The French air accident investigator, the BEA, has released the final report on the 1 June 2009 AF447 accident.
Shortly after two o'clock in the morning Air France flight 447 from Rio to Paris, an Airbus A330, was cruising at FL350 (35,000ft approx) with autopilot engaged. The two pilots were discussing a heading change to avoid one of the cumulonimbus clouds ahead, visible on the weather radar. Such clouds are always present in the inter-tropical convergence zone, which the aircraft was passing though as it approached the equator.
Because of the atmospheric conditions in the vicinity of the clouds, turbulence increased a little, and ice crystals temporarily blocked the pitot tubes that provide airspeed readings to the pilots. The flight management computer, confused by the disparity in airspeed data it was receiving, disconnected the autopilot as it was designed to do, warning the pilots that they were in control through a raucous audio signal that the crews call the "cavalry charge".
The copilot, who was flying, reacted by making a control input that raised the nose of the aircraft dramatically, especially considering that its height was already close to the maximum at which the aircraft could fly safely at that weight. The aircraft zoomed upward at 7,000ft/min rate of climb, speed rapidly decreasing, and less than a minute later a stall warning sounded. That was the first of many stall warnings, but the crew acted as if they did not believe them, continuing to climb and thus lose more speed.
As the aircraft slowed and lost the lift from its wings, it started descending and entered a deep stall. It was eventually falling vertically at more than 3km per minute with the crew still holding its nose high. The BEA says in its report: "The aeroplane went into a sustained stall, signalled by the stall warning and strong buffet. Despite these persistent symptoms, the crew never understood that they were stalling and consequently never applied a recovery manoeuvre."
During the press briefing associated with the report's release today, the BEA said that the crew interpreted the unfamiliar stall buffeting and the high level of unfamiliar aerodynamic noise as indicating an overspeed condition, whereas it was the opposite.
This is what the BEA had to say about why the pilots appeared unable to cope: "The investigation brought to light weaknesses in the two copilots: the inappropriate
inputs by the PF (pilot flying) on the flight controls at high altitude were not noted by the PNF (pilot not flying) through an absence of effective monitoring of the flight path.
"The stall warning and the buffeting were not identified either. This was probably due to a lack of specific training, although their training was in accordance with regulatory requirements. Manual aeroplane handling cannot be improvised and requires precision and measured inputs on the flight controls. There are other possible situations leading to autopilot disconnection
for which only specific and regular training can provide the skills necessary to ensure the safety of the flight.
"Examination of their last training records and check rides made it clear that the copilots had not been trained for manual aeroplane handling of approach to stall and stall recovery at high altitude."
The BEA made a series of recommendations, but the ones most directly related to the circumstances of this accident were that Airbus should provide the pilots with a direct angle of attack readout, and that training should be widened to take into account manual flying at the edges of the flight envelope, particularly handling the high altitude stalling that AF447 encountered. To make this training effective, the BEA wants simulators to be improved to represent - more accurately than they now do - handling characteristics at the edges of the flight envelope.