June 5, 2009 in Nation/World

Jet probe looks at autopilot

Messages show system may have malfunctioned
Ralph Vartabedian Los Angeles Times
 

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

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

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

The sequence of events forced the crew of Air France 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 accident appeared to cast doubt on early reports that the plane encountered severe thunderstorms, lightning and wind gusts. While there were storms, they were almost certainly less intense than those sometimes encountered above the U.S., and lightning was at least 150 miles away, said Greg Forbes, the severe weather expert for the Weather Channel.

Forbes said an examination of weather data for Sunday, including satellite images, indicated that updrafts of perhaps 20 miles per hour, far from the initial reports that pegged those storms at 100 mph. “I wouldn’t expect it to be enough to break apart the plane,” he said.

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

The sequence started with an auto pilot failure and a loss of the air data inertial reference unit, a system of gyroscopes and electronics that provides information on pitch, roll, acceleration and location. That system has been involved in two previous incidents that caused Airbus jetliners to plunge out of control, though pilots were able to recover.

The automatic 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 indicated that multiple failures were occurring, including depressurization 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.

Ditchey said that the software on the Airbus would have left the crew with a very small margin of error, where even minor buffeting by the wind could have compounded the risk of losing control.


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