Understanding Air France 447 by Bill Palmer Page A

of 15° per second. The center position commands a zero roll rate, or in other words, maintain the current bank angle.
    In pitch, Normal Law provides a g-load/pitch-rate demand. This means that for a given amount of sidestick deflection forward or aft, the pilot commands a consistent pitch response. At higher speeds this is best expressed in the g-force as a result of the pitch maneuver, at other times the pitch rate is a better measure. The center position commands 1 g, or unaccelerated flight (which could be in a climb, level, or descent). The result is the flight path is maintained. If flaps are extended or retracted the airplane will automatically adjust the pitch in order to maintain the same flight path. In normal operation this reduces the workload of hand flying the airplane. The stab trim is set automatically because the flight control system holds the airplane as commanded whether it is in trim or not. In Normal and Alternate laws, the pilot does not receive feedback on the trim status, nor does he need any.
    Normal Law provides two key low-speed reference points: Alpha Protect (commonly, Alpha Prot) and Alpha Max. As the name Alpha implies, a term meaning angle of attack, even though they are represented on the airspeed indicator, they are actually referenced to angle of attack.

    When slowing, as the angle of attack reaches Alpha Prot the sidestick transforms from a g-load demand to an angle-of-attack-demand input.
    Alpha Max is an angle of attack just below stall, that is the maximum angle of attack allowed by the low-speed protections.
    The airspeed at which Alpha Prot occurs is lower than normal operating speeds but higher than the stall speed. When operating between Alpha Prot and Alpha Max, the center/neutral sidestick position commands Alpha Prot, and full back commands Alpha Max. Therefore, in Normal Law, the airplane will pitch down on its own if the angle of attack increases to Alpha-Prot, but the pilot retains the authority to command a higher angle of attack. Once the angle of attack reaches Alpha Max, the flight control laws will not allow the angle of attack to increase further and will control the pitch to keep the AOA at or below Alpha Max. Because of this, the pilot can command a maximum performance maneuver by holding the sidestick all the way back.
    For Air France 447, when the transition to Alternate Law was made, these angle of attack limits no longer functioned.
    A Different Relationship
    In an airplane with conventional flight controls the resultant roll or pitch rate would more directly correlate with the force applied to the control wheel, and not the amount of deflection required. At higher speeds less control deflection is required to achieve the desired performance, and the airflow over the control surfaces makes the controls correspondingly stiffer. At low speeds greater control deflection is required and the controls feel “sloppy.”
    On the fly-by-wire system, the sidesticks do not provide tactile feedback of flight control force. Sidestick deflection correlates to the desired performance. The system determines how much to move the actual controls surfaces to satisfy the demand. Therefore, airplane handling and response is consistent throughout the flight envelope.
    The Airbus pilot must occasionally remind himself of this different relationship. I have told many students “you are telling it what to do, not how to do it.” Airbus pilots will have trouble if they forget this principle. One common instance is during approach with turbulence. Most pilots of conventionally controlled airplanes are used to instantly responding with a wind gust that tips a wing with a corresponding lateral control input. With the sidestick in neutral, an Airbus will attempt to maintain a zero roll rate on its own, and will automatically input a roll command in response to the gust. Pilots who react to each bump and gust end up creating their own turbulence by wagging the sidestick back and forth faster