Understanding Air France 447 by Bill Palmer

airplanes, the pilot moves the controls to directly command the position of the flight control surfaces. In most larger aircraft, and some smaller aircraft as well, the pilot is not directly moving the flight control surface, but is doing so through some mechanical means to activate a hydraulic or electric servo that moves the control.
    As an airplane has a wide range of speeds, the effect of a specific amount of control deflection has differing results depending on that speed. At low speed, there is little airflow and the controls are easy to move, yet require more deflection to achieve a given performance, such as a given roll rate. At high speeds there is greater airflow over the control surface which will have a greater effect, and the same roll rate can be achieved with much less control deflection. At higher speeds the force needed to displace the controls into the faster relative wind is greater as well. These forces cannot be felt through a hydraulic system; therefore, an artificial feel system is incorporated to mimic the natural feel as if there was a direct connection.
    At high altitude, a given control deflection provides a faster airplane response than at low altitude due to less aerodynamic damping (i.e., it is easier to move the airplane around in the thinner air). This creates an opportunity for over-controlling the airplane if large control inputs are made.
    In pitch, there is a balance between the center of lift from the wings, the center of gravity, and the aerodynamic forces created by the tail of the airplane. The airplane pivots around the center of gravity, which is about 20-30% of the way back from the average front of the wing (the mean aerodynamic chord, to be precise). This balance is dynamic and changes with the loading as well as the speed of the airplane. The two horizontal components of the tail, the horizontal stabilizer and the elevator hinged to its aft half, move to control the pitch attitude of the airplane.
    The elevator is the primary pitch-control surface and moves immediately with pitch commands. The stabilizer is adjusted to reduce prolonged elevator deflection for both efficiency and controllability. This adjustment is called trim, and the stabilizer is often referred to as the “trimmable horizontal stabilizer” (THS) or simply the “stab,” and the adjustment of it as stab trim.

     
    The trim setting is such that at a given load distribution, the airplane’s trim setting is valid for a particular speed. If the airplane slows down, left on its own the nose will tend to pitch down, which will increase speed, causing the nose to pitch up, causing the airplane to slow down to eventually become stable at the speed the trim is set for. An airplane with this configuration, like commercial jet transports, is considered dynamically stable (it will return to its stable speed).
    Therefore, if an airplane is trimmed to fly at a given speed and the actual speed falls much below that, the pilot has to exert additional control force to keep the nose of the airplane from pitching down. Either that or adjust the trim to the new speed.
    When a new speed is desired, the pilot or autopilot adjusts the trim, so that the airplane is stable at the new desired speed. This occurs repeatedly throughout any flight.
    An additional factor that plays in this balance is the thrust from the engines, especially when they are mounted below the wing and not in line with the center of gravity. An increase in power from below the center of gravity will induce a pitching-up moment and a new balance must be achieved with the trim.
    Operation of Trim
    On Airbus fly-by-wire aircraft, the trim is normally automatic. The automatic trim moves the stabilizer so that the elevator is neutral on average (aligned with the stabilizer). This is both aerodynamically efficient and it provides for a range of elevator movement, and thus control, on either side of the trimmed setting.
    A trim wheel is located next to the thrust