At 1 h 55, the Captain woke the second co-pilot and said "[…] he’s going to take my place". Between 1 h 59 min 32 and 2 h 01 min 46, the Captain attended the briefing between the two co-pilots, during which the PF said, in particular "the little bit of turbulence that you just saw […] we should find the same ahead […] we’re in the cloud layer unfortunately we can’t climb much for the moment because the temperature is falling more slowly than forecast" and that "the logon with Dakar failed". The Captain left the cockpit.
Assume after the seat exchange, the PF noted the air temperature was higher than expected, thus stating his understanding they were thrust limited and therefore could not climb much higher in the warm air. This also seems to verify the previous weather discussions and ice formation at that altitude, related to this accident.
From 2 h 10 min 05, the autopilot then auto-thrust disengaged and the PF said "I have the controls". The airplane began to roll to the right and the PF made a left nose-up input. The stall warning sounded twice in a row. The recorded parameters show a sharp fall from about 275 kt to 60 kt in the speed displayed on the left primary flight display (PFD), then a few moments later in the speed displayed on the integrated standby instrument system (ISIS).
Note 1: Only the speeds displayed on the left PFD and the ISIS are recorded on the FDR; the speed displayed on the right side is not recorded.
Note 2: Autopilot and auto-thrust remained disengaged for the rest of the flight.
At 2 h 10 min 16, the PNF said "so, we’ve lost the speeds" then "alternate law […]".
Note 1: The angle of attack is the angle between the airflow and longitudinal axis of the airplane. This information is not presented to pilots.
Note 2 : In alternate or direct law, the angle-of-attack protections are no longer available but a stall warning is triggered when the greatest of the valid angle-of-attack values exceeds a certain threshold.
Much to discuss here. From ACARS messages, we believe the pitots iced over, causing airspeed disagreement resulting in disconnect of the AP and AT. The airplane responded to AP disconnect with a right roll, which the pilot tried to correct. Given that the pitots froze with the heaters on, the right roll could have been caused by asymmetric ice accumulation, wing fuel tank imbalance, or perhaps by the turbulence they were experiencing. From the text, it sounds like the first 2 stall warnings were brief in nature, and could have been false due to asymmetric ice on the AOA vain, or perhaps briefly induced by turbulent up draft. IAS was apparently 275kt prior to loss of airspeed on the left PFD. Assume the AT disconnect left the engines at the same power settings prior to disconnect. The PF realized the airspeeds had been lost, and that alternate law had been selected.
The PF response to the right roll is problematic. The nose-up component started a climb, which he already knew he did not have excess power for. We have no information that he increased power at this time. Why he pulled nose-up is a mystery. Maybe he thought the 2 brief stall warnings were false, maybe he wanted to try and climb above the icing or the turbulence. However he did not have reliable airspeed indication in these moments, to attempt this.
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. The speed displayed on the left side increased sharply to 215 kt (Mach 0.68). The airplane was then at an altitude of about 37,500 ft and the recorded angle of attack was around 4 degrees.
Control inputs at high altitude are exaggerated due to the thin air. Control inputs therefore have to be smooth, and a progressive pitch increase to 10 degrees and beyond would seem to indicate the PF understood this. Either nose-up control inputs caused the brief zoom climb, or perhaps an updraft bringing up warmer air contributed. What is sure is that by the time he reached 37,500 ft and the left airspeed returned, he only had 215kt, which was too low to sustain flight at that altitude. The left and right roll responses seem to indicate either the aircraft was near a stall, or the rolls were being caused by turbulence, and certainly the quick disappearance of the pitot ice condition indicated some passage from one air mass to another. However the aircraft got to this altitude with this low airspeed, it was now going to stall, no way around it.
From 2 h 10 min 50, the PNF tried several times to call the Captain back.
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. Around fifteen seconds later, the speed displayed on the ISIS increased sharply towards 185 kt; it was then consistent with the other recorded speed. The PF continued to make nose-up
inputs. The airplane’s altitude reached its maximum of about 38,000 ft, its pitch attitude and angle of attack being 16 degrees.
Note: The inconsistency between the speeds displayed on the left side and on the ISIS lasted a little less than one minute.
When the 3rd stall warning occurred, the pilots pushed the throttles to TOGA (the correct move) and then continued to maintained nose-up inputs (bad move). This is inexplicable, and I can only think that maybe they fell back on training where they were taught to power out of a stall with minimum loss of altitude, which is training that has now been changed since this accident. For whatever reason, the continued nose-up inputs (or selecting TOGA perhaps?) caused the THS to trim to +13 degrees up. Then at some point alternate law 2 (abnormal alternate) must have been triggered, since the THS never moved down in response to later nose-down inputs. My thinking is that with the THS stuck at 13 degrees up angle, they were never going to recover the stall, unless they realized they were in abnormal alternate and had to manually trim the THS. Nothing from the BEA report seems to indicate they did this.
At around 2 h 11 min 40, the Captain re-entered the cockpit. During the following seconds, all of the recorded speeds became invalid and the stall warning stopped. Note: When the measured speeds are below 60 kt, the measured angle of attack values are considered invalid and are not taken into account by the systems. When they are below 30 kt, the speed values
themselves are considered invalid. The altitude was then about 35,000 ft, the angle of attack exceeded 40 degrees and the vertical speed was about -10,000 ft/min. The airplane’s pitch attitude did not exceed 15 degrees and the engines’ N1’s were close to 100%. The airplane was subject to roll oscillations that sometimes reached 40 degrees. The PF made an input on the sidestick to the left and nose-up stops, which lasted about 30 seconds.
When the angle of attack exceeded 30 degrees, this may have been the point where abnormal alternate law (alternate 2) kicked in, so no auto trim of the THS after that.
I could go on, but I'll stop here. The great mystery for me is the persistent nose-up inputs. The later nose-down inputs might have worked if they had remembered to manually trim the THS. Too many holes in the Swiss cheese here.