Crash Mystery Could Be Explained
When an airliner crashed into a New York neighborhood two months after the attacks of Sept. 11, 2001, it jolted the nation with fear that terrorists had struck again.
An act of terrorism was soon eliminated, but investigators remained baffled about what caused the plane’s tail fin to snap off in midair. It was believed to have been the first such structural failure in modern commercial aviation.
That mystery may finally be explained this week, and the answer could be surprising: What destroyed American Airlines Flight 587 may be much like what happens when the driver of a sport utility vehicle swerves sharply back and forth in an emergency and the vehicle flips.
On Tuesday, the National Transportation Safety Board is scheduled to meet and report its conclusions about what caused the crash, which killed all 260 people on board and five on the ground in the Queens neighborhood it struck.
The hearing is expected to take place in an acrimonious atmosphere -- rare for aviation investigations.
The airline and a former NTSB aviation safety chief have accused Airbus Industrie, the European manufacturer of the A300, of withholding information that might have helped prevent the accident. The top Airbus safety official has strongly rejected the charge.
What is not in question is what the copilot, who was flying the plane, did just before the catastrophic structural failure. Minutes after taking off without incident from John F. Kennedy International Airport, the copilot apparently reacted to turbulence, which shook the Airbus as it was hit by the invisible wake of a larger plane, by working the rudder controls back and forth in rapid sequence.
The rudder is a large movable flap on the rear of the tail fin. It is operated by a system of pedals, and normally pilots make little use of the rudder in flight.
The copilot’s first rudder command might have been intended to help level the plane. But it jolted the big jet, and his subsequent back-and-forth action on the rudder is believed to have generated the destructive forces that doomed the plane.
The A300’s tail cannot withstand the stress of that maneuver, manufacturers said. It ripped off, sending the plane crashing to earth.
The question at the heart of the investigation is whether the copilot could or should have known about the design limitations.
Airbus memos distributed to the NTSB and the airline well before the crash cautioned against moving the rudder back and forth during emergency maneuvers because it could cause stress on a plane’s tail beyond safety limits. But the admonitions were in papers that dealt with a range of issues, and were not prominently noted.
The NTSB investigation has found that, prior to the crash, airline pilots were generally not aware of the potential for such a structural failure. Most pilots assumed that they could make full use of the rudder and other aircraft controls within normal operating speeds.
Now, new evidence in the investigation could help to explain the actions of copilot Sten Molin.
In a technical report commissioned by the NTSB, a UC Davis aeronautical engineering professor concluded that the accident was “consistent with” a phenomenon that is rare in civilian aviation, though it sometimes occurs with pilots in high-performance military planes.
What happens, according to the expert, Ronald A. Hess, is analogous to a driver rolling over a sport utility vehicle.
Hess said the driver of a top-heavy SUV might make a hard turn to avoid road debris, only to feel the vehicle tilting sideways at an unexpectedly sharp rate. That could prompt the driver to swerve in the opposite direction, only to get the SUV leaning even more. With another sharp turn, the vehicle could flip.
“Somebody observing this ... might wonder why you are deliberately swerving back and forth,” Hess said. “ ‘Why are you doing it? It’s dangerous.’ As the driver, however, you have a considerably different viewpoint: You are doing everything in your power to maintain control of a vehicle that, to you, is acting very strangely.”
Hess also found that the A300’s rudder controls were the most sensitive among comparable airliners, and that the design was “possibly conducive” to the phenomenon.
Flight 587 took off on the clear morning of Nov. 12, 2001, bound for the Dominican Republic. Molin, 34, was at the controls.
About two minutes after takeoff, the plane wobbled slightly as it flew through the wake generated by one of the wings of a Boeing 747 ahead of it. Seconds later, as the A300 was turning left, it wobbled more noticeably as it encountered the wake from the 747’s other wing.
Molin first used his control wheel to try to level the plane by moving panels on the wing. When that apparently did not suffice, he pushed on his control pedals, moving the rudder sharply to the right and causing the plane to jolt sideways.
Perhaps trying to compensate, Molin then moved the rudder back to the left. But that only made matters worse. He went to the right again, then to the left, then right one last time before the tail broke off.
The whole sequence took about seven seconds.
The SUV analogy seemed convincing to aviation consultant Peter Goelz, a former managing director of the NTSB. “The Hess report certainly makes for a very compelling scenario,” he said.
“Was it reasonable to expect that the pilot understood there was grave jeopardy in manipulating the rudder the way he did?” Goelz asked. “If he was not aware of that, and was reacting in a way he thought was not only appropriate, but safe, then that broadens the area of responsibility for this accident.”
Airbus has strongly disputed the findings by Hess. The company said there was nothing wrong with the A300 controls that would make the plane susceptible to problems. It said unorthodox actions by the pilot were the sole cause of the accident.
The NTSB is also expected to address whether Airbus was completely forthcoming about earlier incidents in which back-and-forth rudder use resulted in severe stress on the A300. One such incident, involving a plane that stalled and nearly crashed in 1997, was investigated by the agency.
A memo recently released in a lawsuit indicated that Airbus engineers suspected at the time of the 1997 incident that the aircraft involved had exceeded the design safety limits of its tail.
Airbus did not share that specific piece of information with NTSB investigators, although it made general warnings that back-and-forth rudder movements could produce excessive stresses.
Bernard S. Loeb, then-NTSB safety chief, said the omission was critical. But John Lauber, the Airbus executive responsible for safety, said he didn’t see much difference between the details in the memo and general warnings issued by the company.
At stake in who gets blamed are corporate reputations and possibly millions of dollars in compensation to the families of victims. The NTSB may recommend a redesign of the rudder controls. Or it may decide cautions already issued to pilots would suffice.