What happened
On 10 August 2022, a Bombardier CL-600-2B16 (registration D-AAAY) was climbing through FL190 following a departure from Farnborough Airport. While the aircraft was traveling at approximately 300 KIAS, the crew observed a 'flaps fail' caution on the EICAS display. The aircraft began to pitch nose-up and decelerate as the flaps began to extend without any input from the cockpit control lever.
Crucially, the movement was not stopped by the aircraft's automatic arrest system. The flaps continued to move until they reached the full 45-degree position, even though the aircraft was well above the maximum operating speed for that setting. The crew took control, reduced thrust, and initiated a descent. After deciding to divert back to Farnborough, the crew stabilized the aircraft at a lower speed and landed without further incident. There were no fatalities or injuries among the three crew members or seven passengers on board.
The investigation
An AAIB field investigation examined the aircraft's flap operating system, including the motors, relays, and Brake Detector Units (BDUs). Investigators also reviewed flight data from the incident aircraft and several other Challenger 604 aircraft.
Testing revealed that while the cause of the initial uncommanded movement could not be definitively identified, the failure of the arrest system was linked to a latent defect in the No 1 flap retract relay. This relay had been malfunctioning for at least 64 previous flights, causing the flaps to retract at only half the normal speed. Furthermore, electrical testing showed that the relay contacts had suffered damage due to electrical arcing.
Findings
Investigators found that the relay contacts had been degraded by electrical arcing. This was attributed to the fact that the-relay contacts were subject to voltage spikes when the circuit was interrupted. The investigation also identified that the BDM's electrical system was prone to such spikes. The failure of the relay to function correctly prevented the system from properly managing the electrical load, contributing to the degradation of the contacts.