Originally Posted by
Luc Lion
Effectively, when a pilot discovers that he is unable to trim manually his airplane, he may be tempted to try in the opposite direction for checking if the system is mecanically blocked. I read a description of such an action in a PC-6 accident report.
However, in such a case, the trim movement is brief.
The report states and shows in the diagrams that the stabilizer position moved gradually in the AND direction over a period of 2 and half minutes.
This gradual movement can't be imparted as a manual action.
And, as indicated by fdr, an error in DFDR data calibration would produce a constant offset
My best guess is that the brakes on the trim clutch are not totally effective in an extreme out-of-trim situation at high speed.
The FDR traces are coarse so it is hard to know for sure whether the changes were truly continuous or a series of small steps, the raw data should show which.
Other possibilities, of course could also be 'all of the above'
1: Each (ineffective) manual trim attempt briefly unlocked the brakes allowing an aero load induced movement.
I have no knowledge on the mechanical interaction between manual trim and the brakes so this may not be possible.
2: The pilots went back a bit before attempting to go forward, this can be seen in the mentour video demonstration almost like they were bouncing backwards.
The cables do act as a spring so hard to say if this was enough to actually move the stab.