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Crashes Analyzed for Clues

As the Northwest Airlines jet climbed past 16,000 feet, its speed began increasing mysteriously. A short time later, a horn sounded to alert the pilots that they were flying dangerously fast.

"Just pull her back, let her climb," the captain told the confused co-pilot, suggesting that they could slow the Boeing 727 down by making it climb even steeper.

Within minutes, this 1974 flight crashed into a wooded area in New York, killing all three pilots, the only ones aboard. According to aviation safety experts, it is one of a string of accidents around the world that could offer clues into what might have caused Air France Flight 447 to disappear as it cruised above the Atlantic Ocean on June 1.

The blaring warning horn and speed indicators in the cockpit of the Northwest jet were erroneous, investigators concluded later. The normally highly reliable aircraft was instructing pilots to do the wrong thing.

Instead of speeding up, the jet was actually slowing, the National Transportation Safety Board (NTSB) found. All three devices that measured air speed had become clogged with ice and were telling the pilots they were going far faster than they actually were. Faced with a jarring series of sometimes contradictory warnings, the pilots became confused.

The jet's speed got so slow that its wings no longer could keep it aloft. It plunged from nearly 25,000 feet in 83 seconds. Heavy buffeting began tearing pieces off the plane before it hit the ground.

Worrisome indicators

Though investigators have not yet retrieved the flight data recorders they hope will tell them precisely what happened aboard the Air France Airbus A330, a string of data messages from the jet point to problems with the air-speed indicators. The NTSB announced last week that it is investigating two other recent incidents aboard Airbus A330 jets that appear similar to the problems reported on the Air France flight.

The Air France messages - which were automatically transmitted via satellite to the airline as part of routine flight monitoring - indicate that some or all of the sensors called Pitot tubes that calculate speed had failed. When the Airbus' computerized flight-control system is not sure what the aircraft's speed is, it takes a number of protective steps. The data from Flight 447 show that the computer shut off the autopilot and the automatic throttle, requiring the pilots to fly manually.

By this time, the pilots would have been hearing various chimes and receiving computer-generated text warnings, according to several current and former Airbus pilots.

"There could have been a lot going on on the flight deck," says John Cox, a former airline pilot who flew Airbus jets and now works as an aviation safety consultant.

Pilots are trained to keep the plane level and maintain engine power if they get unusual airspeed readings. After the Air France accident, Airbus urged airlines to remind pilots of the rule.

Cox and other aviation experts caution that it's too soon to conclude what happened aboard Flight 447, but they say accidents like the one in 1974 demonstrate that pilots sometimes get so confused in such conditions that catastrophe can result.

The NTSB studied similar incidents during its 1974 investigation. "It was apparent from these incidents that some pilots who understood the basic principles of air-speed measurement failed to analyze the possible results of a blockage of the Pitot ... systems," the agency said.

Deadly results

Other crashes have resulted from erroneous speed readings, according to accident reports:

-- On Feb. 6, 1996, a charter flight operated by Turkish airline Birgenair crashed in the ocean off the Dominican Republic, killing all 189 people aboard. The Pitot tube that fed the captain's instruments was blocked, making it appear that the Boeing 757 was flying more than 400 mph. The jet's warning systems alerted the crew that they were flying too fast. As the jet was actually slowing to about 200 mph, a warning system alerted the crew that they were flying too slowly. The jet plummeted into the water.

-- On Oct. 10, 1997, a flight by Argentine airline Austral Lineas Aéreas crashed in Uruguay, killing all 74 people aboard the McDonnell Douglas DC-9. The jet's Pitot tubes had become clogged with ice. The result: speed readings that were slower than the plane was actually traveling.

The pilots added power to the engines and began descending to pick up speed. The pilots also deployed panels on the wings that add lift. The jet, however, was flying so fast that the wind tore the panel off one wing and the jet veered out of control.

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