What happened
While en route, the flight crew encountered a failure of the pitch trim control system. The crew found themselves unable to adjust the pitch trim using either the switches mounted on the yoke or the horizontal stabilizer control handles located on the center console. This loss of functionality was specifically noted by the crew when they attempted to extend the slats during the descent phase of the flight.
The investigation
An examination of the horizontal stabilizer pitch trim system revealed that there was no lubrication present on the screw actuator assemblies. Investigators determined that a fuse pin had sheared within the output drive unit of the horizontal stabilizer drive gearbox. This shearing of the fuse pin caused the horizontal stabilizer to lock in its final set position, which is the intended design function of the component upon failure.
Further investigation into the manufacturing process involved a review of Assembly Orders at Douglas Aircraft (DAC). The records showed that both DAC production personnel and DAC inspection personnel had stamped the assembly orders to certify that the lubrication process had been completed and accepted. However, the physical evidence confirmed that no lubrication had actually been applied to the screw actuator nuts.