What happened
While flying at 13,000 feet mean sea level and decelerating to 250 knots, the DC-10-30 experienced a significant flight control malfunction. The horizontal stabilizer became unresponsive to both autopilot commands and the primary and alternate control switches located in the cockpit. Because the horizontal stabilizer remained fixed at approximately 1 degree nose-up, the crew encountered excessive control forces. The flight crew declared an emergency with air traffic control and subsequently performed an uneventful landing on runway 27R at ATL.
The investigation
The investigation focused on the inoperative horizontal stabilizer trim system. It was determined that the failure originated from an improper overhaul of the horizontal stabilizer chain drive unit drive assembly (part number AJH 7337-507, serial number DCA-418). During the overhaul process, a pin (P/N 3D0009-5-4), one nut (P/N MS21043-08), and one washer (P/N NAS 1252-BL) used to secure the drive gear were omitted.
This omission led to a fatigue failure of an additional pin (P/N AJH7378-1) and the shear failure of the single remaining installed pin. These failures prevented the output of the horizontal stabilizer drive assembly from being transmitted to the acme screws of the horizontal stabilizer. Records indicated that the overhauled assembly was installed on April 23, 2006, after having accumulated 2,421 hours and 473 cycles. There had been no previous flight crew reports regarding pitch trim issues for the preceding 93 days.