OPERATIONAL USE OF 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 . It is less useful to the flight crew in the normal operational range. On most FLIGHT OPERATIONS 10 Boeing models currently in production, AOA information is presented in several JOHN E. CASHMAN BRIAN D. KELLY BRIAN N. NIELD ways: , airspeed tape, and pitch limit indicator. Boeing has also devel- DIRECTOR TECHNICAL FELLOW MANAGER 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- WHAT IS ANGLE OF ATTACK? 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 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 A dedicated AOA indicator shown which it can function efficiently. In and instructional material written on the primary flight display (PFD) its simplest form, is a function of 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. During the development of the higher AOA than a light one. Conversely, Dedicated AOA indicators have been new indicator, discussions with as an airplane decelerates, the AOA used on military 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 not been used often on commercial ational range, there is a relationship and engineers provided a unique airplanes. On Boeing models currently among lift, speed, and AOA. opportunity to examine potential uses in production, AOA is used to drive This relationship will change if the AOA, FLIGHT PATH ANGLE, AND PITCH ANGLE 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 cators, and the pitch limit indicator from the upper surface, resulting in display and indication technology. (PLI) on the primary attitude dis- a loss of lift, or a stall. It should be plays. AOA information is combined This article discusses the following: noted that this stall condition could occur at a wide range of speeds with other data and displayed as an 1. Basic principles of AOA. Angle of attack integral part of flight deck displays. (depending on the airplane weight or 2. Airplane performance and AOA. load factor, or g loading) and at any Velocity Pitch angle Recent accidents and incidents 3. AOA measurement. attitude (depending on the flight (flight path vector) have resulted in new flight crew path angle). What is important is the 4. AOA indications and flight crew Flight path angle training programs for upset recovery AOA. Therefore, it is imperative to Angle of attack (AOA) is the (still air) procedures in current Boeing and terrain avoidance, and these in know when the wing is approaching angle between the oncoming air or production models. the stall AOA and to take steps to turn have heightened industry relative wind and a reference line Horizon interest in AOA as a useful flight 5. Design and uses of a separate avoid it. on the airplane or wing. Sometimes, parameter for commercial aviation. AOA indicator. However, many other parameters the reference line is a line connect- influence the lift that a wing produces. ing the and trailing relative to the ground will be less The most basic is the configuration Flight path angle is defined in 1 LIFT VERSUS AOA edge at some average point on the two different ways. To the aerody- than that referenced to the air. On of the wing, specifically the position of wing. Most commercial jet airplanes FIGURE namicist, it is the angle between the newest commercial jet airplanes, the trailing-edge flaps, leading-edge use the centerline or flaps or slats, and spoilers. As the the flight path vector (where the this angle can be displayed on longitudinal axis as the reference Maximum lift trailing-edge flaps are extended, the airplane is going) and the local the primary flight display and is line. It makes no difference what curvature (or camber) and area of atmosphere. To the flight crew, calculated referenced to the ground Lifting range Stalled the wing are increased, and the wing the reference line is, as long as it it is normally known as the angle (the inertial flight path angle). is used consistently. will produce more lift at the same between the flight path vector and AOA is the difference between Lift AOA (fig. 2). Note that although the AOA is sometimes confused with the horizon, also known as the pitch angle and flight path angle maximum lift is increased, the AOA at pitch angle or flight path angle. climb (or descent) angle. Airmass- when the flight path angle is refer- which stall occurs is actually less Pitch angle (attitude) is the angle referenced and inertial-referenced enced to the atmosphere. Because of because the wing cannot sustain the between the longitudinal axis flight path angles are the same only the relationship of pitch angle, AOA, higher lift levels up to the same AOA. (where the airplane is pointed) in still air (i.e., when there is no and flight path angle, an airplane The airflow separates earlier. and the horizon. This angle is dis- wind or vertical air movement). For can reach a very high AOA even with Wing-mounted speed brakes or played on the example, in a headwind or sinking the nose below the horizon, if the spoilers have the opposite effect. Angle of attack or artificial horizon. air mass, the flight path angle flight path angle is a steep descent. They reduce the lift at a given AOA;

