Air Accidents Investigation Branch
In the climb, after departing Farnborough Airport, D-AAAY had 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 flaps extended where it landed without further incident.
An uncommanded and unarrested flap movement requires the flaps to move without movement of the flap lever and then for a failure in the flap arrest system to stop this movement. The flap surfaces are moved by two drive motors that are commanded by the sequencing of four extend and retract relays. These four relays also form part of the system to arrest an uncommanded flap movement.
The reason for the uncommanded movement of the flaps during the flight, and later during fault finding on the ground, could not be determined.
It was established that there had been a latent failure in the No 1 flap retract relay for at least the previous 64 flights, which caused the flaps to retract at half their normal retraction speed and prevent the arrest of an uncommanded flap movement. The failure of the relay resulted from damage to the D contacts which provide electrical power to the flap Brake Detector Units. This damage was caused by electrical arcing resulting from an unsuppressed back-EMF generated when the Brake Detector Units were de-energised to apply the flapbrakes when the flaps reached their selected position. The AAIB published two Special Bulletins in which four Safety Recommendations were made: S2/2022 on 22 September 2022, and S1/2023 on 2 March 2023. A number of Safety Actions have been taken by Transport Canada and Bombardier Aviation, and additionalaction is planned in response to the recommendations.