The A300/A310, like most other aircraft (except for some small general-aviation aircraft), has an adjustable-incidence-angle horizontal stabiliser, which is used to trim the aircraft in pitch (for instance, to change the aircraft’s speed or compensate for shifts in its center of mass) without the need for large elevator deflections (which would increase the stress placed upon the elevator control linkages and actuators, shortening their lifespan, and, if the autopilot was flying the aircraft, would result in large pitch excursions upon autopilot disconnect as the elevator returned to its neutral position).
There are three methods by which the horizontal stabiliser can be moved:
- The A300/310’s autopilot, like those of most other autopilot-equipped aircraft, automatically trims the aircraft in pitch when engaged; it moves the stabiliser by applying power to a set of electric motors which rotate the stabiliser jackscrew, moving the stabiliser’s leading edge up or down.
- In manual flight, the aircraft is usually trimmed by using two sets of electric trim switches (one on each of the two control yokes), which, like the autotrim system, apply power to a set of electric motors which rotate the stabiliser jackscrew.
- As a backup, a manual pitch trim wheel is mounted on the center console between the pilots; this wheel is connected to the stabiliser jackscrew via a cable mechanism, and, when used, manually rotates the jackscrew directly.
If the autopilot is engaged, the electric pitch trim switches are inhibited, and cannot be used to trim the aircraft; the aircraft can only be manually trimmed by rotating the manual trim wheel, which disconnects the autopilot. As a result, in the event of an autotrim runaway, the pilots need to take one hand off the yoke to manually trim the aircraft against the malfunctioning autotrim. This used to be especially critical in two specific autopilot modes (LAND and GO-AROUND), where, prior to 1997, pulling or pushing on the yoke to use the elevators to manually counter the autotrim-induced pitching moment would not disconnect the autopilot if the aircraft was less than 400 feet AGL (and, originally, would not do so no matter what the height AGL); indeed, doing so would cause the autotrim to trim further and further against the pilots’ control inputs, exacerbating the problem at a time when maintaining pitch control of the aircraft was (due to the very limited altitude available with which to work with) far more critical than in most other phases of flight.
So why are the yoke-mounted electric pitch trim switches inhibited and inoperative whenever the autopilot is engaged?