Incident Overview
The incident occurred as the airplane was descending through 14,500 feet. An uncommanded autopilot disconnect took place, followed by an abrupt pitch-up from 5 degrees nose down to 15 degrees nose up—a phenomenon described as pitch reversion. Both pilots applied forward pressure on their control columns to regain control. The flight crew successfully re-established a descent and found the airplane controllable in the landing configuration. The flight landed without further incident.
Examination Findings
Post-incident examination of the airplane revealed that the right elevator trim control wheel had been improperly installed. This installation allowed the autopilot trim servo to request nose-down trim until the trim tab actuators reached their physical limits. The autopilot trim servo continued to request nose-down trim, loading the elevator trim system cables, until the autopilot disconnected. Additionally, the examination noted that the trim tab deflections and cable tensions did not meet manufacturer specifications.
Maintenance History
Review of maintenance records identified two instances when the elevator pitch trim system underwent maintenance. On April 14, 1998, the right side elevator trim control wheel was removed and reinstalled "while aligning stop arm into wheel." Continental Express reported that the trim tabs were adjusted during this maintenance. On August 11, 2000, a maintenance write-up was recorded in the aircraft flight log stating: "Elev trim will not go full forward, stops at two units." The flight log indicated that the corrective action taken was to "lub trim wheel for elev trim." Continental Express reported that the trim wheel was not removed during this corrective action.