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Air France jet's flight-control system under scrutiny

Automated messages before the crash point to a failure of the system that flies the plane most of the time, experts say. Weather looks like less of a factor.

June 05, 2009|Ralph Vartabedian

A sophisticated flight-control system that relies on electronic instruments and computers came under growing scrutiny Thursday as investigators tried to unravel the mysterious crash of an Air France Airbus 330 into the Atlantic.

A series of messages sent automatically by the jet moments before it plunged into the ocean late Sunday with 228 passengers and crew members aboard has raised speculation that the crash might have involved a malfunction of the automated system that flies the plane most of the time.

One of the messages reported that one of the plane's navigational control units had failed and that, almost simultaneously, the autopilot system had disengaged.

The sequence of events forced the crew of Flight 447 to fly the jet manually, a difficult task on an Airbus traveling at high altitude near its maximum speed, aviation experts said. Any significant change in airspeed could have caused the plane to lose lift or stability, both potentially deadly conditions.

Meanwhile, new analysis of the weather in the vicinity at the time of the crash appears to cast doubt on earlier reports that the plane encountered severe thunderstorms, lightning and wind gusts. Though there were storms, they were almost certainly less intense than those sometimes encountered above the United States, and lightning was at least 150 miles away, said Greg Forbes, severe-weather expert for the Weather Channel.

Forbes said an examination of weather data for Sunday, including satellite images, indicated updrafts of perhaps 20 mph, far from the initial reports of 100 mph.

"I wouldn't expect it to be enough to break apart the plane," Forbes said.

Though experts generally agreed Thursday that weather alone did not explain the crash, USC aviation safety expert Michael Barr said the investigation was still wide open.

"You can never disregard any possibility until you can prove what happened," Barr said. "The key here is to determine what the crew could have done after the initial event. Or was there nothing they could have done and they were just along for the ride?"

Air France executives said the plane had sent out a series of messages indicating technical failures, confirming news reports in Brazil and data that U.S. aviation experts had already gained access to.

A series of serious electronic breakdowns occurred on the Airbus over a four-minute period before the jet plunged into the sea, said Robert Ditchey, an aeronautical engineer, pilot and former airline executive.

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

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