The sequence started with an autopilot failure and a loss of the air data inertial reference unit, a system of gyroscopes and electronics that provides information on speed, direction and position. That system has been involved in two previous incidents that caused Airbus jetliners to plunge out of control, though the pilots were able to recover.
The automated messages then indicate that a fault occurred in one of the computers for the major control surfaces on the rear of the plane. Such a failure would have compounded the problems, particularly if the pilots were flying through even moderate turbulence.
The last message indicates that multiple failures were occurring, including pressurization of the cabin. Such a message would have reflected either a loss of the plane's pressurization equipment or a breach of the fuselage, resulting in rapid decompression.
All of these issues would have made the plane difficult to control.
When cruising at high altitude, a plane must fly within a fairly small window of speed, said Robert Breiling, an aviation safety expert in Florida. If speed drops even slightly, the plane can lose lift. If the speed is too high, it causes instability over the control surfaces.
"Flying a big jetliner at high altitude without autopilot, you have your hands full," Breiling said.
Ditchey said the Airbus software would have left the crew with a very small margin of error, where even minor buffeting could have boosted the risk of losing control.
"As they got into a degraded regime, they probably got into a bigger and bigger pickle," Ditchey said.
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ralph.vartabedian @latimes.com
