OPERATIONAL USE by hedongchenchen

VIEWS: 16 PAGES: 7

									                                                                                                                    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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