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The Douglas tests indicate that the unit in this abnormal condition would be capable of operating throughout the normal required range. However, since the test rig had no means to introduce appropriate air loads, neither the torque forces required to start and sustain rotation of the unit, nor that actual rate of drive were realistic when compared with the normal design values. It is believed that operation of the unit varied from these normal values, however, the variation would be nominal. The geometry of the drive system and the wear patterns in evidence strongly suggest a unit which would not attract attention to its abnormal condition until it failed completely.
Based on the evidence contained in the recovered horizontal stabilizer drive unit, and the tests performed by DACO, the Board concludes that the drive unit was installed by EAL maintenance personnel in September 1963, with the support bushing in the inverted position. This would allow the bushing to fall free at some point in time after installation and the drive shaft to drop down from its normal position. In this instance it dropped 1/4 inch and was operated in this position for an extended period of time. Since the wear rate would be dependent on the number of actuations, as well as the associated loads imposed, there is no way to determine the exact point in time of the commencement of this condition.
It is logical to assume that the drive unit was functioning prior to departure from New Orleans since the crew would have to position the stabilizer for takeoff. If the drive unit failed prior to takeoff, the crew would have had the difficulty corrected. The EAL DC-8 Flight Manual in use at the time of the accident indicated a stabilizer setting of one degree ANU for takeoff and since a normal trim correction toward AND is experienced as the aircraft is rotated and then "cleaned up," the drive unit was operating after the aircraft became airborne and started to climb as the assigned cruise level.
The stabilizer position of two degrees AND, whether placed there intentionally or unintentionally by the pilot, or by malfunction, is symptomatic of an abnormal flight condition. Consequently, the Board has focused on the possible reasons for the stabilizer position and the attendant conditions produced by this setting.
On at least two occasions tobacco tar, dust, and other material from the cabin have collected in the fairleads of DC-8 rear pressure bulkheads. On these occasions when actuation of the stabilizer was initiated by the pilot, the cables stuck in the fairleads and the pilot was unable to stop the stabilizer at an intermediate position. Once the full nosedown or noseup position was reached he was then able to actuate the control in the opposite direction. This also resulted in the control running to the full travel position. The rear pressure bulkhead of N8607 was not available for examination so the Board must rely solely on maintenance records which show that this area was cleaned a week before the accident. If the stabilizer cable fairleads were in fact cleaned at that time, and there is no reason to suspect otherwise, it is doubtful that the full AND position was produced by fairlead contamination.
Testimony by a DACO Aerodynamicist revealed that the extreme AND range of the stabilizer was provided to allow pilots, who so desire, to maintain a pull force under certain loading conditions during acceleration after takeoff. While it
