What happened
While descending through 14,500 feet, the aircraft experienced an uncommanded autopilot disconnect. This event triggered a pitch reversion, where the airplane abruptly transitioned from a 5-degree nose-down attitude to a 15-degree nose-up attitude. In response to the sudden change, the captain and first officer applied forward pressure on their control columns to stabilize the aircraft. The crew successfully re-established a descent and confirmed the airplane remained controllable in the landing configuration. The flight subsequently landed without further incident.
The investigation
Post-incident examination of the aircraft revealed that the right elevator trim control wheel had been improperly installed. This specific installation allowed the autopilot trim servo to request nose-down trim until the trim tab actuators reached their physical limits. The servo continued to request nose-down trim, which loaded the elevator trim system cables until the autopilot disconnected.
Investigators also found that the trim tab deflections and cable tensions did not meet manufacturer specifications. A review of maintenance records identified two previous involvements with the elevator pitch trim system:
- On April 14, 1998, the right side elevator trim control wheel was removed and reinstalled while aligning the stop arm into the wheel; during this period, the trim tabs were adjusted.
- On August 11, 2000, a flight log entry noted that the elevator trim would not move full forward, stopping at two units. The recorded corrective action was to lubricate the elevator trim wheel, though the wheel was not removed during this procedure.