General Discussion

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.

If the forklift had been positioned incorrectly, the engine/pylon assembly would not be stable as it was being handled, causing it to rock like a seesaw and jam the pylon against the wing's attachment points. Forklift operators were guided only by hand and voice signals, as they could not directly see the junction between the pylon and the wing. Positioning had to be extremely accurate, or structural damage could result. Compounding the problem, maintenance work on N110AA did not go smoothly. The mechanics started disconnecting the engine and pylon as a single unit, but a shift change occurred halfway through the job. During this interval, even though the forklift remained stationary, the forks supporting the entire weight of the engine and pylon moved downward slightly due to a normal loss of hydraulic pressure associated with the forklift engine being turned off; this caused a misalignment between the engine/pylon and wing. When work was resumed, the pylon was jammed on the wing, and the forklift had to be re-positioned. Whether damage to the mount was caused by the initial downward movement of the engine/pylon structure or by the realignment attempt is unclear.  Regardless of how it happened, the resulting damage, although insufficient to cause an immediate failure, eventually developed into fatigue cracking, worsening with each takeoff and landing cycle during the eight weeks that followed. When the attachment finally failed, the engine and its pylon broke away from the wing. The structure surrounding the forward pylon mount also failed from the resulting stresses.