mark which AOA can be used to increase further increases in pitch attitude if Indication of maximum L/D or or configuration. These factors are taken s Take inventory of reliable for stall warning may behave erratically knowledge and airmanship depends, of needed, while avoiding stick shaker range, detection of weight errors, into account in determining the size information. in a pitot or static failure state, as may course, on the approach taken by the activation. The PLI was developed and a check of fuel consumption of the green approach reference band. stick shaker, PLI, and speed tape amber airline in training its flight crews and s Find or maintain favorable primarily with these purposes in mind To keep the size of the green band during cruise. As shown in the section and red bands. However, the AOA the use of the indicator in training sce- flying conditions. and works well. On all current produc- from becoming too large, these varia- on airplane performance, AOA is not the needle and digits will remain stable, narios for nonnormal procedures. Some s Get assistance from others. tion models, PLI is shown when flaps appropriate parameter for optimizing tions were root-sum-squared because and the indicator itself still will be of the limitations are discussed below. are down. At this time, PLI is avail- cruise flight, because of the strong of the low probability that they would s Use checklists. able with flaps retracted on the 717, all add in the same direction at any useful as a backup for unreliable influence of Mach number on airplane airspeed, provided the AOA Recognition of a problem 767-400, 777, and MD-11. Work is performance. Because AOA is not very one time. The resulting green band is vanes are undamaged. KEY POINTS TO EMPHASIZE IN TRAINING will be accomplished by under way to make this capability sensitive to speed or weight changes about 2 deg wide for the 777 and 3 deg instrument scanning and available on other Boeing-designed at cruise speeds, even large gross for the 737. The band is centered at an A variety of potential uses for s AOA is most useful in high-AOA, low-speed cross-check practices or crew models currently in production. weight errors may not be detectable. AOA equivalent to Vref +5 kt, assuming AOA were examined during the a nominal gross weight, mid-CG, no parts of the envelope; it is less useful at most alerts, depending on the The first steps in windshear escape A 0.5-deg error in AOA is equivalent design of the new AOA indicator: sideslip, a stabilized 3-deg glideslope normal speeds. design of the system in the and terrain avoidance procedures to 30,000 lb on a 757-200, or approxi- s Improved situational airplane. In this respect, AOA involve applying maximum certified mately 14 percent of the maximum thrust level, and no system error. s Airspeed and Mach are still the primary sources awareness and flight crew for performance data for reasons of precision, instruments can be useful as thrust and control of airplane pitch takeoff weight. A 20,000-lb weight error on a 757, training. regulatory basis, system redundancy, and integrity. an additional cross-check. attitude to an initial target, while corresponding to approximately s AOA backup indication Therefore, if the AOA indicator is used, flight crews Present procedures for unre- honoring stall warning. AOA margin to Cross-check to detect weight or 10 percent of maximum landing gross should cross-check with other instruments, just liable airspeed call for flying stick shaker, whether shown with the configuration errors on approach weight or about a 40 percent error in following pitot or static as they would with airspeed. the airplane by reference to PLI or the AOA display, is a secondary to reduce the probability of tail payload, yields a change in AOA of system failures. pitch attitudes, and refer reference during this part of the strikes on landing. AOA can be used 1.7 deg. So, it can be seen that even s The AOA approach reference green band may be s Reference during upset the pilots to reference tables maneuver, not the primary target. As during approach as an extra cross-check relatively large weight errors may not used as a cross-check for configuration errors, recovery, windshear mentioned in the section on PLI, pitch- for errors in configuration, weight, or be enough to move the needle out of reference speed calculation errors, or very large showing pitch attitudes for escape, and terrain ing up by sole reference to AOA-based reference speed calculation. Proximity the green band. Conversely, it is also errors in gross weight. Normal variations in AOA various configurations, avoidance maneuvers. indications can result in excessively of the barber pole to the reference possible that flying at the proper speed measurement dictate the width of the green band. weights, and altitudes that high pitch attitudes if the maneuver speed on the airspeed tape can be and configuration may yield an AOA s Indication of maximum Also, because approach speed in some cases can will result in safe angles of is entered at sufficiently high speeds. used in a similar manner because it is that is outside the reference band. L/D or range, detection be determined by issues not related to or sensed attack and speeds. AOA could Because the AOA display is separate based on AOA margin to stick shaker. Figure 13 illustrates how errors can be by AOA, increasing or decreasing approach speed by be useful if the relevant data of weight errors, and a targeting the center of the green band can result is included in the pitch and check of fuel consump- tion during cruise. in inappropriate approach speeds. power tables that already 13 AOA GAUGE UNDER VARIOUS CONDITIONS ON APPROACH TO LANDING s Pulling to stick shaker AOA from a high-speed con- exist in the nonnormal FIGURE s Cross-check to detect dition without reference to pitch attitude can lead checklist procedures. AOA weight or configuration to excessive pitch attitudes and a higher probability would be most useful in fly- errors on approach to of stall as a result of high deceleration rate. ing the airplane in multiple reduce the probability of failure conditions where all 4.7 6.9 7.5 6.7 6.2 tail strikes on landing. pitot or static sources are AOA backup indication following affected, making all airspeed indicators AOA can be used for some of these pitot or static system failures. The unreliable. purposes, but it does not work as AOA instrument described in this article Care should be taken when flying Weight error 0 23,000 0 0 16,000 23,000 0 well for others. From the standpoint is useful as a backup for unreliable the airplane by reference to AOA in lieu (14%) (9%) (14%) of flight operations, some of the goals airspeed indication caused by pitot or of airspeed. Control should be made by can be met with certain caveats that static source blockage because the cal- reference to pitch attitude, using AOA CG mid mid mid fwd mid mid aft take into account the principles and culation of indicated AOA is not greatly as a cross-check to ensure that the limitations of AOA measurement affected by pitot or static pressure Speed Vref + 5 Vref + 5 Vref + 5 Vref Vref + 5 Vref + 5 Vref + 8 pitch attitude results in the desired and aerodynamic performance of modern inputs for its calibration, and the dis- speed or AOA. Attempting to follow Speed brake down down 40% down down down 40% commercial jet airplanes. played value has not been normalized. AOA or speed indications too closely AERO AERO 20 21 masked or canceled out by variation in the other parameters. For these and other reasons, the AOA indicator can be used as an additional means to check for large errors in weight or configuration, but it should not be used as a substitute for current proce- dures to establish approach speeds and verify configurations. To determine the approach speed based solely on placing AOA in the green band can cause situa- tions of excessively high or low approach speeds, depending on a variety of circumstances. SUMMARY AOA is a long-standing subject that is broadly known but one for which the details are not broadly understood. While AOA is a very useful and important parameter in some instances, it is not useful and is potentially misleading in others. s The relationship between AOA and airplane lift and performance is complex, depending on many factors, such as airplane configur- ation, Mach number, thrust, and CG. s AOA information is most impor- tant when approaching stall. s AOA is not accurate enough to be used to optimize cruise performance. Mach number is the critical parameter. s AOA information currently is displayed on Boeing flight decks. The information is used to drive the PLI and speed tape displays. s An independent AOA indicator is being offered as an option for the 737, 767-400, and 777 airplanes. The AOA indicator can be used to assist with unreliable airspeed indications as a result of blocked pitot or static ports and may provide additional situation and configuration awareness to the flight crew.
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