What happened
On 10 August 2022, a Bombardier CL-600-2B16, registration D-AAAY, was performing a commercial passenger charter flight from Farnborough Airport to Málaga. While climbing through FL190 at approximately 300 KIAS, the flight crew observed a 'FLAPS FAIL' message on the EICAS display. Although the flap control lever remained in the flap 0 position, the crew noted the aircraft was pitching nose-up and decelerating as the flaps began to extend.
Because the flap lever had not been moved from the 0 position, the aural flap overspeed warning did not activate. The flaps continued to extend at roughly half their normal speed, reaching the 45-degree position while the aircraft was still traveling at 234 KIAS, which was 45 knots above the maximum permitted speed for that setting. The commander took control, reduced thrust, and initiated a descent to stabilize the aircraft. The crew eventually decided to divert back to Farnborough, where the aircraft landed safely without further incident.
The investigation
Investigators examined flight data from the FDR, which revealed that the flaps had been retracting at half-speed during the 64 flights preceding the incident. Technical inspections of the aircraft's flap system identified a malfunction in the No 1 motor retract relay. A CT scan of the relay revealed a contact anomaly.
It was determined that while the uncommanded movement protection system did detect the movement and trigger a 'FLAP FAIL' message, the faulty relay prevented the system from effectively arresting the extension. Specifically, the No 1 motor continued to operate, preventing the wing tip brakes from engaging. This allowed the flaps to continue extending until they reached their limit stop.
Findings
- The uncommanded flap extension was not arrested by the aircraft's protection system due to a failure in the No 1 motor retract relay.
- The flap overspeed warning failed to sound because the warning is triggered by movement of the flap lever rather than the actual position of the flaps.
- A contact anomaly in the No 1 motor retract relay caused the flaps to retract at half-speed, a condition that had been present for at least 154 flight hours.
- There is no cockpit alert to notify the crew if the flaps are operating at half-speed or if a relay is malfunctioning.
Safety action
- Bombardier is issuing an Advisory Wire to operators of the Challenger 600 series to communicate the circumstances of this event.
- The manufacturer is reviewing existing maintenance tasks to ensure operators can identify if flaps are operating at half-speed.
- A safety recommendation has been made for Bombardier to provide operators with specific instructions on how to manage an uncommanded flap operation in flight.