Continental Express Embraer EMB-120ER incident

The airplane was descending through 14,500 feet when an uncommanded disconnect of the autopilot occurred, and the airplane "abruptly" pitched up from 5 degrees nose down to 15 degrees nose up (pitch reversion). Simultaneously, the captain and first officer applied forward pressure on their control columns to regain control of the airplane. The airplane was re-established in a descent and the flight crew found that the airplane was controllable in the landing configuration. Subsequently, the flight landed without further incident. Post incident examination of the airplane revealed that the right elevator trim control wheel was improperly installed. The installation allowed the autopilot trim servo to request nose down trim until the trim tab actuators had reached their physical limits. The autopilot trim servo continued to request nose down trim, loading the elevator trim system cables, until the autopilot disconnected. During the examination, it was also noted that the trim tab deflections and cable tensions did not meet manufacturer specifications. Review of maintenance records revealed 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 the maintenance. On August 11, 2000, the following maintenance write-up was recorded in the aircraft flight log: "Elev trim will not go full forward, stops at two units." The flight log indicated that the following corrective action was taken; "Lubed trim wheel for elev trim." Continental Express reported that the trim wheel was not removed during this corrective action.

Verbatim NTSB analysis and probable cause from the NTSB dataset