Originally Posted by
fizz57
Wrong again
On the A320-330-340 they are full wheels that are mechanically connected to the trim actuator and turn when the stab moves.
They are in fact a final backup for longitudinal control should all electrics fail. Still need hydraulics though, like the 747 and all later Boeings - the 737-o-saurus must be the only airliner of any size still flying with a direct mechanical connection to the THS. On the A350-380 they're gone completely and replaced by switches.
Ok, but they are moving hydraulic valves via a cable and not moving the surface directly so there is no force feedback for the amount of trim discrepancy. That has to be a low-friction system, otherwise it will detect a problem and report as a jammed stabilizer.
Can the system decouple the THS actuator from the trim wheel input in the case the cable becomes jammed and not the stabilizer?