Common Themes

Organizational Lapses

Prior to the failure at American Airlines, Continental Airlines had developed a similar procedure for removal of the engine and pylon as a unit, in order to conduct the inspections required by the McDonnell-Douglas service bulletin. Twice, during service bulletin-related maintenance, Continental caused failures in the aft bulkhead, but discovered them prior to completion of the procedure. The failures were successfully repaired, and Continental attributed the failures to maintenance errors. As a result, they did not inform McDonnell-Douglas, or the FAA, and information was not disseminated outside Continental Airlines. Had the disposition and reporting process been more robust, American Airlines could possibly have learned of the failures at Continental, and taken measures to prevent similar failures on their own DC-10 fleet.

Human Error

As stated by the NTSB in the official accident report, "A minor mistake by the forklift operator while adjusting the load could easily damage the aft bulkhead and its upper flange." "....maintenance personnel had to be extremely cautious while they detached and attached the pylon." A seemingly minor error could result in the pylon attach fittings impacting the wing attachment structure, and could cause damage to the pylon aft bulkhead sufficient to cause the failure of the fitting, and separation of the engine/pylon when subjected to flight loads.

Flawed Assumptions

Airplane Design - McDonnell-Douglas, designed the DC-10 leading edge slat system to be hydraulically locked when extended, and did not provide a mechanical means to lock the system in the extended position. It was assumed that sufficient hydraulic isolation had been provided to prevent the asymmetric retraction of the leading edge slats.

Airplane Maintenance - American Airlines assumed that it would be safe to employ a forklift in conducting service bulletin-related maintenance, and that it was safe to remove the engine and pylon as a single unit, contrary to maintenance manual procedures. Damage to the just-inspected structure was the primary cause of the accident.

Preexisting Failures

This accident was a direct result of a failure that occurred, and was undiscovered during maintenance. The preexisting failure of the wing/pylon attachment structure led to the separation of the engine and pylon during a takeoff.

Unintended Effects

The McDonnell-Douglas service bulletins were intended to improve the safety of the fleet. American Airlines developed a service bulletin related maintenance procedure in order to reduce the time associated with removing and inspecting the engine pylon attachment structure. The procedure resulted in damage to, and the ultimate failure of, the part that had just been inspected. The ultimate result was to negate the safety improvement intended by the service bulletins, and actually reduce the safety of the DC-10 fleet.

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