Originally Posted by
MickG0105
I found the following [...] It details the different operation of the stabilizer trim main electric motor (turns in only one direction and drives the stabilizer trim actuator through two electro-magnetic clutches, one for NU, another for ND) and the autopilot trim system (turns in either direction and drives the stabilizer trim through a single clutch).
[...]
Does anyone know if that described motor/clutch arrangement has been carried on through the NG/MAX line?
based on the bits of AMMs I (unofficially) happen to have, the answer to that appears to be - no. Or more precisely "only some of it". Comparing drawings:
the "autopilot trim servo" is totally missing on the NG. Block diagrams also show both autopilot and main electric trim signals going to the same actuator.
The actuator does seem to be the same with multi-speed single direction drive and dual clutch. A duel between manual electric and auto trim doesn't seem to be possible, but one clutch sticks "on" and the other engages
might be possible, then, my guess, the whole lot would jam which would most likely unstick the dodgy clutch fairly rapidly - the not properly disengaged clutch would be the weakest point.
Worth noting that this run-on issue on classics was only in high speed mode, and the trim motor is (should be) in high speed mode
only when flaps are down. MCAS is (should be) on
only with flaps up, therefore with low speed mode and much less loading on the whole actuator system. Also looking at ADs the binding clutch issue was fixed with a new (actuator) part number, before the NG was in service.