maintenance operation used by American and Continental Airlines. Both operators had devised special programs to replace the forward and aft bulkhead's spherical bearings. The manufacturer's service bulletins recommended that the maintenance be performed during an engine removal and that the engine be removed from the pylon before the pylon was removed from the wing. Both American Airlines and Continental Airlines believed that it would be more practical to comply with the service bulletin when an aircraft was scheduled for major maintenance-maintenance which would not necessarily otherwise necessitate engine removal. Therefore, American and Continental devised a procedure which they believed to be more efficient than that recommended by McDonnell Douglas-removal of the engine and pylon as a single unit. An engine stand and cradle were affixed to the engine and the entire weight of the engine and pylon, engine stand, and cradle was supported by a forklift positioned at the proper c.g. for the entire unit. The pylon-to-wing attaching hardware was removed, and the entire assembly was lowered for access to the spherical bearings. These were replaced and the entire unit was then raised and the attaching hardware reinstalled.

A close examination of these maintenance procedures disclosed numerous possibilities for the upper flange of the aft bulkhead, or more specifically the bolts attaching the spar web to this flange, to be brought into contact with the wing-mounted clevis and a fractureproducing load applied during or after removal of the attaching hardware in the aft bulkhead's fitting. Because of the close fit between the pylon-to-wing attachments and the minimal clearance between the structural elements, maintenance personnel had to be extraordinarily cautious while they detached and attached the pylon. A minor mistake by the forklift operator while adjusting the load could easily damage the aft bulkhead and its upper flange. The flange could be damaged in an even more insidious manner; the forks could move imperceptibly as a result of either an internal or external pressure leak within the forklift's hydraulic system during pylon removal. The testimony of the mechanics who performed the maintenance on the accident aircraft confirmed that the procedure was difficult.

The number one engine and pylon assembly separated after the flightcrew was committed to continuing the takeoff. Witnesses saw the pylon and engine assembly travel up and over the left wing after it separated, and the deformation of the pylon's forward bulkhead was consistent with their observations. The left wing's leading edge skin forward of the pylon's front bulkhead was found on the