Air Accidents Investigation Branch
Introduction
This Special Bulletin provides an update on the progress of the investigation into the uncommanded and unarrested flap extension above the maximum flaps extension speed that occurred on a Bombardier Challenger 604 aircraft, registration D-AAAY, on 10 August 2022. It follows publication of an earlier Special Bulletin, which provided preliminary information on the event and included a description of the flap operating system. The investigation established that a failure in the System 1 retract relay prevented the system from arresting the uncommanded flaps extension. This failure also caused the flaps to retract at half speed during the previous 64 flights recorded on the FDR, without the pilots being aware. A failure of the retract or extend relays on either motor channel would have a similar effect on the flap speed.
Following this serious incident, the aircraft manufacturer issued an Advisory Wire on 26September 2022 to advise operators of this event, and on 29 December 2022 issued fiveService Bulletins (SB) for operators to check the flap system on the Challenger600 series of aircraft. On 10 February 2023, Transport Canada issued an Airworthiness Directive requiring the initial operational test detailed in the SB to be carried out within 100flight hours or 15 months.
Summary
While actioning the SB, the operator of D-AAAY identified two further aircraft where the flaps had been operating at half speed over a number of flights. The investigation has established that 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 (BDU) was de-energised. The four flap extend and retract relays form part of the system to arrest an uncommanded flap movement.
Two Safety Recommendations have been made in this Special Bulletin to the Manufacturer to introduce a life policy for the relays, and a modification to protect the contacts from damage caused by the back-EMF. A third Safety Recommendation is made to the Regulator to reassess the safety case for the flap operating system. A Safety Recommendation had previously been made to the Manufacturer on 19 September 2022 to inform operators of the actions to take in the event of an uncommanded flap operation in flight.