Originally Posted by
fox niner
Very often, as I placed the stab trim full nose down with the trim wheel, the following would regularly happen.
It regularly was VERY hard to return the stab trim. INCLUDING using the trim wheels. You had to yank and pull the trim wheels quite a few times so that they would snap free, and the it was usable again and trimmable.
And this was at the gate/de-icing platform. Airspeed was zero.
If this “snap free” hardware problem turns out to be the issue here, than...Boeing is in for a hardware modification. That requires a totally new certification.
I suspect the MCAS is capable of driving the trim wheels all the way forward and capable of causing this same “snap free” lockup as I experinced regularly at the gate.
[SLF & retired s/w engineer]
Sounds like an "interesting" failure mode. However it wouldn't show up in the current-simulator based testing unless Boeing have modelled it, which I really doubt.
PS As this failure-mode might occur whenever the trim end-stops are reached it would be comforting to know that a "responsible adult" in the FAA has investigated
the likelihood and consequences.
PPS As the problem happened regularly during de-icing it should be easy to recreate on ground tests of the linkage. Do the maintenance engineers know about it?