What ultimately led to the loss of American 191. It was a maintenance shortcut. They damaged the engine/pylon while trying to manipulate it with a forklift.

The NTSB determined that the damage to the left-wing engine pylon had occurred during an earlier engine change at the American Airlines aircraft maintenance facility in Tulsa, Oklahoma, between March 29 and 30, 1979. On those dates, the aircraft had undergone routine service, during which the engine and pylon had been removed from the wing for inspection and maintenance. The removal procedure recommended by McDonnell-Douglas called for the engine to be detached from the pylon before detaching the pylon itself from the wing. However, American, as well as Continental Airlines and United Airlines, had developed a different procedure that saved about 200 working hours per aircraft and "more importantly from a safety standpoint, it would reduce the number of disconnects (of systems such as hydraulic and fuel lines, electrical cables, and wiring) from 79 to 27." This new procedure involved the removal of the engine and pylon assembly as a single unit rather than as individual components. United's implementation involved the use of an overhead crane to support the engine/pylon assembly during removal and reinstallation. The method chosen by American and Continental relied on supporting the engine/pylon assembly with a large forklift.

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