I agree that that's a key question, poit. The BEA's report is pre-occupied with 'noseup inputs' on the part of the PF, but even the report credits the PF with a correct response to the 'zoom':-
"The airplane’s pitch attitude increased progressively beyond 10 degrees and the plane started to climb. The PF made nose-down control inputs and alternately left and right roll inputs. The vertical speed, which had reached 7,000 ft/min, dropped to 700 ft/min and the roll varied between 12 degrees right and 10 degrees left."
That was the first phase of the upset. The next phase started with the stall warning sounding. The PF appears to have responded with the correct drill at the time - 'TO/GA power and seek to maintain altitude':-
"At 2 h 10 min 51, the stall warning was 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) passed from 3 to 13 degrees nose-up in about 1 minute and remained in the latter position until the end of the flight."
This was the crucial 'second phase.' Whether this was the result of TO/GA power pushing the nose up, the PF's inputs, the THS going to 'full up,' or the thin air at 38,000 feet, or whatever, we don't know; my own feeling is that it was probably the result of a combination of all those factors. In any event, the aeroplane appears to have 'sat on its tail' and entered a deep stall.
We don't know what instruments the pilots had available at that time. However, I suspect that they were at first pre-occupied with the rapid loss of altitude, and formed the view that the aeroplane was in a dive rather than a stall. After all, the stall warning had stopped, that may have given them the impression that they had successfully 'avoided' any stall. It annoys me that the BEA probably KNOWS, from the CVR, what the pilots reckoned 3was goingon (unless they descended over 20,000 feet without saying anything at all to each other?).
There was in fact a third phase. In that connection I next have to mention a 'leak' published early on by 'Der Spiegel'; not the most reliable source, obviously, but it has never been denied:-
"The BEA report, in its current form, only provides the angle of the stabilizer but provides no explanation as to why. The report merely indicates that it was at this moment that Captain Marc Dubois re-entered the cockpit.
"Exactly what orders he issued are not part of last Friday’s report. But sources close to the investigation are saying that he said: “This is a stall. Reduce power and nose down!”
Indeed, a changed (and more correct) approach is indeed referred to in the BEA report, while the aeroplane still had over 10,000 feet in hand:-
"At 2 h 12 min 02, the PF said "I don’t have any more indications", and the PNF said "we have no valid indications". At that moment, the thrust levers were in the IDLE detent and the engines’ N1’s were at 55%. Around fifteen seconds later, the PF made pitch-down inputs. In the following moments, the angle of attack decreased, the speeds became valid again and the stall warning sounded again."
However, as we all know, the aeroplane never recovered from the deep stall. In this connection I have to quote the BEA again; saying that, even though the PF duly (even if belatedly) applied 'nosedown inputs,' "the THS remained in the latter position until the end of the flight." That is, remained at 13 degrees up.....
I'm afraid that that opens up the possibility that the THS didn't just go to 13 degrees up; but that, like the THS on that Alaskan Airlines MD80, it then jammed there?