Originally Posted by
Tomaski
As I understand the system, if the stab was truly jammed in place - either by aerodynamic loads or some physical malfunction - the electric trim motor would turn, the clutch would slip, and the stab would not move. That being said, I know of no actual case of this happening on a 737.
To expand a little :-
There will be an out of trim combination of HS AND position and aircraft speed which generates forces on the jack screw that are so large they can't be overcome by using the trim wheel.
In this condition the stab motor can't overcome these forces either. ( the design principle is rightly that the trim wheels have mechanical authority). Selecting the cutout switches back to normal would then be counterproductive as MCAS can then drive the stab motor in the AND direction actually assisted by the air loading forces it has itself created.
At this point the situation is not recoverable other than possibly by using the unloading manoeuvre. Great if you know about it, have practiced it and have the altitude to implement it.
The ball nut which runs on the jack screw is an excellent low friction mechanism for the trim actuator to move the HS but the converse also applies so the ball nut will produce a back turning force on the jack screw from the HS. ( I believe there is a braking mechanism in the stab gearbox to prevent 'creep back' even in normal flight conditions).
I may have mis-remembered but I think there was indication of a small uncommanded AND change of HS position towards the end of ET302 which would be indicative of HS creep under what was probably a very large air loading condition.
I think arguments above are correct in principle so may help to explain some of what happened in the cockpit of ET302. They do, of course, depend on actual data for the forces involved. We don't have these data and I have an uncomfortable feeling that Boeing didn't.