What happened
On 28 February 2020, a Beech 2/PT6A-41 (registration G-FSEU) was performing a commercial air transport flight from Doncaster-Sheffield Airport to Wick Airport. During the climb through 8,800 ft, the crew detected an electrical burning smell in the flight deck, which was followed by the appearance of grey smoke within the cabin. The commander declared a MAYDAY and requested an ILS approach to Runway 20.
While the co-pilot searched for the source of the smoke, the commander attempted to deploy the flaps for the approach. However, the flaps remained in the retracted position and failed to move. The aircraft landed safely at Doncaster-Sheffield Airport, where the airport Rescue and Fire Fighting Service met the aircraft. Using thermal imaging, responders identified a hotspot beneath the cabin floor near the flap motor location, though no active fire was present. There were no injuries to the two crew members on board.
The investigation
Investigators examined the aircraft's flap control system, which utilizes an electric motor and gearbox to drive the flaps via flexible shafts and jackscrews. The system is designed to disconnect power via a limit switch once the flaps reach the desired position.
Post-incident inspections by maintenance engineers revealed heat damage and a burnt odor emanating from the flap motor. Furthermore, testing of the flap motor circuit breaker (CB) revealed that the component was operating intermittently.
Findings
- Wear within the flap cam and link-arm assembly prevented the flap up limit switch from triggering when the flaps were retracted.
- Because the switch failed to operate, electrical power was continuously supplied to the motor even after the flaps reached the up position.
- The continuous electrical supply caused the motor to overheat, resulting in the smoke and smell observed by the crew.
- The circuit breaker failed to trip immediately to protect the motor due to an intermittent fault in its operation.