Confess to still being ‘bugged’ by the issue of the THS going to ‘full up.’ Unfortunately the BEA doesn’t say much about it in the latest report – and what it does say is rather contradictory.
Page 10 - "At 2 h 10 min 51, the stall warning triggered again. The thrust levers were positioned in the TO/GA detent and the PF maintained nose-up inputs. The recorded angle of attack, of around 6 degrees at the triggering of the stall warning, continued to increase. The trimmable horizontal stabilizer (THS) began a movement and passed from 3 to 13 degrees pitch-up in about 1 minute and remained in the latter position until the end of the flight."
That’s just a repeat of the section in the earlier report.
Page 75 - "Despite some nose-down inputs, the PF maintained nose-up inputs overall. Pitch attitude fluctuated between 11° and about 18° and the angle of attack between 11° and 23°. The THS began a movement that was consistent with the PF’s inputs and reached 13° nose-up about a minute later. It should be noted that in alternate law, the auto trim is still active. On the other hand, it is difficult for the crew to know the trim position and there is no warning to the crew that it is moving."
The important thing there, to me, is that autotrim was still in operation.
Page 77 - throughout the flight, the movements of the elevators and the THS were consistent with the pilot’s inputs,
That’s the contradictory bit. As far as I can see, from the chart on page 111, the THS movements simply weren’t ‘consistent’ with the pilots' inputs at all. Indeed it appears only to have made the one movement – to ‘full up’- during the whole episode.
On Page 111, the pale blue line under ‘STABILIZER POSITION’ appears to show that it didn’t move at all at first. Then, at about ‘2.10.50,’ it began to pull up. This movement seems to have started during a period when the PF (shown as ‘CAPTAIN’) was putting in quite a lot of small movements both ways – none of which seem, on the face of it, to have been large enough to trigger such a huge movement by the THS? The THS movement finished (i.e. reached ‘full up’) at about ‘2.11.50. From then on the THS seems just to have remained at ‘full up,’ and didn’t respond to any of the further stick inputs made by both the pilots?
Can anyone with more knowledge of the aircraft and its systems – bearing in mind that the BEA says that autotrim was still in operation - explain why the THS would have behaved in this (on the face of it, very strange) way?
Is it possible, for instance, that the autotrim was responding not to the pilots’ inputs but to the very low Indicated Air Speed, or to other irregularities ‘reported’ by the sensors?