Understanding Air France 447 by Bill Palmer Page A

levers on the center pedestal. The trim wheel provides a direct link with the THS hydraulic control. Whenever the THS moves the trim wheel moves, and vice versa. Its operation is silent and usually goes unnoticed. The pilot can adjust the stabilizer manually with the trim wheel, and manual movement of the trim wheel control overrides the automatic function. However, except in case of a malfunction, there is no reason to do so.

     
    But Airbus fly-by-wire airplanes are operated by a different set of rules than a conventional airplane. Those rules are called flight control laws: Normal, Alternate (with two versions), and Direct.
    In practicality, the trim operation is completely behind the scenes. Trim operation in Normal and Alternate Law does not affect the way the airplane handles, which is quite a departure from conventional airplanes.
    Assuming the pitch controls laws are operative (Normal, Alternate 1, or Alternate 2), the elevators will move to maintain the g-force/pitch rate requested by the pilots via the sidestick. The trim does not play a part in the feel of the airplane. If the pilot pitches up to 15° and lets go of the sidestick, the pitch will remain there, whether the stabilizer has trimmed or not. The only difference is how much elevator the flight control computer will command in order to maintain that attitude. As the stabilizer trims, less elevator will be held, but the pitch attitude and sidestick position will remain unchanged.
    In a case where the automatic trim fails, the pilots (directed by procedure) will trim the stabilizer by referencing the flight control display on the ECAM, and trim the stabilizer until the elevator is in the neutral position. When this is done, the sidestick remains in neutral and no change is felt with the sidestick or actual aircraft pitch. Once the stabilizer and elevator are aligned, the elevator’s ability to carry out a pitch up or pitch down order is assured. If this were not done, and the elevator were near the limit of its range (due to stabilizer being positioned in the opposite direction), the elevator would not be able to move further in order to carry out a pitch change. We describe this scenario as running out of elevator.
    If the pitch laws have degraded to Direct Law, the elevators are directly controlled with sidestick position. If the sidestick must be held against the centering spring then trim is called for. When properly trimmed the sidestick will be in the center position and the elevator and stabilizer aligned. If other than neutral elevator is required, the pilots must hold the sidestick deflected and move the stabilizer with the trim wheel until sidestick no longer needs to be held out of center.
    Stalls
    There are limits to how slow an airplane can fly. The slower an airplane moves through the air, the greater amount the wing must be deflected against the direction of movement in order to achieve the required amount of lift to stay in the air. The angle between the wing and the relative wind is the angle of attack (AOA).

    Above a critical angle (in the lower atmosphere it is typically around 15°) the air can no longer flow smoothly around the wing, the airflow becomes turbulent and lift rapidly decreases with any increase in the angle of attack. This is called a stall. The stall angle of attack is considered constant for a given configuration, but is decreased by flap extension and generally for Mach numbers above 0.3 (about 200 knots).

    The stall is accompanied by characteristic behaviors of the airplane such as loss of effectiveness in the controls and a buffeting of the airplane from the turbulent airflow. Pilots experience this in training, albeit briefly.
    In primary flight training, pilots are taught that an airplane always stalls at the same angle of attack (for a given wing configuration), no matter what the speed, weight, or attitude. That is true enough throughout the normal operating range of airplanes that pilots learn to fly in. However,