initial event in the accident sequence was the structural separation of the number one engine and pylon assembly from the aircraft's left wing. Witness accounts, flight data recorder parameters, and the distribution of the major structural elements of the aircraft following the accident provided indisputable evidence that the engine and pylon assembly separated either at or immediately after rotation and about the same time the aircraft became airborne. At that time, the flight crew was committed to take off, and their decision not to attempt to discontinue takeoff was in accordance with prescribed procedures and was logical and proper in light of information available to them.
The investigation and analysis were concentrated primarily in two major areas. First, the investigation sought to identify the structural failure which led to the engine-pylon separation and to determine its cause; second, the investigation attempted to determine the effects the structural failure had on the aircraft's performance and essential systems, and the operational difficulties which led to the loss of control. In addition, the investigation went beyond these primary areas and probed such areas as the vulnerability of the DC10's design to maintenance damage, the adequacy of the DC-10's systems to cope with unique emergencies, the quality control exercised during DC-10 manufacturing and aircraft assembly, the adequacy of operator maintenance practices, the adequacy of industry communications of service and maintenance difficulties, the extent of FAA!s surveillance of overall industry practices, and the adequacy of an accepted operational procedure.
About eight weeks before the accident, the No. 1 pylon and engine had been separated from the wing of the accident aircraft in order to replace the spherical bearings in compliance with McDonnell Douglas'service bulletins 54-48 and 54-59. The four other American Airlines and two Continental Airlines aircraft, in which cracks-were detected in the aft bulkhead's upper flange, had also been subjected to the same programmed maintenance during which the engine and pylon was removed. Further corroboration that the cracks had been produced during these maintenance operations was obtained when it was learned that Continental Airlines had, on two occasions before the accident, damaged the upper flange on the aft bulkhead as pylons were being removed or reinstalled. In these two instances, the damage was detected; the bulkheads were removed and repaired in accordance with a method approved by McDonnell Douglas.
Therefore, the evidence indicated that the overstress cracks in the aft bulkhead's upper flange were being introduced during a