Incident Description
While en route, the flight crew experienced a failure of the pitch trim control system. The crew attempted to change the pitch trim using the yoke-mounted trim switches and the center console-mounted horizontal stabilizer control handles, but neither method was effective. The crew stated that when the slats were extended during descent, they were unable to change the stabilizer trim.
Investigation Findings
A thorough examination of the horizontal stabilizer pitch trim system was performed. The examination revealed that no lubrication was present on the screw actuator assemblies. Additionally, the fuse pin in the output drive unit of the horizontal stabilizer drive gearbox had sheared. As designed, the sheared fuse pin locked the horizontal stabilizer in its last set position.
Assembly Order Discrepancy
The investigation included a review of Assembly Orders at Douglas Aircraft (DAC). It was discovered that both DAC production personnel and DAC Inspection personnel had stamped the assembly orders, confirming that lubrication had been completed and accepted. However, the investigation found that no lubrication had actually been applied to the screw actuator nuts.