What happened
On 7 July 2006, a Boeing 737-377, registration G-CELA, was performing a cargo flight from Newcastle Airport to Stansted Airport. During a climb to FL270 with the autopilot engaged, the aircraft failed to capture the selected altitude, eventually overshooting the target and reaching 27,450 ft.
Upon realizing the aircraft was not leveling off, the commander disconnected the autopilot. Following this disconnection, the commander experienced significant difficulty controlling the aircraft' and noted a vibration through the control wheel when applying aft force. Due to the difficulty in maintaining level flight, the commander declared a 'PAN' and later upgraded the situation to a 'MAYDAY', requesting radar vectors for an emergency descent to Stansted. The aircraft landed safely without injuries to the two crew members.
The investigation
The AAIB investigation focused on why the aircraft failed to capture the altitude and the cause of the subsequent pitch control issues. Investigators examined the flight data recorder, cockpit voice recorder, and radar recordings.
Technical examinations of the aircraft revealed that a circuit breaker for the autopilot stabiliser trim actuator had tripped. While engineers could not identify a specific technical cause for the tripping of this breaker, they confirmed that the autopilot stabiliser trim system was inoperative at the time of the incident. Furthermore, while the mechanical elements of the elevator system were found to be serviceable, the investigation looked into whether the loss of the trim system contributed to the commander's difficulty in managing the aircraft's pitch.
Findings
- The failure of the aircraft to level off at the selected altitude was caused by the autopilot stabiliser trim actuator being inoperative due to its circuit breaker being tripped.
- The loss of the autopilot stabiliser trim meant the horizontal stabiliser could not automatically compensate for elevator movements, limiting the aircraft's pitch authority.
- The commander's difficulty in controlling the aircraft in pitch may have been exacerbated by overcontrolling the aircraft following the initial altitude overshoot.
- The location of the autopilot stabiliser trim circuit breaker on the P18 panel, positioned behind the commander and partially obscured by a map light and the seating position, likely hindered the crew's ability to identify and rectify the tripped breaker.