Flight 587 crash investigation keys on structural failure

New York-Evidence is implicating failure of the composite material vertical tail (stabilizer) and its attachment to the fuselage in the crash of an American Airlines Airbus A300 in New York last Monday.

While discussion is centering on wake turbulence encountered from another aircraft after takeoff, Wolf Czaia, a former American Airlines captain and check airman (clearing others to fly) for the 767 and 757, gives good insight to Design News as to possible crash circumstances. "A vertical tail cannot come off, at least not with the speed they had, through wake turbulence alone," he notes. "However, if both engines are at full power and one slams into reverse, you generate a sideslip that will most likely cause first the vertical tail to come off and eventually the engines to come off," which is the sequence that occurred. "If the tail had come off due to structural failure because of the way it is attached and there are cracks that haven't been detected, I would think the aircraft would still be flyable-corrected for by differential thrust. But one engine going into reverse, or the wrong corrective action in case of loss of an engine can generate excessive sideslip."

In the possible accident scenario, perhaps triggered by a deployed thrust reverser or wake turbulence, the vertical stabilizer separated from the aircraft. This caused a lack of stability and control in yaw (left-to-right rotation of the aircraft). The Airbus then experienced alternating side forces of 0.3 to 0.8g due to aerodynamic loads from extreme yaw angles, which could tear the engines from their mounts. These angles also likely cut off smooth, straight in airflow to the engines, which were set for high power, causing them to fail.

When recovered from Jamaica Bay, the undamaged-looking vertical tail showed an extremely clean break from the airplane. While the carbon composite used in the tail has a very high strength-to-weight ratio, it exhibits very little yield (deformation) before failing catastrophically-consistent with the break exhibited.

Photos of the recovered vertical tail on the National Transportation and Safety Board (NTSB) website (http://www.ntsb.gov/Events/2001/AA587/tailcomp.htm) show failures in the front attachment lugs (there are three lugs, front, middle, and rear on each side of the base of the vertical stabilizer). The center and aft attachments visible in the photos show intact lugs still attached to the rear fuselage (empennage) with the failures occurring in the structure above the attachments.

While probably not a structural design flaw in the stabilizer or its attachment (no other Airbus has had this problem), any defect may have been an isolated manufacturing problem or the result of specific in-service damage or aging. The airplane that crashed was involved in an air turbulence incident in 1994 in which many passengers had to be treated for injuries. American Airlines is inspecting the vertical tails of its remaining European-made Airbus A300 fleet for any defects and the FAA and French authorities are issuing directives for all the aircraft's operators to do so.

This crash points to the value in the future of onboard video recording from cameras that view an airplane's control surfaces and engines, not just for accident investigation but also permitting the crew to see parts of the plane not be visible from the cockpit. Additional cameras could also monitor the passenger cabin and cockpit to add to flight data recorder information. Likewise, airports could have continuous camera coverage to record any incidents at the field or in the vicinity.