DW
Franzl - it looks to me (quickly doing the maths in my head) like the elevator trace looks like it takes THS position into account and is relative to the THS value.
It may look like that, but it is not relative to it. If the loadfactor demand would have been achievable, let´s say with a THS trim of 10° NU, then the elevators would be back in neutral after SS release. The elevators are used by the FCPC to change the loadfactor demand (short term), when achieved and stick is back in neutral the THS trim (long term) will trim out the load of the elevators , bringing those back to neutral.
The FCPC will command what it is told to, this is true - but letting go of the stick should at least allow the aircraft to follow natural trajectory -
That is wrong again, the natural trajectory of the stalled AF447 was 45° down, but the intended trajectory commanded to the FCPC´s was 10° climb, and thats the only order the FCPC´s will try to follow. Releasing the stick to neutral does reduce the load factor demand to 1 g, which in turn does not lead to follow natural trajectory but to an order maintaining a ordered trajectory with one g.
The trajectory you describe would be less than one g.
ít wouldn't be enough to get the nose down, and I'm not arguing that it would,
see above
but the elevators were being held in position by the inputs and came back down when the stick was held forward for a few seconds and then held around neutral.
Elevators were being held by the FCPC´s in order to follow the loadfactor demand, which could be changed only by SS ND.
Elevators came out of the full NU position to 15° NU, but never ND position, because the FCPC+s where not ordered to do so.
The question of whether the underlying systems were in G-loading mode or pitch command mode (which IIRC becomes active below a certain speed) is important when discussing this, because if it was holding G then the FCPC might have held the elevators there and if it was in pitch command mode the elevators should have relaxed.
This pitch commanding mode was mentioned several times, however no reference to it surfaced until now. Would be interesting to know, when does it self-employ and how would it work.
Either way this doesn't help understanding to a great degree, because at the point the elevators returned from full-up, the aircraft was beyond saving. Why did it take so long?
That is the one million dollar question, some answer with adressing poor airmanship and some try to look deeper to prevent it from happening in the future.