The exact cause of an uncommanded flap movement on a Bombardier Challenger 604 departing London-area Farnborough Airport has not been fully resolved by the UK’s Air Accident Investigation Branch (AAIB). A field investigation report published on Thursday provides details on some latent failures in the No. 1 flap retract relay, which it said meant it did not arrest the malfunction with the flaps.
At the start of a charter flight to Malaga in Spain on Aug. 10, 2022, the German-registered Challenger (tail number D-AAY) experienced an uncommanded flap movement above the maximum flap extension speed, during which the flaps moved to their fully extended position. The aircraft returned to Farnborough with the flap extended, but subsequent testing on the ground could not determine the reasons for the malfunction. It was established that a latent failure in the No. 1 flap retract relay had occurred on at least the previous 64 flights.
According to the AAIB investigators, the failure of the relay resulted from damage to the D contacts, which provide electrical power to the flap brake detector units. They concluded that the damage was caused by electrical arcing resulting from an unpressurized back electromotive force generated when the brake detector units were de-energized to apply the flap brakes when the flaps reached their selected position.
In September 2022 and March 2023, AAIB published a pair of special bulletins with four safety recommendations issued in light of the incident. Bombardier issued five related service bulletins at the end of December 2022.
In its report, the agency summarizes progress in implementing these and indicates in a couple of cases that it does not consider the issues closed. It indicated that Bombardier is working to resolve these issues with Transport Canada, with various deadlines set during 2024. In March and April 2023, the manufacturer advised of three occasions in which uncommanded flap movement occurred on a second Challenger 604.
The report says that, after realizing the flaps had malfunctioned, the Challenger crew first requested clearance to land at London Gatwick Airport before deciding it would be more direct to return to Farnborough, where it landed safely. The crew chose not to declare an emergency when prompted by air traffic controllers. While AAIB said this call had no operational impact, it would not have been advisable had there been a higher density of traffic that day in the London terminal area.