After discovering the black boxes earlier this month from the Air France flight that disappeared at sea in 2009, French investigators have released a report that identifies a serious instrument malfunction as the cause of the crash.
What they found was chilling: the co-pilot had disengaged the auto-pilot while the Captain was not in the cockpit, in order to turn slightly to the left to avoid an area notorious for storms and turbulence. What happened next was a malfunction of the pitot tubes, the instruments that tell the pilots how fast they are going.
The pilots experienced the first signs of trouble at 4 hours and 10 minutes into the flight. The black boxes indicate that the pilot switched off the auto-pilot in order to bypass the turbulent area. It was at this point that a stall warning sounded, and as the co-pilot pushed the yoke forward to generate lift, the speed sensors plunged and then spiked erratically. “So, we’ve lost the speeds,” the co-pilot calmly noted.
The Captain finally returned to the cockpit after one and a half minutes, but by then it was too late. The airplane was pitching violently from side to side, and it was already losing altitude quickly – falling at a gut-wrenching 10,000 feet a second. According to the boxes, the crew followed all procedures as they attempted to recover the aircraft from its plunge from the sky.
The plane was in a free fall for 3.5 minutes before slamming into the ocean in the middle of the pitch-black night, killing all 228 people on board.
Without the aircraft’s speed, the pilots could not determine the appropriate thrust to keep the plane from stalling – there was not enough lift to keep the plane in the air. So as the co-pilot disengaged the auto-pilot, he inadvertently made it impossible to control the plane. It remains to be seen why the auto-pilot was able to successfully fly the plane if these instruments were damaged.
From the report:
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.