SkyWest Airlines Flight 5353

The elevator trim froze in flight. During the initial descent from 22,000 feet mean sea level (msl) to 14,000 feet msl, the trim wheel moved to the forward stop. The trim wheel made a "snapping" motion midway through the movement, and then moved aft. The trim wheel continued to move back and forth, so the crew disconnected the autopilot and trim. The airplane wanted to pitch up, and the captain had to apply moderate forward pressure to maintain pitch control. The trim wheel was still jammed, but one crewmember could control the airplane. At 12,000 feet msl, and approximately 23 degrees Fahrenheit outside air temperature, the trim remained immovable. Between 5,000 feet msl and 4,000 feet msl, the trim wheel became moveable. By the time the airplane reached the traffic pattern altitude, the trim wheel moved freely. The outside temperature at that time was 46 degrees Fahrenheit. Examination of the elevator trim units revealed that one trim actuator had leaking shaft seals. Its packing had cracked, and a bearing had seized. The second actuator was dirty, and minor corrosion was on the hardware. The shaft seals leaked, and a rigging device was installed. Service Difficulty Report (SDR) data from August 30, 1990, to February 29, 2000, contained 19 reports of the elevator trim freezing at altitude. Nine of the reports noted that the trim operated normally after descent to lower altitudes and warmer temperatures. Eleven additional reports referenced binding or stiffness at altitude. Some of the airplanes experienced multiple events. During some of the events, the airplane pitched violently. The manufacturer discovered moisture and corrosion in some of the actuators that were submitted for repair after these occurrences. They suspected that leaking seals allowed moisture into the actuators, which froze at altitude. They developed a modification that included installation of additional seals inside the actuator. However, Skywest maintenance records indicate that both unmodified (Part Number 5299) and modified (Part Number 5299-1) actuators experienced problems. Both actuators on the incident airplane were unmodified units. Total time on the airplane was 3,757 hours.

Verbatim NTSB analysis and probable cause from the NTSB dataset