What happened
On 10 August 2022, a Bombardier CL-600-2B16 (604), registration D-AAAY, experienced an uncommanded and unarrested flap extension while climbing from Farnborough Airport. The aircraft, operating a commercial passenger flight with three crew and seven passengers, moved beyond its maximum flap extension speed without the system successfully halting the movement. There were no fatalities and no injuries during the event.
The investigation
The AAIB investigation focused on the failure of the system designed to arrest uncommanded flap movement. Testing on D-AAAY and several other Challenger 600 series aircraft revealed that the flap extension speed was significantly reduced during various flights. Specifically, on D-AAAY, the flaps had been extending at half speed for 64 previous flights without the crew being notified.
Laboratory examinations of the extend and retract relays used in the flap operating system showed significant physical damage. Using scanning electron microscopy and CT scans, investigators found that the D contacts—which switch power to the Brake Detector Unit (BDU) solenoids—exhibited evidence of welding, metal transfer, and erosion. In one inspected aircraft, the contacts were found to be welded together, which would cause the flaps to retract uncommanded if the arrest system were activated.
Findings
Testing of the BDU solenoids demonstrated that when the solenoids are de-energised, a transient voltage spike of up to 300V is generated. This unsuppressed back-electromotive force (back-EMF) is a characteristic of the inductive load in the system. Because the flap operating system lacks protection to suppress this spike, the resulting electrical arcing caused the degradation and eventual failure of the relay contacts.
Safety action
Following the investigation, the AAIB issued three Safety Recommendations:
- To Bombardier Aviation: To implement a modification protecting the D contacts from back-EMF arcing.
- To Bombardier Aviation: To introduce a life policy for the flap operating system relays.
- To Transport Canada: To reassess the safety case for the flap operating system on the Challenger 600 series to ensure it meets required safety standards.