What happened
During an initial descent from 22,000 feet mean sea level (msl) to 14,000 feet msl, the elevator trim wheel moved to the forward stop. The movement included a "snapping" motion midway through before moving aft. Because the trim wheel continued to move back and forth, the crew disconnected both the autopilot and the trim system.
The aircraft exhibited a tendency to pitch up, requiring the captain to apply moderate forward pressure to maintain control. At 12,000 feet msl, with an outside air temperature of approximately 23 degrees Fahrenheit, the trim remained immovable. As the aircraft descended between 5,000 feet and 4,000 feet msl, the trim wheel became moveable again. By the time the aircraft reached the traffic pattern altitude, the trim wheel moved freely, at which point the outside temperature was 46 degrees Fahrenheit.
The investigation
An examination of the elevator trim units revealed that one actuator had leaking shaft seals, cracked packing, and a seized bearing. The second actuator was found to be dirty with minor corrosion on the hardware.
Review of Service Difficulty Report (SDR) data from August 30, 1990, to February 29, 2000, identified 19 reports of elevator trim freezing at altitude. Nine of these reports noted normal operation after descending to warmer, lower altitudes, while eleven other reports referenced binding or stiffness at altitude. Some aircraft involved in these reports experienced multiple events, including instances of violent pitching.
The manufacturer found moisture and corrosion in several actuators submitted for repair following such events, leading to the suspicion that leaking seals allowed moisture to enter the units and freeze at high altitudes. While a modification was developed to install additional internal seals, maintenance records indicated that both unmodified (Part Number 5299) and modified (Part Number 5299-1) actuators had experienced issues. Both actuators on this aircraft were unmodified units. The aircraft had a total time of 3,757 hours.