Summary
On 7 July 2006 at 2350 UTC, a Boeing 737-377 cargo aircraft, registration G-CELA, experienced an altitude capture failure while climbing to FL270 en route from Newcastle Airport to Stansted Airport. The aircraft, operated as a public transport cargo flight, had two crew members on board and no passengers. No injuries or damage occurred. The commander took control and reported difficulty maintaining pitch, leading to a PAN and later a MAYDAY declaration. The aircraft landed safely at Stansted.
History of the Flight
The flight was the third of four scheduled flights. Earlier flights were uneventful except for a reported anomaly with the thrust reversers on landing at Newcastle, which subsequent ground tests showed to be operating correctly. For the incident flight, the first officer was the pilot flying (PF) and flew manually using the flight director. After an uneventful takeoff and climb to FL210, autopilot B was engaged. The aircraft was cleared direct to Manchester and to climb to FL270.
As the aircraft approached FL270 with LNAV and VNAV selected, the first officer observed 'FMC SPD' and 'VNAV PATH' annunciations but noted no change in attitude. The commander, who was on radio, saw the aircraft approximately 300 ft above the selected altitude and still climbing, with the altitude alert light illuminated. He disconnected the autopilot and autothrottle, manually flying the aircraft back to FL270. Around that time, both crew members briefly saw the 'STAB OUT OF TRIM' light, but were uncertain whether it appeared before or after autopilot disengagement.
The commander found it difficult to control the aircraft in pitch and could not achieve the correct pitch trim using electric or manual trim. He informed the first officer and, after no improvement, contacted Manchester ATC, declaring a PAN and requesting vectors to Stansted. He upgraded to MAYDAY shortly after. During descent, the commander used Level Change and manual thrust, noting a vibration in the control wheel when applying aft force or increasing thrust. At about 400 ft agl, he perceived an uncommanded control wheel input to the left, which he corrected. The landing was normal with a surface wind of 260° at 6 kt.
Investigation Findings
The flight data recorder showed that during the altitude exceedance, the VNAV mode changed from PATH to SPD, the control column moved forward, and pitch attitude reduced. Autopilot B disengaged at approximately 27,400 ft, and the thrust levers were retarded. The aircraft peaked at 27,450 ft before descending to 27,000 ft.
Investigation revealed that a tripped circuit breaker for the autopilot stabiliser trim actuator caused the failure of the aircraft to capture the selected altitude. No malfunction was found to explain the commander's difficulty in accurately controlling the aircraft in pitch. The recorded horizontal stabiliser trim data was invalid due to sensor or wiring problems, and lack of a discrete parameter replicating the 'STAB OUT OF TRIM' light limited the usefulness of the recorded data.
Both pilots confirmed they had not checked the circuit breakers following the altitude capture failure. The commander's flying experience was 3,395 hours total, with 2,575 on type; the first officer had 756 hours total, with 180 on type. Weather conditions were stable with no turbulence, and the ceiling was above 8,000 ft amsl. ATC provided full assistance throughout the emergency.