mm43, HazelNuts39, thanks so much for the informative replies.
Quoting HazelNuts39:-
My understanding from the Perpignan report is that, with autotrim in operation, the THS only moves when the elevator goes past the neutral position: nose down elevator will command the THS to move more nose down. The elevator follows the (g-driven) SS orders. So a small nose-down command from the SS that causes the elevator to move from nose-up to less nose-up without going to nose-down will not cause the THS to move.
I read the BEA's Perpignan report (on a accident back in 2008) but missed the point you very kindly brought up. To quote part
of the report:-
"From 15 h 44 min 30 the automatic trim function displaced the stabiliser as far as the electric nose-up thrust stop (- 11 degrees). The stall warning sounded at 15 h 45 min 05. The nose down commands applied by the Captain on the sidestick brought the elevators, due to the load factor, to the neutral position, without however pushing them to the stops. Consequently, the trimmable stabilizer did not move even though the flight control law was normal. From 15 h 45 min 15 until the end of the flight, the automatic trim function remained unavailable. In fact, the direct law was active from 15 h 45 min 15 to 15 h 45 min 40 and the Abnormal attitude law phase 1 (without auto-trim) remained active till the end of the flight.
"Footnote:- The elevators must go beyond the neutral position before the auto trim function adjusts the position of the stabilizer.
"When the stall warning sounded, the Captain reacted by placing the thrust levers in the TO/GA detent and by pitching the aeroplane down, in accordance with procedures.
"The nose-down input was not however sufficient for the automatic compensation system to vary the position of the horizontal stabilizer, which had been progressively deflected to the pitch-up stop by this system during the deceleration."
So, as I read that, in order to get the THS even to start moving back to a sensible angle, the Perpignan pilots (and later the AF447 ones) would have needed to shove the stick full forward and hold it there for quite a while, until the THS 'accepted the situation' and started moving back from 'full up.'
But, of course, on AF447, every time they attempted that, the airspeed increased and they got another 'stall warning.' And presumably thought they were 'doing it wrong' (what, apart from an engine falling off, can be more serious than an impending stall?) and instinctively relaxed the stick pressure.......
It'd be interesting to know whether Airbus, Air France, or the BEA had warned pilots that this sort of 'impasse' could occur; and indeed had occurred, as far back as Perpignan in 2008? I very much doubt it........
Anyway, thanks again - I now know, to my own satisfaction, the primary causes of this latest accident. And, in my view, they weren't entirely, and possibly weren't even mainly, 'pilot error.'