Understanding Air France 447 by Bill Palmer Page B

than the airplane can respond. With each movement of the sidestick, the pilot is asking for a different performance (roll left, roll right), and it takes some time for this to all happen. It is not difficult to get into a situation where the inexperienced Airbus pilot is reacting to his previous sidestick input.
    Despite how it may sound, the transition to this different flight control/performance relationship is quite easy. Pilots do not normally think of how much control deflection they need to achieve a given pitch or roll rate, but will use as much as required with the the airplane’s response as a cue. Likewise with the sidestick, the pilot inputs as much control input as necessary to achieve the desired performance and do not give it a second thought.
    Another situation where this different relationship becomes significant is during crosswind landings. During the approach the airplane is crabbed into the wind, flying somewhat sideways to the runway. It is a normal procedure during the pre-touchdown stage of a crosswind landing to use the rudder to align the airplane with the runway so that the wheels touchdown in line with their axis of rotation. When the airplane is aligned with the runway, drift is controlled with the input of a slight bank angle.
    A properly input bank angle balances out the effect of a crosswind. To maintain this balance rudder and opposite aileron must both be deflected. In a conventional airplane, this is done be holding the rudder and the control wheel both deflected in opposite directions, referred to as a cross-control input. But on Airbus fly-by-wire aircraft, the sidestick commands rate of roll and not aileron deflection. Therefore, if the sidestick was held as in a conventionally controlled airplane, the airplane would continue to increase its bank angle as long as the sidestick was held deflected.
    The Airbus pilot must learn to use the sidestick only to establish the bank angle, and then return it to neutral in order to maintain the bank angle. This is not particularly difficult, but it does require an conscious understanding of what the sidestick is commanding.
    As an interesting contrast to the Airbus fly-by-wire design, the Boeing 777 (also a fly-by-wire airplane) directly commands aileron deflection for roll so that airplane mimics one that is conventionally controlled.
    The B-787 on the other hand, uses a rate-of-roll demand for lateral inputs, similar to Airbus aircraft. But the engineers also designed in a roll response to rudder pedal input so that in a cross control situation the pilot inputs can be conventional. The simultaneous control wheel and rudder pedal commanded roll rates cancel each other out. This provides the excellent handling characteristics of a roll-rate demand with the crosswind handling that pilots are used to.
    As mentioned earlier, Normal Law in addition to providing well behaved and consistent handling characteristics, also provides the highest level of protections. These protections are designed to prevent loss of control and exceeding the normal operating parameters. Protections are provided for g load, pitch and roll attitudes, speed, and angle of attack.
    Some naysayers describe the protections as the pilot wanting something and the computers deciding if they will allow it, inferring that the pilot always knows better and should always be obeyed. The pilot commands a performance not a control command. There is no reasonable scenario where a transport category airplane should be rolled, looped, stalled or significantly oversped. There is however, an accident history where these parameters were inadvertently exceeded ending in disaster. The protections are there for a good reason.
    In roll, the sidestick commands a rate up to 15° per second, which is quite fast. In airline operations bank angles rarely exceed 30°, and Normal Law reflects that. At bank angles up to 33° if the stick is returned to neutral, the airplane will maintain that bank angle all day