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Fri, May 27, 2011

BEA Releases Findings From AF447 Voice And Data Recorders

A330 Apparently Stalled, Impacted The Ocean Descending More Than 10,000 FPM

The French accident investigating agency BEA has released a document culled from the initial findings from the Air France Flight 447 accident which occurred nearly two year ago. The report indicates the Airbus entered a deep stall from which the cockpit crew was unable to recover. While icing on the speed sensors is not  specifically mentioned, the pilots do indicate that "we've lost the speeds."

File Photo

All times are expressed in UTC.

At 0155, the Captain woke the second co-pilot and said "[...]He's going to take my place". The Captain attended the briefing between the two co-pilots, during which the PF (Pilot Flying) 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.

The airplane approached the ORARO point. It was flying at flight level 350 and at Mach 0.82 and the pitch attitude was about 2.5 degrees. The weight and balance of the airplane were around 205 tonnes and 29% respectively. Autopilot 2 and auto-thrust were engaged.

At 0206:04 the PF called the cabin crew, telling them that "in two minutes we should enter an area where it'll move about a bit more than at the moment, you should watch out" and he added "I'll call you back as soon as we're out of it".

At 0208:07 the PNF (Pilot Not Flying) said "you can maybe go a little to the left ". The airplane began a slight turn to the left, the change in relation to the initial route being about 12 degrees. The level of turbulence increased slightly and the crew decided to reduce the speed to about Mach 0.8.

From 0210:05, 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). BEA notes that 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, and that autopilot and auto-thrust remained disengaged for the rest of the flight.

At 0210:16 the PNF said "so, we've lost the speeds" then "alternate law". (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. This information is not presented to pilots.) The airplane's angle of attack 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.

From 0210:50, the PNF tried several times to call the Captain back. At 0210:51, 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.

BEA Graphic Showing Route Of 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. BEA notes that the inconsistency between the speeds displayed on the left side and on the ISIS lasted a little less than one minute.

At around 0211:40, the Captain re-entered the cockpit. During the following seconds, all of the recorded speeds became invalid and the stall warning stopped. 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.

At 0212:02, 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.

At 0213:32, the PF said "we're going to arrive at level one hundred". About fifteen seconds later, simultaneous inputs by both pilots on the  sidesticks were recorded and the PF said "go ahead you have the controls". The angle of attack, when it was valid, always remained above 35 degrees.

The recordings stopped at 0214:28. The last recorded values were a vertical speed of -10,912 ft/min, a ground speed of 107 kt, pitch attitude of 16.2 degrees nose-up, roll angle of 5.3 degrees left and a magnetic heading of 270 degrees.

At this stage of the investigation, as an addition to the BEA interim reports of 2 July and 17 December 2009, the following new facts have been established:

  • The composition of the crew was in accordance with the operator's procedures.
  • At the time of the event, the weight and balance of the airplane were within the operational limits.
  • At the time of the event, the two co-pilots were seated in the cockpit and the Captain was resting. The latter returned to the cockpit about 1 min 30 after the disengagement of the autopilot.
  • There was an inconsistency between the speeds displayed on the left side and the integrated standby instrument system (ISIS). This lasted for less than one minute.

After the autopilot disengagement:

  • The airplane climbed to 38,000 ft.
  • The stall warning was triggered and the airplane stalled.
  • The inputs made by the PF were mainly nose-up.
  • The descent lasted 3 min 30, during which the airplane remained stalled. The angle of attack increased and remained above 35 degrees.
  • The engines were operating and always responded to crew commands.
  • The last recorded values were a pitch attitude of 16.2 degrees nose-up, a roll angle of 5.3 degrees left and a vertical speed of -10,912 ft/min.

In publishing this initial report, the BEA says "This note describes in a factual manner the chain of events that led to the accident and presents newly established facts. The initial analyses will be developed in a further interim report that is scheduled to be published towards the end of July."



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