May 27, 2011

AFR447: High-Altitude Stall

The preliminary report of the French BEA (their transportation incident investigatory agency) is in, and early indications are that the A330 comprising Air France flight 447 crashed into the Atlantic Ocean due to a high-altitude stall. That refers to a wing stall, not a jet engine compressor stall.

An aircraft wing lifts the airframe when sufficient lift can be generated. Lift is generated by passing air over the wing surface, causing a difference in air pressure between the upper and lower surfaces. That in turn pushes the wing up. If the wing moves through the air too slowly, it stalls, causing the airframe to descend rapidly.

In the case of AFR447, the crew made nose-up inputs to the flight controls and the aircraft climbed from its cruise altitude of 35,000 ft MSL to 38,000, its maximum. Continued nose-up input caused the wing to stall. For the next three and a half minutes the aircraft fell at a rate of about 11,000 feet per minute, until it impacted the ocean. It’s important to note that the aircraft was in the midst of storm clouds and was likely being thrown around by turbulence, adding to the disorientation felt by the pilots as they navigated by instruments alone in the dark of night.

Suspicion rests on frozen pitot tubes, which could have provided the flight management software with invalid airspeed indications leading to erroneous pilot inputs. The aircraft’s engines and flight controls performed correctly until the moment of impact. It would appear that a fully intact and functional widebody transport was flown into the ocean because the pilots were responding to bad data from a single component.

I wonder why the advanced-navigation equipment (GPS and/or INS) didn’t provide a backup against which the software could make a comparison, and alert the pilots to the discrepancy. Seems there might be a failure of software design here in addition to a failure of pitot tube industrial design.

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