Free Understanding Air France 447 by Bill Palmer Page A

constant, at least not at Mach numbers above 0.3. Therefore, to act as a replacement in case of loss of airspeed/Mach number, the Mach number actually needs to be known to know the stall AOA, or conservative assumptions made. It is a catch-22. That is not to say it would be useless. In the case of AF447, it would have shown an obviously excessive AOA, and perhaps would have allowed the crew to answer the question both first officers posed: “What’s happening?” It might have led to earlier attempts to recover from the stall with pitch. However, in so much as the pilot flying seemed to be ignoring the more fundamental indications of pitch attitude and altitude, along with numerous stall warnings, one could question what difference a rarely used AOA gauge would have made.
    AOA indications are more useful at low altitudes (where the stall angle is constant) for higher AOA flight regimes like approach. Precision in the approach and climb phases is more critical and an AOA reference is appropriate from a aerodynamic perspective. Military jets equipped with AOA indications use them in those flight phases and high performance maneuvering, but not at high-altitude cruise. The military attitude/AOA critical carrier approach also uses a “back side of the power curve” technique not compatible with transport category flight director and autothrust operating. 19 Additionally, the stall AOA is also influenced by flap and speed brake position. The addition of flaps actually reduces the stall AOA. 20 Other factors such as CG, required body angle clearances, gust factors, and minimum control speeds (not AOA related), combine so that no single AOA can be targeted to ensure proper speed or landing attitude margins.
    Measuring AOA is also more complicated than it may first appear. The airflow around the fuselage, where transport airplane angle-of-attack vanes are mounted, is not identical with the airflow experienced at the wing. Boeing cites Mach number, flap and gear position, side-slip angle, pitch rate, ground effect, fuselage contour, radome damage, installation error, sensor inaccuracies, contamination, and damage among the factors that add errors to the measurement of AOA.
    Some transport category aircraft do have AOA indications (e.g., late models of B-737, 767, and 777). The indication provides a green approach band which represents the normal range for approach operations. The band is intended not as a target reference for the approach, but a tool to detect configuration errors, reference-speed calculation errors, and very large errors in gross weight, as not all approach speed parameters are related to or sensed by AOA.
    Airbus does offer an angle-of-attack based speed replacement display called the Back Up Speed Scale (BUSS). The BUSS provides a green target area based on angle of attack and replaces the barometric altitude display with GPS altitude data. However the display only comes on after all three ADRs (Air Data Reference units) are shut off by the pilot, and its use is not recommended above 25,000 feet.
    Boeing notes an additional hazard: “Pulling to stick shaker AOA from a high-speed condition without reference to pitch attitude can lead to excessive pitch attitudes and a higher probability of a stall as a result of a high deceleration rate.”
    For a more complete discussion of these AOA integration issues for transport airplanes, see the excellent issue of Boeing AERO magazine at: http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html
    As part of the investigation’s certification recommendations, the BEA recommended that “EASA (the European Aviation Safety Agency) and the FAA evaluate the relevance of requiring the presence of an AOA indicator directly accessible to pilots on board airplanes.”
    I think it is indicative of the complexity of the issue that the agency that does not concern itself with the cost or technological barriers associated with many of its recommendations has only called