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Wednesday, February 2, 2000
Early stages of crash investigation will focus on Flight 261's stabilizer
If instrument is at fault, 'this could be precedent-setting'
By Matthew Fordahl
Associated Press
LOS ANGELES - Early theories on the Alaska Airlines crash focus on the horizontal stabilizer, a small, tail-mounted wing that helps point the jet's nose up or down. But experts said other, still-unexplained factors probably played a major role, too.
Shortly before the airliner went down, a crew member reported that the horizontal stabilizer had jammed. Minutes later, the MD-83 jetliner, with 88 people aboard, slammed into the Pacific as it was flying to Los Angeles for an emergency landing.
Problems with the device are rare and can usually be resolved by switching its motors off, said Barry Schiff, an aviation consultant and former TWA pilot. The system has not been tied to any other crashes involving the MD-80 series of jets.
"This could be precedent-setting," he said. "That's a pretty rugged hunk of machinery back there, and it's designed not to fail."
The horizontal stabilizer is used to balance the plane's up-and-down motion in a process called trimming. A plane can fly without such a system, but the crew would have to push extremely hard on the yoke to maintain level flight.
Pilots routinely train for an unusual scenario called runaway trim, in which an electrical short or some other glitch causes a loss of control of the stabilizer, said Dugan Blechschmidt, an Alaska Airlines pilot who checks the qualifications of the airline's flight crews.
"All of a sudden the airplane wants to pitch up without any input from the pilot," he said. "We have procedures to deal with that."
Catastrophic possibilities
In the 11 minutes between the first call of trouble and the crash, the crew should have had time to deal with the problem, including using other controls or even cutting power to the system altogether without losing control, experts said.
Other factors, however, could have complicated the situation.
"What if something catastrophically occurred back in the tail that rendered one of these controls inoperative? Then you could have a real problem on your hands," Schiff said.
"Or what if a piece of the tail actually came off and then jammed the stabilizer?"
Signs of corrosion
In May, the Federal Aviation Administration ordered operators of MD-80s and related jets to undergo inspection for signs that hinges connecting parts of the plane's tail were corroded.
Alaska Airlines was in the process of inspecting its fleet and had not checked out the plane that crashed Monday.
Airline spokesman David Marriott said 10 planes that were inspected showed "nothing of any significance." He said the aircraft involved in the crash had been scheduled to be checked in June.
Feathers on the plane
A horizontal stabilizer has the same effect as the feathers on an arrow or the fins of a dart. It counteracts the aerodynamic effects that cause bobbing during flight. Another stabilizer controls left-right motion.
"If you fired an arrow and there were no feathers back there, the arrow would wobble through the air," Schiff said.
"So we put tail feathers on an airplane for the very same reason - to stabilize the airplane so it doesn't wobble."
The system is controlled by flight-deck switches on the pilot's and first officer's steering columns. Electrical motors controlled by the switches can move the stabilizer up or down by 14 degrees, said John Thom, a spokesman for the Boeing Co., which in 1997 bought McDonnell Douglas, maker of the MD-80s.
"Trimming" the stabilizer is an action a pilot uses to bring it into balance.
Beyond the crew's capacity
If the crew lost control of the horizontal stabilizer, it would have no way to keep the nose pointed at the proper angle up or down, though it is unlikely there would be a sudden change in the plane's pitch. Aerodynamic forces would cause the plane to gradually point toward the ground.
"The fact that they had at least a few minutes to deal with the situation, whatever started was probably subtle," said William Waldock, associate director of the Center for Aerospace Safety Education at Embry-Riddle Aeronautical University in Arizona.
"It sounds like something happened relatively quickly at the end," he said. "Whatever it was, it was obviously beyond the capability of the crew to overcome."
It will probably be weeks or months before any conclusions can be made. Flight recorders will shed light on the condition of the instruments, and radar tracking data might reveal how quickly the plane dropped.
"What we're doing is interpolating between that comment and the crash itself to try to create some scenario that would all of this to happen," he said. "If you have a catastrophic failure in some way, that could lead us to the accident site."
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