Air France Flight 447: Pitot Icing, Alternate Law and the Stall

This is Mentour Pilot's long explainer on Air France Flight 447, the Airbus A330 that disappeared over the Atlantic on the night of 31 May to 1 June 2009 with 228 people on board. The narrator, an airline captain and training pilot, spends the first third of the video on the systems that matter to the story and the rest on the flight itself, minute by minute, using the BEA's final report and the cockpit voice recording.

The crew and the route

The crew had arrived in Rio on 28 May for a three-day stop. Because the flight back was about 11 hours long, three pilots were rostered: a 58-year-old captain and two first officers, so that each could take a rest period. The weather was fine in Rio and in Paris, but the route crossed the intertropical convergence zone, where tall thunderstorms are common. The video explains why that area matters: at high altitude the storm tops hold ice crystals, which do not cause the airframe icing pilots usually worry about, but can clog the pitot probes that measure airspeed.

Pitot probes and alternate law

The narrator explains how the pitot probes feed the air data computers, and why losing them takes away the autopilot and many of the protections on an Airbus. In normal law the aircraft cannot be stalled. In alternate law, which is what the A330 changed to when the speeds became unreliable, the stall protection is gone. He also describes the trim system, which follows the pilot's pitch commands automatically, and why that matters later. Air France had already begun replacing this type of probe on its A330s, and F-GZCP was due to have its probes changed when it got back to Paris.

Into the clouds

At about 02:00 UTC the captain called the relief pilot to the cockpit and left for his rest without saying clearly who was in charge. The aircraft was at 35,000 feet in cloud. The first officers discussed the weather radar and made a small 12-degree turn to the left. Shortly after, the sound of ice crystals could be heard in the cockpit, the speeds dropped, and at 02:10:05 the autopilot disconnected.

The climb and the stall

The pilot flying took control and, as the video describes, made sharp roll inputs and pulled up. The aircraft climbed steeply, losing speed. The relief pilot read the warnings out in a rushed way and asked him several times to go back down. The stall warning then sounded continuously while the pilot flying kept pulling back and selected take-off/go-around thrust. The narrator explains why that response, which is correct for a windshear or terrain escape at low level, made the stall worse at 38,000 feet, and how the stabiliser trimmed fully nose-up as a result.

The last three minutes

The aircraft fell at about 10,000 feet per minute with its nose still up and an angle of attack near 40 degrees. When the measured speed fell below 60 knots the stall warning stopped, and it came back when the nose was lowered, which may have pushed the crew away from the correct action. The captain returned about 90 seconds into the emergency but had not seen the start. The two first officers made opposite inputs at the same time, and the side-stick inputs were averaged. The ground proximity warning sounded and the recording ended at 02:14:28.

The investigation

The video covers the debris search, the discovery of the wreckage in April 2011 and the recovery of the recorders. It then summarises the BEA's conclusion: no component had failed apart from the temporary loss of airspeed, and the crew had not recognised or recovered from a stall. The recommendations covered pitot probes, flight recorders, the stall warning and search and rescue. The narrator closes with what he thinks pilots should take from the accident: knowing the aircraft's systems well enough to use them under stress, and flying pitch and power first when the instruments disagree.

For the investigation and the search for the wreckage, see the Mayday episode on this page. Two other accidents involving unreliable instruments and loss of control are Birgenair Flight 301 and Aeroperú Flight 603.