It used to be said that the only really silly question is the one you don't ask.
I'm wondering WHY the autotrim 'decided,' in both accidents, to go to 'THS full up' in the first place? And then STAY there, even though both pilots were in the end pushing the stick forward? Anyone know what the 'system logic' that could cause that reaction is?
The BEA gives us no guidance at all on the point. For the Perpignan accident - even though it's the final report - the BEA just says:-
The auto-trim system gradually moved the horizontal stabilizer to a full nose-up position during the deceleration. The horizontal stabilizer remained in this position until the end of the flight.
In the case of AF447, it says even less - it just tells us 'what' happened, and doesn't even tell us when it happened (i.e.
which minute):-
The trimmable horizontal stabilizer (THS) passed from 3 to 13 degrees nose-up in about 1 minute and remained in the latter position until the end of the flight.
No mention of 'why?' in either case.
So here's my 'silly question.' Despite the fact that the autopilot and autothrust had 'signed off' on AF447, and the pilot at Perpignan was most certainly seeking to get the nose down all the time, and the AF447 one much of the time - is it possible that the autotrim in both cases was still trying to regain the previously-commanded altitude?
Never flown anything with autotrim (in fact, I've only flown anything with an autopilot twice in my life). But as far as I know, that (keeping the aeroplane exactly to a given altitude) is the main 'everyday' function of an autotrim?