AERO AERO 12 13 that the wing must provide enough is usually slower than that for best EFFECT OF TRAILING-EDGE EFFECT OF LEADING-EDGE POSITION AIRPLANE PERFORMANCE 2 AND SPEED BRAKE POSITION 3 lift to compensate for the download 2 AND AOA lift-to- (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 a close-in obstacle may not be cleared. Takeoff the stall speed is increased. where ground effect and crosswinds do Thrust also can affect lift in three not affect the sensor reading, AOA Cruise wing Lift will provide valid information. Cruise ways. First, the component of thrust Cruise. Range is a function of both the Lift During takeoff climb, there is no aerodynamics of the airplane and the Increasing Contaminated that acts in the lift direction offsets 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 climb speeds (hence, AOA) are limited Angle of attack Angle of attack airflow around the wing and flaps, optimal AOA will vary as Mach number by stall speed, tail clearance, and mini- which does not usually have a large is changed. mum control speeds. The higher speed effect on jet transport airplanes. Third, they also reduce the maximum lift On most transport category airplanes, the resultant increase in Mach number and greater thrust of an all-engine The fuel-flow characteristics of the thrust affects airplane trim, usually achievable but, surprisingly, increase the lift that the wing produces is also will cause a stall at a higher speed and takeoff reduce the AOA significantly engines are not affected by AOA, but 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 (see previous paragraph on CG). (drag), Mach number, and temperature. Leading-edge devices, such as Krueger as the airplane approaches transonic and landing speeds with the flaps down. engine-out climb speed (V2). 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 increased (the speed of sound changes of contamination, such as ice or dents, weight and wing lift (fig. 5). Therefore, parameters must be considered and on flight at these speeds. Following rota- shows the fuel mileage of a 757-200 at with temperature), lift will increase. on the leading edge. Contamination the on the horizontal tail accounted for in the indications and tion at VR, V2 is the resulting engine- an altitude of 35,000 ft as a function 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 5 EFFECT 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 as gross weight, this potential variability in stall AOA 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 EFFECT OF MACH NUMBER 4 AOA = 2.5 deg FIGURE 180,000 lb Fuel mileage, 200,000 lb Angle of attack mile/ Angle of attack pound of fuel 220,000 lb Lift Wing lift Gross thrust 1 to 2 deg Lift Increasing Mach number Wing Tail lift V(Flight path vector) ■ As CG moves forward, tail lift (down) for trim is greater. A component of thrust ■ Wing lift must increase to compensate. acts to increase airplane lift. Angle of attack ■ 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 es were flown on a daily basis by refer- FIGURE for the variation Any such device has inherent errors 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 AOA AOA at the most con- strike would be greater than the current local vicinity, not at the wing. Stall vane vane servative altitude. practice of using approach airspeeds. warning devices have been mounted on 0.4 deg 0.5 deg The speeds also 0.4 deg Angle of In addition, variations in thrust will affect the wing, but most modern commercial attack allow for the most the approach AOA-speed relationship. jet airplanes have movable leading edges adverse CG (forward) that would interfere with such an instal- AOA AOA that requires the From the discussion above, it can be lation. Most have the sensor located body body 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, AOAvane Gear down AOAvane approach speed. minimizing the required probe height. 0.5 deg AOA MEASUREMENT 0.5 deg can be seen that the optimal long-range In addition, the approach speed 3 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 AOAbody AOAbody 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. by movement of the CG. So, during the wing is not known. It only can be AOA. It is apparent that flying a Also, the sensitivity to changes in an approach at Pitch rate Ground effect constant AOA will not yield optimal AOA is greater, so a 1-deg change the aft CG, if the performance. If a flight crew tried to fly 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 effect fly at the same trailing-edge flap position has an influ- AOAvane AOAvane mileage could be 3 percent or more. 0.5 deg AOA as required ence on a typical AOA sensor calibra- 0.3 deg tion, as has position (in Wind is a more fundamental consid- for the forward 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 Cruise AOA AOA faster than the speed for best range in minimum control around the nose and therefore changes body 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 neous readings at the sensor. While the discussed above can be compensated the air data probes, certain conditions, computer (FMC) that accounts for air- sideration is the nose is pitching up (as in a turn), the for and, depending on the airplane, such as radome damage or loss, may plane, engine, and wind characteristics clearance of the 1 kt local flow angle is reduced, causing many have been. It should be noted, cause erroneous measurement of AOA and can compute the optimal speed aft body from the the reading to be too low. Although the however, that each correction has its as well as airspeed. to be flown. ground as the airplane lands. Stall 0.5 deg sensors are placed to minimize the own inherent uncertainty and can also effect of sideslip, it is not eliminated cause erroneous readings if the input AOA INDICATIONS AND FLIGHT Approach speed. Approach speed is Some airplanes, Angle of attack and can be quite significant at sideslip data is incorrect. 