Board Orders New Airliner Design Rules
After a three-year investigation, federal air safety investigators announced Tuesday that an unusual combination of human and mechanical factors caused the crash of American Airlines Flight 587 and called for new design requirements that would prevent mistakes by pilots from causing an airliner’s tail fin to rip off in flight.
The National Transportation Safety Board determined that the direct cause of the crash, which occurred two months after the Sept. 11 attacks and raised fears of terrorist sabotage, was the copilot’s “unnecessary and excessive” use of the rudder on the Airbus A300-600 jet to deal with fairly routine turbulence. The rudder, a movable flap on the tail fin, is a powerful control that is normally used sparingly.
However, the board also cited as contributing factors a “highly sensitive” rudder control system that remained “susceptible” to hazardous misuse by pilots, and airline training that might have conditioned the copilot to resort to the rudder without full awareness of the possible dangers.
“All three elements blended together in a way that caused a unique accident,” said NTSB Chairwoman Ellen Engleman Conners.
Flight 587 crashed into a neighborhood in Queens on Nov. 12, 2001, shortly after takeoff from John F. Kennedy International Airport. It was the second worst air disaster on U.S. soil, killing all 260 people aboard the jet and five people on the ground.
The NTSB recommendations called for the Federal Aviation Administration to assess the structural risks that sharp side-to-side movements would cause to airliners and to issue safety standards limiting the sensitivity of rudder controls.
The board also called on the FAA and European authorities to review options for modifying rudder controls on hundreds of Airbus A300-600 and A310 jets, which have the same system. The NTSB urged modifications to “provide increased protection from potentially hazardous” rudder control commands.
Airbus Industrie, a European consortium that vies with Boeing as the world’s leading aircraft manufacturer, reacted sharply to the NTSB’s findings.
“We do not believe the facts of the investigation point to a sensitivity of the rudder as contributing to the accident,” the company said in a statement. It had contended that the copilot flying the plane and the airline’s training program were at fault.
American Airlines spokesman John Hotard said the airline had already made significant changes in its training program, and he continued to blame Airbus. “They never told us about the rudder’s sensitivity,” he said.
The FAA reacted cautiously to the NTSB’s recommendations for new safety standards on rudder sensitivity. “We’ll look very carefully and closely at it,” said spokesman Les Dorr.
The board criticized the agency for failing to set forth clear emergency maneuver guidelines for pilots and airlines.
Throughout the inquiry, investigators wrestled with how to apportion responsibility among the copilot, the airline that trained him and the manufacturer that designed a jet with uniquely sensitive rudder controls.
When Flight 587 took off from New York, bound for the Dominican Republic, copilot Sten Molin, 34, was at the controls.
His reputation as an aviator was excellent, but two pilots who had previously flown with him told investigators that he was keenly sensitive to turbulence. On a previous occasion, a captain had criticized him for using sharp back-and-forth rudder commands to counteract turbulence. According to that captain, Molin replied that he was following American’s training recommendations.
The trouble on Flight 587 began when the plane, making a left turn, crossed the second of two wakes generated by a larger jet ahead. When the Airbus wobbled, Molin turned his control wheel sharply to the right and pushed on his right rudder pedal. That jolted the plane. The copilot went left, right, left and right again, generating forces beyond the strength of the tail fin.
It took about seven seconds before the tail broke off. Several witnesses saw fire shooting from the jet seconds before it crashed, leading to speculation that terrorists had attacked.
But in its report, the NTSB said the fire was probably caused by fuel leaking after the plane had started to break up. The FBI also investigated and found no evidence of terrorist involvement in the crash.
If Molin had eased up on the controls, he and everybody else on the plane might be alive now, investigators said. “If the first officer had stopped reacting ... the accident would not have occurred,” testified Malcolm Brenner, an NTSB expert who analyzed the crash.
But Molin was probably “surprised and confused” by the way the plane jolted, Brenner said, and might have believed that it was caused by turbulence and not by his actions. He was not unlike the driver of a sport utility vehicle who swerves to maintain control and ends up flipping the vehicle.
The design of the rudder controls made them increasingly sensitive at higher speeds, Brenner said. “To me, it is an inherently unfriendly design,” he said.
In ranking the contributing factors, the NTSB staff initially had recommended listing American’s training program ahead of the rudder controls. But the board voted 3 to 2 to emphasize the sensitivity of the plane’s controls.
“This agency’s role is to advocate change to prevent this type of accident from happening again,” said board member Mark V. Rosenker.
Although NTSB recommendations do carry weight, only the FAA has the legal authority to order changes. That process could take several more years, as was the case in another accident concerning rudder systems.
That investigation involved the 1994 crash of USAir Flight 427 near Pittsburgh, killing all 132 people aboard. That plane was a Boeing 737, the world’s most widely used airliner.
The investigation pointed to an extremely rare malfunction that could cause the rudder to go in the opposite direction commanded by a pilot. The FAA and Boeing initially questioned the NTSB conclusion, but later relented and ordered a fix.
Flight 587 presented the NTSB with several new challenges. It was the first time the tail fin of a jetliner had come off in routine flight. And the fin was not made out of metal, but from composite materials that are increasingly common in commercial aviation.
The strength of the composite tail fin turned out not to be the issue. Indeed, the stresses generated by Molin’s actions were far above the fin’s design limits. Yet investigators concluded that the rudder control system helped generate those forces.
