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Sideways Jolts Shook Jet After It Hit Turbulence

Probe: Data show pilots lost control within seconds as the plane's tail fin apparently fell off.

November 16, 2001|ERIC MALNIC and RICARDO ALONSO-ZALDIVAR | TIMES STAFF WRITERS

NEW YORK — A routine encounter with turbulence from another jetliner turned deadly with stunning swiftness as the pilots on American Airlines Flight 587--who had trained for just such an event--lost control of the plane within seconds, federal investigators said Thursday.

Information from the Airbus A300's flight data recorder showed three rapid sideways movements after Flight 587 crossed the wakes of a larger plane ahead. The movements were well within what the A300 was designed to handle, investigators said. But seconds later, the pilots lost control as the plane's tail fin apparently fell off.

"This is a very significant lateral acceleration we are talking about," said National Transportation Safety Board Chairwoman Marion C. Blakey.

The movements may be the key to understanding how the plane's tail fin broke away without any evidence of outside damage--an event virtually unheard of in commercial aviation. "Possibly that was the precursor," said Greg Feith, a former senior air crash investigator for the NTSB.

Blakey said the Federal Aviation Administration and its French counterpart are preparing to order mandatory inspections of A300 tail fins, a sweeping action that would affect all 91 of the wide-body planes in service in the United States and more than 400 worldwide. The A300 is built by Airbus Industrie, a European consortium based in France.

As the probe focuses on the loss of the 27-foot tail fin--known as the vertical stabilizer--the inspections could play a critical role. The results could help establish whether the plane that crashed suffered from a one-of-a-kind problem, or whether there is a broader safety issue. Blakey repeated Thursday that there were no indications of sabotage or a bomb.

Flight 587 took off early Monday from John F. Kennedy International Airport, climbing into clear skies. Less than three minutes later, the plane rolled and nose-dived into a residential neighborhood in Queens. All 260 aboard perished in the fiery crash, as did five people on the ground.

Pilots Likely Didn't Know Tail Was Gone

The flight data recorder indicated that Capt. Edward States and First Officer Sten Molin strived in vain to control the aircraft. The pilots probably did not know they had lost the tail fin, which helps keep a plane flying straight.

"They thought they were flying against wake turbulence," Feith said. "They didn't know the tail was off the airplane."

The first notable event on the recorder occurred approximately 48 seconds before the final impact. At that point, Flight 587 crossed the wake of a Japan Air Lines Boeing 747 some four to five miles ahead. Though both are classified as "heavy" aircraft, the Airbus weighed about 350,000 pounds, while the 747 weighed 800,000 pounds.

Like all planes, the 747 creates two wakes as it flies, horizontal tornadoes that whirl from each wingtip. But the Airbus A300 is big enough that the "wake vortices" of the 747 should not have been a problem. Flight 587's pilots had trained on how to recover from an upset caused by a 747. Indeed, Blakey said that Flight 587 crossed the first wake without incident.

"The airplane attitude did not change much during the first encounter," Blakey said.

Second Wake Proved Disastrous

The data do not indicate that the wake was particularly severe. "Airliners encounter wake turbulence every day around the globe," Blakey said.

Twenty seconds later, Flight 587 crossed the second wake of the 747, which was similar to the first.

As it emerged from that wake, it was shaken by three sideways forces, three to four times as strong as the movements measured during the wake turbulence. Two were in the same direction and the final one in the opposite direction. Blakey did not specify whether the movements were first to the left or to the right.

But investigators said these movements occurred simultaneously with changes in the position of the plane's rudder, which is part of the tail fin assembly and is used to help steer.

"These accelerations basically follow rudder movement," said Tom Haueter, NTSB's lead investigator on the crash. "The precise sequencing is still being examined."

A few seconds later, there was a sharp jolt--about eight times as strong as those measured during the wake encounters.

Nonetheless, Haueter said, the forces "were well below what the plane should be able to handle."

At the same time, rudder position information from the flight data recorder became unreliable. "The vertical stabilizer and the rudder were probably already off the airplane," Feith said.

Flight 587 began quickly turning to the left, while the left wing dipped down at a 25-degree angle, Blakey said. The plane pitched nose down at a 30-degree angle.

The pilot, meanwhile, was turning his control wheel to the right. "What they were trying to do is turn opposite to the roll," Feith said. "They were trying to counteract it." But having lost a tail fin in the midst of a rapid climb, the plane would have been all but impossible to control.

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