Incident Overview
After leveling at flight level (FL) 310, the flight crew experienced a loss of elevator trim. To maintain level flight, the crew was required to hold continuous control pressure. The total air temperature at that altitude was -50 degrees Celsius. The captain decided to continue the flight to the intended destination. Normal elevator trim operation was restored when the aircraft descended through 16,000 feet on approach, where the total air temperature was -7 degrees Celsius. The flight subsequently landed without further incident.
Postflight Inspection
Following the flight, inspectors from the Federal Aviation Administration (FAA) conducted a postflight inspection of the elevator trim actuators. The inspection revealed paint overspray on the elevator trim actuator push rods, as well as scoring and corrosion on the push rods. Additionally, white desiccant windows were observed on the actuators.
Maintenance History
The airline's FAA principal maintenance inspector (PMI) reported that there had been 10 unscheduled removals of DO-328 elevator trim actuators over the preceding two and a half years. These removals were attributed to moisture and/or freezing in flight. The PMI noted that the maintenance check cards did not include instructions for checking the desiccant windows for moisture contamination or criteria for evaluating the color of the desiccant windows.
Vendor Findings
Teardown reports from the vendor, AVIAC Technologies of France, indicated that the paint overspray on the push rods was damaging the actuator quad rings during operation. The PMI stated his belief that the damage to the quad rings allowed moisture to enter the actuator housings.