What happened
While maintaining flight level 310 (FL 310), the crew experienced a loss of elevator trim. To maintain level flight, the crew was required to apply continuous control pressure. At cruise altitude, the total air temperature was -50 degrees C.
The captain decided to proceed to the destination. As the aircraft descended through 16,000 feet during its approach, where the total air temperature was -7 degrees C, normal elevator trim operation was restored. The flight landed without further incident.
The investigation
Postflight inspections conducted by FAA inspectors on the elevator trim actuators revealed several mechanical issues. Findings included paint oversprey on the elevator trim actuator push rods, as well as scoring and corrosion present on those rods. Additionally, the desiccant windows on the actuators were found to be white.
The airline's FAA principal maintenance inspector (PMI) noted that there had been 10 unscheduled removals of the DO-328 elevator trim actuators over the previous 2.5 years due to moisture or freezing during flight. The investigation found that maintenance check cards failed to include instructions for checking desiccant windows for color criteria or moisture contamination.
Teardown reports from the vendor, AVIAC Technologies of France, indicated that the paint overspray on the push rods was causing damage to the actuator quad rings during operation. It was believed that this damage to the quad rings allowed moisture to enter the actuator housings.