4 CREW PROCEDURES IN CURRENT critical to landing performance and is particularly those angles that may occur on short final In the philosophy of “keep it simple,” BOEING PRODUCTION MODELS established during the airplane certifi- with stretched 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 , have An error in AOA corresponds to a much higher error data, the more robust the AOA system will at high angles of attack to show the only by margin above stall speed but increased approach in airspeed or gross weight at high speeds than at low Even variations in the contour of the be. For example, Mach number affects margin to stall or stall warning. All also may be increased by consideration speeds to reduce speeds. For a mid-sized airplane, such as the 757-200, skin near the sensor can subtly affect the sensor calibration. While this rela- indications driven by AOA—stick shaker, of minimum control speed and the AOA and a 0.5-deg error in AOA results in the following errors: the local flow angle. Many of these tionship could be compensated for, this PLI, and speed tape indications—are tail clearance at touchdown. hence the pitch 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 vides adequate Takeoff/landing 2 kt 6,000 lb multiple of the stall speed. Because for error. The combination of installa- calculated Mach number and therefore warning system is required for airplane clearance between stall speed is a function of Mach num- Stall warning <1 kt <4,000 lb tion error, zero bias, and aerodynamic the calibrated AOA reading would be certification if the natural prestall the body and the ber, stall-limited approach speed will inaccuracy can total 0.5 deg or more. incorrect. This would affect the useful- buffet characteristics of the airplane ground at the occur at a different AOA at different *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 STALL WARNING SCHEDULE OPTIONAL AOA GAUGE FOR 737-600/-700/-800/-900, 767-400, 777 10 nally was developed as part of an industry Mach number, a PLI on airplanes with 12 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 warning schedules that adjust the Angle of calculated from the difference between 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 and roll relative to the horizon, while as well as when flaps are up if speed 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 30 deg of pitch attitude, regardless of Approach reference band liners, standard practice has been to displayed with flaps up. It should be noted from figure 10 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 primary focus of the flight crew during Speed tape indications. Soon after airplanes also have employed stick high Mach numbers. The buffet here is windshear escape or terrain avoidance the introduction of the PLI, a vertical Digital nudgers or stick pushers to improve caused by Mach buffet, or too high a procedures, extremely high pitch scale was developed readout stall avoidance and stall characteristics. speed. Setting the stall warning system attitudes could be reached before stall and added to electronic flight displays. All these indications have been driven to activate at this point may lead the warning if the maneuver is entered This offered the opportunity to by an AOA threshold, which is usually flight crew to believe the airplane is with sufficient speed. Therefore, the calculate and place airspeed-related a function of flap configuration, landing near stall and increase, rather than PLI shows the lesser of either margin data such as maximum, minimum, to cruise. The position of the amber parameters throughout the flight deck. gear configuration, or both. decrease, speed. to stick shaker, or 30 deg of pitch. maneuvering, and reference speeds on and red bands is always a function of 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 position as a function of Mach number PRIMARY FLIGHT DISPLAY STALL WARNING MARGIN INDICATIONS The speed tape is designed to pro- cally was set at a conservative level to 11 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 thresholds were not set to be effective airplanes currently in production, these maximum placard speeds or minimum selected. The range of the approach indicator (PLI) at cruise altitudes and speeds because indications are based on sensed AOA stall warning speed, as well as the reference band accounts for normally 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 The stick shaker was set at an AOA with fixed AOA stall warning sched- DESIGN AND USES OF A barber pole Many AOA indicators used in the past effective for low altitudes but at too 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 is shown as an AOA of one. Normalized Later stall warning systems used Based on (fig. 12). The upper right location by the FMC. AOA on a commercial jetliner would Mach number from the pitot or static AOA – AOA was chosen as one that can be accom- ss current require that Mach number be intro- air data system to adjust the stall Boeing-designed, On newer models, such as the plished without significant rearrange- duced into the calculation of AOA warning AOA threshold down as Mach series-700 777 and 767-400, the amber and red ment of the existing PFD or electronic because stall AOA and buffet margins models bands show margin to stall warning flight display formats. The indicator number increased. This provided the are a function of Mach number. flight crew with a stall warning related at all times because the stall warning itself consists of an analog scale and 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 problem (see “Erroneous Flight the flight crew’s understanding of the Reference during upset recovery, as the flight crew’s primary focus or Normal variations in AOA as a result of of blockage or a fault in the system. Instrument Information,” Aero no. 8, physics of flight and their general windshear escape, and terrain target during such maneuvers. the regulatory requirements on approach The pointer of a normalized indicator in Oct. 1999): 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. 