Incident Overview
During a flight at 13,000 feet mean sea level (msl), while decelerating to 250 knots, the horizontal stabilizer on a DC-10-30 airplane could not be moved by either autopilot commands or the primary and alternate control switches in the cockpit. The stabilizer remained at approximately 1 degree airplane nose-up, resulting in excessive cockpit control forces. The flight crew declared an emergency with air traffic control and subsequently landed without further incident on runway 27R at Hartsfield-Jackson Atlanta International Airport (ATL).
Maintenance History
The horizontal stabilizer drive assembly, part number AJH 7337-507 and serial number DCA-418, had been overhauled on April 30, 1999. It was installed in the incident airplane on April 23, 2006. At the time of the incident, the airplane had accumulated 2,421 hours and 473 cycles since installation of the overhauled assembly. Review of flight crew write-ups found no entries related to pitch trim for the preceding 93 days.
Cause of Failure
Investigation determined that the inoperative horizontal stabilizer trim system was caused by improper overhaul of the horizontal stabilizer chain drive unit drive assembly. During the overhaul, one pin (part number 3D0009-5-4), one nut (part number MS21043-08), and one washer (part number NAS 1252-BL) were omitted. These parts were necessary to secure the drive gear of the horizontal stabilizer drive assembly. The omission led to fatigue failure of another pin (part number AJH7378-1) and shear failure of the single installed pin (part number 3D0009-5-4). As a result, the output of the horizontal stabilizer drive assembly could not be transmitted to the acme screws of the horizontal stabilizer, rendering the trim system inoperable.