What happened
On 11 November 2005, a Bombardier CL600-2B16 Challenger 604, registration VP-BJM, was operating a flight from Lagos, Nigeria, toward London. Approximately four and a half hours into the journey, the aircraft's autopilot pitch trim failed, followed by a failure of the stabiliser trim system itself. During the incident, the stabiliser began to move into a nose-down position despite the crew's attempts to apply nose-up trim commands. The trim eventually reached a position of almost full nose down.
To prevent the aircraft from pitching down uncontrollably, both members of the flight crew were forced to maintain continuous aft pressure on the control column. Due to the significant physical effort required to maintain level flight, the commander decided to divert to London Heathrow. Although the Quick Reference Handbook (QRH) specified a 20° flap setting for a stabiliser trim failure, the crew opted to land with the flaps retracted to avoid the increased control column loads that would have resulted from deploying them.
The investigation
The AAIB investigation focused on the failure of the stabiliser trim channels. Investigators established that while the crew attempted to re-engage the trim channels, channel 2 showed no response to commands, and channel 1 functioned only intermittently. The investigation examined the Horizontal Stabiliser Trim Control Unit (HSTCU) and identified that electrical shorting had occurred within the unit.
Findings
- The investigation determined that the runaway trim was caused by electrical shorting within the HSTCU due to moisture contamination.
- This moisture likely resulted from humid air condensing on the cooling motherboard during the long-duration flight at altitude.
- The contamination caused a common cause failure because the two trim control channels were located in close physical and functional proximity within the unit.
- The HSTCU design lacked sufficient protection against environmental moisture ingress.
- There was no independent mechanical backup system to provide separation from the failed electronic channels.
- The investigation noted that the manufacturer had been aware of previous similar occurrences involving HSTCUs.
Safety action
- A safety recommendation was issued to Transport Canada to ensure Bombardier Aerospace implements measures to eliminate the risk of contamination affecting the horizontal stabiliser trim control system on the Challenger 604 and similar aircraft.
- A recommendation was also made to EASA, in coordination with the FAA and Transport Canada, to update airworthiness requirements regarding the design and installation of electronic components to better protect against moisture and fluid contamination as a source of common cause failures.