Sensors crucial to jet speed drawing scrutiny in disaster

Iced instruments may have robbed pilots of key data

A Mass at Candelaria Cathedral in Rio de Janeiro yesterday drew many friends and relatives of the 228 passengers and crew presumed killed in Sunday's Air France crash. A Mass at Candelaria Cathedral in Rio de Janeiro yesterday drew many friends and relatives of the 228 passengers and crew presumed killed in Sunday's Air France crash. (Antonio Scorza/AFP/Getty Images)
By Bradley Brooks and Joan Lowy
Associated Press / June 5, 2009
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RIO DE JANEIRO - Investigators trying to determine why Air France Flight 447 broke apart in a violent storm over the Atlantic are looking at the possibility that the jet's speed sensors failed, two aviation industry officials said yesterday.

Meanwhile, Brazil's Navy issued a statement saying that wreckage recovered by a helicopter crew earlier in the day was not from the plane. The military earlier said it had pulled a cargo pallet from the water where the Airbus A330 went down Sunday night off the country's northeastern coast, killing all 228 people aboard - the worst aviation disaster since 2001.

Officials with knowledge of the investigation and independent analysts all stressed they do not know why a plane that seemed to be flying normally suffered a catastrophe just minutes after the pilot messaged that he was entering an area of extremely dangerous storms.

Investigators have little to go on until they recover the plane's black box flight data and voice recorders, now probably on the ocean floor miles beneath the surface.

Other hypotheses - even terrorism - haven't been ruled out, though there are no signs of a bomb. Officials have said that a jet fuel slick on the ocean's surface suggests there was no explosion.

Two officials with knowledge of the investigation said they are looking at the possibility that an external probe that measures air pressure may have iced over. The probe provides onboard computers with data used to calculate air speed and altitude. Another possibility is that sensors inside the aircraft that read the data malfunctioned.

If the instruments were not accurately reporting information, it is possible the jet would have been traveling too fast or too slow as it entered turbulence from towering bands of thunderstorms, according to the officials.

"There is increasing attention being paid to the external probes and the possibility they iced over in the unusual atmospheric conditions experienced by the Air France flight," one of the industry officials explained, speaking on condition of anonymity because he isn't authorized to discuss the investigation publicly.

Meteorologists said the Air France jet entered an unusual storm with 100 mile-per-hour updrafts that acted as a vacuum, sucking water up from the ocean. The incredibly moist air rushed up to the plane's high altitude, where it quickly froze in minus-40 degree temperatures. The updrafts also would have created dangerous turbulence.

The jetliner's computer systems ultimately failed, and the plane broke apart as it crashed into the Atlantic on the flight from Rio to Paris.

Independent aviation specialists said it is plausible that a problem with the external probe, called a pitot tube or sensors that analyze data collected by the tube could have contributed to the disaster.

The tubes have heating systems to prevent icing, but if those systems somehow malfunctioned, the tubes could quickly freeze at high altitude in storm conditions.

Other analysts outside the investigation said it is more likely that the sensors reading information from the tubes failed.

"When you have multiple system failures, sensors are one of the first things you want to look at," said John Cox, a Washington-based aviation safety consultant and former crash investigator for the Air Line Pilots Association.

Jetliners need to be flying at just the right speed when encountering violent weather, pilots say: too fast and they run the risk of breaking apart; too slow, and they could lose control.

"It's critical when dealing with these conditions of turbulence to maintain an appropriate speed to maintain control of the aircraft, while at the same time not overstressing the aircraft," said Bill Voss, president and CEO of the Flight Safety Foundation in Alexandria, Va.

Two buoys - standard emergency equipment on planes - were spotted from the Atlantic Ocean about 340 miles northeast of Brazil's northern Fernando de Noronha islands by the helicopter crew, which was working off a Brazilian navy ship.

Other debris spotted so far includes a 23-foot chunk of plane, an airline seat, an oil slick, and several large brown and yellow pieces that Cardoso said probably came from inside the plane. But a cargo pallet picked up yesterday that was originally thought to be from the jet turned out not to be, Brazilian officials said.

Brazilian Air Force General Ramon Cardoso said the plane was not carrying wooden pallets.

The pilot of a Spanish airliner flying nearby at the time reported seeing a flash of white light plunging to the ocean, said Angel del Rio, spokesman for the airline, Air Comet.