Friday, November 16, 2001 - Page updated at 12:00 AM

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Turbulence from 747 likely caused New York crash

Los Angeles Times

NEW YORK — A routine encounter with turbulence from another jetliner apparently turned deadly with stunning swiftness as the pilots on American Airlines Flight 587 lost control of the plane within seconds, federal investigators said yesterday.

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 the pilots lost control seconds later as the plane's tail fin apparently fell off.

"This is a very significant lateral acceleration we are talking about," said Marion Blakey, chairwoman at the National Transportation Safety Board (NTSB).

How tail fin works

The tail fin that separated in flight from the body of American Airlines Flight 587 moments before it crashed in New York was only one component of the Airbus A300's tail assembly. Here are the six tail "control surfaces" and their functions:

Vertical stabilizer, or tail fin: Situated atop and perpendicular to the fuselage, it provides lateral stability that keeps the plane moving in a straight line and supports the rudder. The most recognizable part of a commercial aircraft, it usually carries the company logo.
Rudder: Attached to the rear of the vertical stabilizer, it is moved hydraulically at the pilot's command to help turn the aircraft — "right rudder" to turn right, "left rudder" to turn left. A plane that loses its rudder still may be turned by engine power boosts, but the rudder is important to slow or brake the turn.
Horizontal stabilizers: The two small wings on either side of and below the tail fin. These steady the aircraft's tail section against up and down forces and provide platforms for the elevators.
Elevators: Attached to the rear of the horizontal stabilizers, these surfaces are raised and lowered at the pilot's command to climb or descend. Air flowing past the raised elevators pushes the plane upward, air passing over depressed elevators forces it downward.

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

Blakey said the Federal Aviation Administration and its French counterpart are preparing to order mandatory inspections of A300 tail fins. The A300, 91 of which are in service in the United States, is built by Airbus Industrie, a European consortium based in France.

As the investigation focuses on the loss of the 27-foot tail fin — known as the vertical stabilizer — the inspections could help establish whether Flight 587 suffered from a one-of-a-kind problem or whether there is a broader safety issue. Blakey repeated yesterday that there were no indications of sabotage or a bomb.

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

Flight-recorder data indicated that Capt. Edward States and First Officer Sten Molin strove 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.

The first notable event on the recorder occurred approximately 48 seconds before impact. At that point, Flight 587 crossed the wake of a Japan Air Lines Boeing 747 four to five miles ahead.

Like all planes, the 747 creates two wakes as it flies, horizontal tornadoes that whirl from each wingtip. 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.

Data do not show the wake was severe — just enough to rattle coffee cups.

Twenty seconds later, Flight 587 crossed the second wake, 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.

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

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 turning quickly to the left, while the left wing dipped 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.

About 20 seconds from impact — 2,900 feet above the ground — the flight-data recorder stopped working. Blakey suggested that it lost power at that point.

Haueter cautioned that it's very unlikely that any single factor is to blame.

"Almost every crash I've worked on was the result of a combination of events," he said.

Investigators said they have found no problems with the engines, an initial focus of concern. Instead, officials increasingly are concentrating on a composite material in the tail fin.


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