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the sprockets to the jackscrews. The dual sprocket assembly was replaced, but the rivets sheared again during the ground test operation. It was noted that the shearing occurred without contacting the stop in either direction, so both the jackscrews and sprocket assembly were replaced this time. One of the removed jackscrews had a rusted thread and the lubrication on the bearing was poor. The mechanic who performed this work, an authorized inspector, reported that "…it is the custom in EAL Maintenance to change the complete assembly and not (disassemble) the sprocket housing. Line Maintenance procedures require the replacement of units, not parts, to keep from overhauling units on the line." There was no further maintenance work recorded on the stabilizer drive unit of N8607 until the accident.
The review of the aircraft records disclosed a recent history of PTC difficulties. The PTC computer on this aircraft had been changed eight times, four had been used during the last week of operation. On February 18, although the PTC was reported as operative, the indicator failed to show extension. There was no maintenance performed on the indicator mechanism following this writeup. Computer S/N 268D, installed at the time of the accident, had been removed from various aircraft 15 times, beginning in April, 1960. Six of these removals were for unwanted extensions. No discrepancies were ever found during the shop inspection of this component. Following the accident it was discovered that functional tests by EAL and other operators could not detect certain computer malfunctions. This was demonstrated when a serviceable unit in EAL stock failed to pass the manufacturer's complete test procedures. Two other instances were discovered at other operators. The PTC computer of N8607 was changed the last time on February 24, in Miami. The aircraft was flown to Philadelphia and no flight crew complaints on the PTC were entered in the log. The flight engineer on the outbound flight to Mexico City noted that the PTC failed to check on the ground. Maintenance personnel performed a ground check of the system and confirmed the engineer's findings. The check performed was: activation of the test circuit and watching for movement of the indicator or control yoke. No inspection of the actuator position or operating capability of the indicator system was made. The aircraft was dispatched with a request that the crew check the PTC operation during the flight. This check was performed at cruising speed and altitude between Washington and Atlanta. It was determined that the PTC was inoperative.
The flight maintenance logs also revealed eleven autopilot malfunctions in the last 30 days of operation. Two discrepancies involved yaw, six referred to longitudinal control problems, and three reported automatic disconnects.
The review disclosed that both artificial horizons failed simultaneously on February 18, 1964. This was corrected by replacement of the instrument switching unit, which is a common point in the wiring of both instruments.
1.16 Tests and research
The Douglas Aircraft Company (DACO) performed a functional test of a stabilizer drive unit to determine what effect the omission of the drive sprocket support bushing P/N 2652666 would have on the sprocket drive gear assembly which did not have production shear rivers installed. Their reported dated May 29, 1964, indicated that while full stabilizer trim capability existed, a comparable wear pattern was reproduced on the test assembly. On June 1, 1964, the FAA issued a
