Intact tail fin offers clue to Air France crash

On June 9, the front page of this newspaper carried a photograph of a red, white and blue object floating, like some sort of gaily colored raft, in a blue-black ocean. To pilots, it brought a chilling sense of deja vu. In November 2001, a similarly shaped and colored object floated in Jamaica Bay, just off Long Island. It was the vertical stabilizer -- colloquially, the "tail fin" -- of an American Airlines Airbus, Flight 587, that had broken up shortly after taking off from JFK. That fin was practically undamaged; it had parted at the root, each of the massive fittings that attach it to the fuselage torn neatly in half. Here was another such fin: seemingly intact, snapped cleanly from the vanished Air France Flight 447.

The National Transportation Safety Board took almost three years to untangle the mystery of the American Airlines crash. It eventually concluded that the first officer had caused the breakup by stepping too vigorously on the airplane's rudder pedals, and that the rudder pedals of Airbus airplanes were more susceptible to over-control than those of rival Boeing's jets.

The rudder is the movable portion of the vertical fin. Unlike the rudder of a boat, it is not used to turn. In fact, the rudders of jets are seldom used at all, except when landing in a strong crosswind or to hold the airplane straight after an engine failure. In this case, the NTSB thought, the pilot had tried to use the rudder to steady the plane in the wake of a 747 several miles ahead and had managed to break the vertical tail off instead.

Pilots were incredulous. The airplane had just taken off and was climbing; it was flying well under its "maneuvering speed," the speed below which a pilot should be able to use the flight controls in any way without risk of damaging the airplane. How, then, could this pilot possibly have broken the airplane with its own controls?

The New York crash uncovered a gaping misunderstanding among pilots, manufacturers and the FAA, which sets standards for structural safety and certifies compliance. Even though pilots believed in the absolute protection of maneuvering speed, and informational publications from the FAA and from manufacturers supported that belief, it turned out that if you read the certification regulations carefully, you would discover an exception: The vertical fin did not have to be strong enough to allow the rudder to deflect fully when the airplane was in a "yawed" position -- that is, when the back end of the plane had swung to one side, most likely because of a gust of wind.