What happened
On 11 November 2005, a Bombardier CL600-2B16 Challenger 604, registration VP-BJM, was operating a flight from Lagos, Nigeria, to Farnborough, UK. Approximately four and a half hours into the flight while at FL400, the crew received an autopilot pitch trim caution. Despite following procedures to attempt to re-engage the system, the stabiliser trim system subsequently failed.
During the descent, the stabiliser began to trim nose-down intermittently, eventually reaching an almost full nose-down position. This uncommanded movement could not be corrected through the trim switches. To prevent the aircraft from diving, both the pilot and co-pilot had to apply continuous, heavy aft pressure on the control column. Due to the extreme physical loads required to maintain level flight, the commander decided to land at London Heathrow with the flaps retracted, bypassing the standard procedure which would have required 20° of flap.
The investigation
The AAIB examined the Horizontal Stabiliser Trim Control Unit (HSTCU) to determine the cause of the malfunction. The investigation focused on the electrical and mechanical integrity of the trim channels and the environmental conditions during the long-haul flight.
Findings
- The investigation established that electrical shorting caused by moisture contamination within the HSTCU was the primary cause of the failure.
- The moisture likely resulted from humid air condensing on the unit's cooling motherboard during the flight at high altitude.
- The electrical shorting affected both control channels because they were located in close physical proximity within the same unit.
- The design of the HSTCU lacked sufficient physical separation between the channels and lacked a mechanical backup system to protect against such environmental contamination.
- The airworthiness standards at the time did not sufficiently account for moisture or fluid ingress as a potential source of common-cause failures in electronic components.