Safety Assumptions

  • It was assumed that a forklift was safe to use in removing/reinstalling the pylon and engine. American Airlines thought they had developed a procedure employing a forklift to lower the engine and pylon as a unit that would be safe and efficient. In retrospect, it produced an unforeseen, difficult to detect problem that damaged the airplane and led directly to the crash.

  • McDonnell-Douglas incorrectly assumed that the leading edge slats would always retract symmetrically. They may also have assumed that a full leading edge asymmetry was always controllable. They didn't address what would happen if a wing stalled due to the asymmetry, and they were not required to examine the effect of an asymmetry in the takeoff region of the flight envelope.

  • It was assumed that slat asymmetry due to loss of hydraulics would not occur. McDonnell-Douglas thought they had provided sufficient system isolation to preclude loss of all hydraulic systems, and thereby prevent slat retraction. They did not foresee loss of hydraulics to the leading edge slats as the result of an engine/pylon separation. The slat system was reliant on the retention of hydraulic pressure to maintain slat position. There was no alternate locking system to prevent retraction of the slats on loss of hydraulics to the slat system.

  • V2 assumes that only the thrust from a single engine is lost.  The weight of the engine, and the wing's ability to generate lift, will be unaffected.

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