Air France 447 Probe Raises Questions about Stall Warning Ergonomics

 - July 23, 2012, 12:20 PM
The pilots did not recognize that pitch-up control inputs on the sidestick were robbing the A330 of its kinetic energy before the stall, the BEA said, thus making them even less prepared for the upset that was to follow.

The final report on the crash of Air France Flight 447 is giving ergonomics specialists food for thought. One area of particular focus has been the stall warning, which the report says the crew ignored. BEA investigators have suggested several factors that might explain why the crew did not react to the alarm, despite its sounding more than 70 times as the Airbus A330-200 descended at up to 11,000 feet per minute, at angles of attack ranging between 35 and 45 degrees.

One explanation centers on the possibility that the crew might have thought the buffeting, aerodynamic noise and even an acceleration cue on the primary flight display were symptoms of overspeed. The report questions whether the crew even heard the alarm. When it sounded, the pilots were already highly stressed and perhaps so absorbed with the onslaught of visual information that their brains failed to process the aural cues, suggested Sébastien David, head of the investigation's human-factors group. Against this background, the BEA is recommending a dedicated visual stall warning. In the case of flight AF447, the speed tape had disappeared from the display and therefore could not indicate the stall.

Moreover, the report continues, the pilots received “very little [exposure] to stall situations” in their training, hence a lack of familiarization with the stall warning and the actions it should have triggered.

More generally, the BEA believes a better understanding of the overall physics of flight at high altitude would have helped the pilots. They would have recognized that pitch-up control inputs before the stall were robbing the airplane of its kinetic energy and thus could have “anticipated the rapid degradation of the situation.”

Had the crew identified the stall after the onset of buffeting, a “straightforward pitch-down input” on a sidestick would have taken the A330 back into its flight envelope, according to tests conducted jointly with Airbus, said investigator-in-charge Alain Bouillard.

The accident, on June 1, 2009, killed all 228 people on board as the aircraft plunged into the Atlantic Ocean while en route from Rio de Janeiro to Paris.