The UK Air Accidents Investigation Branch (AAIB) yesterday issued three safety recommendations related to an uncommanded and unarrested flap extension above the maximum flaps extension speed while Bombardier Challenger 604 D-AAAY was on climbout from Farnborough Airport on August 10. Two of the recommendations in the AAIB’s follow-on Special Bulletin are for life limits on the aircraft’s four flap extend and retract relays, while the third calls for the UK Civil Aviation Authority to “reassess the safety case for the flap operating system.”
According to the AAIB, a failure in the System 1 retract relay prevented the system from arresting the uncommanded flaps extension. This failure also caused the flaps on D-AAAY to retract at half speed during the previous 64 flights recorded on the flight data recorder—without the pilots being aware. Bombardier has since issued five service bulletins addressing this issue and Transport Canada published a related airworthiness directive on February 10.
The cause of the failure was damage to the D contacts in the flap extend relay, which resulted from an unsuppressed back-EMF generated when the flap brake detector unit was de-energized, the AAIB said. The four flap extend and retract relays form part of the system to arrest an uncommanded flap movement, it added.
The AAIB continues its investigation into the incident and will issue a final report “in due course.”