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, ■ 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 further increases in pitch attitude if or configuration. These factors are taken which AOA can be used to increase ■ Take inventory of reliable Indication of maximum L/D or for stall warning may behave erratically needed, while avoiding stick shaker into account in determining the size knowledge and airmanship depends, of information. range, detection of weight errors, 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. ■ Find or maintain favorable stick shaker, PLI, and speed tape amber airline in training its flight crews and primarily with these purposes in mind during cruise. As shown in the section To keep the size of the green band flying conditions. and red bands. However, the AOA the use of the indicator in training sce- and works well. On all current produc- on airplane performance, AOA is not the from becoming too large, these varia- needle and digits will remain stable, narios for nonnormal procedures. Some ■ 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 are down. At this time, PLI is avail- cruise flight, because of the strong of the low probability that they would of the limitations are discussed below. ■ Use checklists. useful as a backup for unreliable able with flaps retracted on the 717, influence of Mach number on airplane all add in the same direction at any 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 available on other Boeing-designed for the 737. The band is centered at an A variety of potential uses for instrument scanning and at cruise speeds, even large gross ■ AOA is most useful in high-AOA, low-speed models currently in production. AOA equivalent to V +5 kt, assuming AOA were examined during the cross-check practices or crew weight errors may not be detectable. ref parts of the envelope; it is less useful at most a nominal gross weight, mid-CG, no design of the new AOA indicator: alerts, depending on the The first steps in windshear escape A 0.5-deg error in AOA is equivalent normal speeds. design of the system in the and terrain avoidance procedures to 30,000 lb on a 757-200, or approxi- sideslip, a stabilized 3-deg glideslope ■ Improved situational thrust level, and no system error. ■ Airspeed and Mach are still the primary sources airplane. In this respect, AOA involve applying maximum certified mately 14 percent of the maximum 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 ■ 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 following pitot or static 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 PLI or the AOA display, is a secondary to reduce the probability of tail payload, yields a change in AOA of system failures. as they would with airspeed. the airplane by reference to reference during this part of the strikes on landing. AOA can be used 1.7 deg. So, it can be seen that even ■ The AOA approach reference green band may be pitch attitudes, and refer ■ during approach as an extra cross-check relatively large weight errors may not Reference during upset used as a cross-check for configuration errors, the pilots to reference tables maneuver, not the primary target. As for errors in configuration, weight, or be enough to move the needle out of recovery, windshear reference speed calculation errors, or very large showing pitch attitudes for mentioned in the section on PLI, pitch- reference speed calculation. Proximity the green band. Conversely, it is also escape, and terrain errors in gross weight. Normal variations in AOA various configurations, ing up by sole reference to AOA-based of the barber pole to the reference possible that flying at the proper speed avoidance maneuvers. measurement dictate the width of the green band. weights, and altitudes that indications can result in excessively high pitch attitudes if the maneuver speed on the airspeed tape can be and configuration may yield an AOA Also, because approach speed in some cases can will result in safe angles of ■ Indication of maximum is entered at sufficiently high speeds. used in a similar manner because it is that is outside the reference band. be determined by issues not related to or sensed attack and speeds. AOA could L/D or range, detection 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- in inappropriate approach speeds. power tables that already AOA GAUGE UNDER VARIOUS CONDITIONS ON APPROACH TO LANDING tion during cruise. 13 ■ Pulling to stick shaker AOA from a high-speed con- exist in the nonnormal FIGURE ■ 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 Speed V + 5 V + 5 V + 5 V V + 5 V + 5 V + 8 limitations of AOA measurement affected by pitot or static pressure pitch attitude results in the desired ref ref ref ref ref ref ref 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. AOA has been used In general, fighters operate workload by giving a simple target AOA has proved particularly techniques are used, where airplanes would be contrary to For these and other reasons, the AOA as a primary performance more often at the extremes of the to fly. AOA is accurate enough for useful for approach to aircraft glide path is controlled primarily the pitch commands provided indicator can be used as an additional means to check for large errors in weight parameter for years on some envelope, often flying at maximum these applications. In addition, the carriers, where it is important to by changes in thrust while the by the flight director bars, and or configuration, but it should not be military aircraft, particularly lift for minimum radius turns. For higher sweep and lower aspect maintain a consistent approach aircraft is held at a fixed AOA. to the speed hold mode of the used as a substitute for current proce- on fighters. There are many other applications, AOA minimizes ratio of the wing reduce the sen- attitude for each landing. In this Use of this technique during , which is often dures to establish approach speeds and verify configurations. To determine the good reasons for this. the pilot (usually single-place) sitivity to AOA errors. case, “backside” approach approach on commercial jet used during approach. 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. MILITARY SUMMARY APPLICATIONS 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. ■ 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. ■ AOA information is most impor- tant when approaching stall. ■ AOA is not accurate enough to be used to optimize cruise performance. Mach number is the critical parameter. ■ AOA information currently is displayed on Boeing flight decks. The information is used to drive the PLI and speed tape displays. ■ 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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