What happened
On 11 November 2005, a Bombardier CL600-2B16 Challenger 604, registration VP-BJM, was operating a private flight from Lagos to Farnborough. While cruising at FL400, the crew observed an 'AUTO PILOT PITCH TRIM' caution. Shortly thereafter, 'STAB TRIM' and 'MAC TRIM' warnings appeared in the cockpit.
Attempts to re-engage the stabiliser trim were initially successful but were followed by further warnings. During the descent to FL320, the crew attempted to use manual nose-up trim switches on the yoke; however, these commands resulted in the horizontal stabiliser moving in a nose-down direction. The stabiliser eventually reached a position of -2.1º, which was near its full nose-down limit.
Due to the extreme physical effort required to maintain control and the significant nose-down pitching moment, the crew elected to perform a flapless approach and landing at London Heathrow Airport. The landing was completed successfully through the coordinated efforts of the flight crew and an off-duty pilot who assisted with the thrust levers.
The investigation
Investigators examined the aircraft's flight recorders, which captured the stabiliser position and trim command parameters. The flight data recorder showed that the 'Horizontal Stab Trim Actuator (HSTA) valid' parameter fluctuated before becoming invalid for the rest of the flight.
Upon inspection of the aircraft, the horizontal stabiliser trim system itself showed no faults during testing. However, a detailed examination of the Horizontal Stabiliser Trim Control Unit (HSTCU) by the manufacturer revealed heavy contamination on the printed-circuit board pins of the rear external connectors. Specifically, the lower connector was found to be heavily contaminated, causing several pins to short together.
Findings
- The investigation established that the uncommanded nose-down trim was caused by the shorting of pins on the HSTCU due to fluid contamination.
- The contamination affected pins responsible for both nose-down and nose-up commands, as well as the inputs and outputs for both independent trim channels.
- Because the inputs and outputs for both command channels converged at these contaminated pins, a single point of failure was created.
- The aircraft's stabiliser trim system is entirely 'fly by wire' with no mechanical backup.
- The operator had previously reported two other incidents involving the stabiliser trim system on VP-BJM.
Safety action
- The AAIB has issued Safety Recommendation 2005-147, advising the safety regulator to ensure that Bombardier Aerospace eliminates the risk of contamination affecting the horizontal stabiliser trim control system in the Challenger 604 and similar aircraft.