Originally Posted by studi
... as was said before, we don't know why Bonin pulled.
Sure, but there are some fairly strong indications that, from the moment the PF assumed control after A/P disconnect, his overriding concern was the avoidance of overspeed.
Section 2.1.3.3.1 of the final report deals with the PF's reaction from the triggering of the STALL 2 warning, as perceived by the HF group (I think the crucial moment is when he starts to pull just before the SW):
Some of the PF’s actions may be interpreted as indicative of a perception of a risk or of a diagnosis of overspeed. Firstly, the PF reduced the thrust during the seconds preceding the activation of the STALL 2 warning and the onset of buffet. Secondly, 51 s after the triggering of this warning, the PF said “I have the impression we have speed” then moved the thrust levers to the IDLE detent. He reformulated his impression a few seconds later, combined with an attempt to extend the speedbrakes. Other factors which may have prompted the PF to fear an overspeed situation were:
- The display on the ECAM (max speed 330/.82) combined with the reconfiguration to alternate law which may have been read;
- The fact that, in cruise, the upper red strip on the speed tape (MMO) is about ten knots above the current speed, whereas VLS is barely visible at the bottom of the tape (thirty knots less);
- The dangers associated with overspeed situations embedded in the collective consciousness of pilots.
The next two paragraphs discuss the PF's reaction to the stall warning. I agree with the first paragraph, but the second seems to me to be somewhat in conflict with the first:
Nevertheless, the PF was also confronted with the stall warning, which conflicted with his impression of an overspeed. The transient activations of the warning after the autopilot disconnection may have caused the crew to doubt its credibility. Furthermore, the fact that the flight director was advising a nose-up attitude may have confirmed the PF’s belief that the stall warning was not relevant. During previous events studied, crews frequently mentioned their doubts regarding the relevance of the stall warning (see 1.16.8.4).
The application of maximum thrust was probably the consequence of the perception of the stall warning. However, the PF may have assimilated the triggering of the warning as a consequence of the reduction in thrust, which he had applied four seconds earlier; he should then have applied full thrust to return to the earlier situation. A few seconds later, the PF said “I’m in TOGA, right?”. Either he was unsure whether or not he had set the thrust controls to the TOGA detent, as he intended, or he did not understand why this action was ineffective in clearing the stall warning. This second case might therefore indicate that the PF had built an erroneous mental representation of the aeroplane’s flight model, and that he had hoped that he could resolve the situation by applying TOGA thrust at high altitude and a pitch attitude oftwelve degrees, a strategy similar to that recommended at low altitudes. The fruitless result of his actions possibly heightened his mistrust of the warning.
It seems to me that, when the FD's reappeared, the PF 'remembered' the first line of the memory items: TOGA / 15° as the proper way to avoid overspeed. When the nose finally dropped below 15° with full NU sidestick, he feared that would result in overspeed, he pulled the thrust levers back and attempted to extend the speed brakes.
Just my opinion, FWIW.