Why are the A300/310’s electric pitch trim switches inhibited with the autopilot engaged? 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 is especially critical in two specific autopilot modes (LAND and GO-AROUND), where pulling or pushing on the yoke to use the elevators to manually counter the autotrim-induced pitching moment will not disconnect the autopilot if the aircraft is less than 400 feet AGL (and, originally, would not do so no matter what the height AGL); indeed, doing so will 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 is (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?
Like with two human pilots, you want control1 to be explicitly in one hands. With autopilot, it's the same: it should be either autopilot or you.

In the case of 'breaching' the normal transfer protocol for whatever reason ("I have control" - "You have control", which in case of autopilot would be the autopilot disconnect button), there are two possible courses of action when the pilot just starts to control the aircraft:


Immediately give control by explicitly disconnecting the autopilot.
Keep control by ignoring/overriding the pilot as much as possible.


Apparently, Airbus espoused the second strategy. It made sense for them to ignore the pilot (whether for trim or main control) until s/he explicitly requests control. (There is not much the aircraft can do when the pilot starts to trim the wheel manually, so it still disconnects).

If the trim switches were not ignored, there could be two options.


Disconnect the autopilot. But this changes the whole ideology, and other things would need to be changed to be consistent.
Just change the trim. But the autopilot (which is still in control) would have to fight against it in order to keep doing its job (e.g. flying level), either by trim or elevator.2 This makes no sense and creates dangerous confusion.


Arguably, completely ignoring the pilot is not a very good idea, so Airbus had to change the algorithm at least in some situations.



1 By 'control' I mean guidance control, excluding things like stability augmentation systems.

2 Unless this is a simple autopilot which does not actuate trim at all and may then need help from the pilot. This is the case for some GA aircraft, but not for A300/310.