DC3 wrote:
I would guess that on some modern airliners the trim wheels use encoders to tell the computer systems where to set the trim (via hydraulics, servos, or ...)
However, I actually have no knowledge of this, it is only speculation based on knowledge of other modern day control systems. Feedback, i.e. turning the trim wheel when the autopilot is controlling it) might be through a stepper motor or some similar item.
Just my speculation....
I don't know every airplane there is of course, but typically the trim wheels, where included, are parts of mechanical linkage. In A320 for instance, cables run from the trim control wheels all the way back to the THS actuator. There is an override mechanism, that allows the manual command to cancel any electrical command driving the THS, the THS itself is actuated by hydraulic motors driving a ball screw. The position sensing is done at the actuator.
When we come to A350, the manual trim wheels are removed, and there is no manual pitch trim possibility in flight, under normal control laws. There are manual pitch trim switches that operate only on ground, or in case the auto-trim is lost. The THS is driven electrically, by 540 VDC Motor Brake Assemblies, as they are called.
-Esa