The UK’s Aviation Accident Investigation Branch (AAIB) has issued a special bulletin containing a recommendation calling on Bombardier to inform 600-series Challenger operators of actions to take in the event of an uncommanded flap deployment in flight.
This recommendation stems from the AAIB’s ongoing investigation of an incident on August 10 in which a German-registered Challenger 604 experienced an uncommanded flap extension above the maximum flaps-extended speed. The event occurred while the jet was climbing after departing Farnborough Airport, where it returned and landed without further incident. Three crewmembers and seven passengers were aboard the intended charter flight.
Testing identified that one of the two motor-retract relays was not working properly. The fault with the No. 1 retract relay meant the No. 1 motor continued to operate and the wingtip brakes did not engage. Consequently, the flaps continued to extend uncommanded at half speed until they reached the limit stop. Further, the uncommanded flap movement protection system did not work.
The aircraft’s master caution display warned the pilots of an unspecified flaps failure, but there is no annunciation for a failed relay. During the aircraft’s 64 previous flights (154 flight hours) recorded on the flight data recorder since July 4, flap retraction occurred at half-normal speed.
According to the AAIB, it is possible “that other aircraft in the fleet may be operating with a similar latent failure that could render the uncommanded flap movement protection system ineffective.” Bombardier is expected to issue an advisory wire addressing the issue next month.