What happened
On 25 April 2015, a Dassault Falcon 20D, registration G-FRAK, was conducting a special operations flight for target towing over the English Channel. The aircraft was recovering a 53 kg target approximately 20 nm south of Portland Bill as part of a military exercise.
During the final stages of the recovery, as the cable length reached approximately 40 metres, the target suddenly accelerated toward the aircraft. The target struck the leading edge of the left wing, causing damage to the wing's droop leading edge and the winch carrier, before detaching and being lost at sea. Despite the impact, the crew of three sustained no injuries, and the aircraft was able to perform a safe, flapless landing at Bournemouth Airport.
The investigation
The AAIB investigation focused on the mechanics of the target winch and the sequence of events captured on the Target Tow Officer's (TTO) onboard video. The investigation established that while the crew followed standard recovery procedures, the target's speed increased significantly during the final 40 metres of the reel-in.
Engineers examined the Meggitt RM30/ASL winch system and performed continuity checks on the vent door components. Testing revealed that the closed limit switch for the vent doors exhibited high impedance and failed in an open circuit state during testing. This failure prevented the servo motor from closing the vent doors as commanded.
Findings
- The target accelerated toward the aircraft due to an overspeed of the target winch.
- The acceleration was caused by a faulty closed limit switch on the vent doors, which prevented the doors from closing.
- Because the doors could not close, the turbine spooled up, increasing torque and causing the winch to accelerate.
- The TTO was unable to intervene using the stop switch or cable cutter because the target was visible on the monitor for only approximately one second before impact.
- The crew had adhered to all standard operating procedures prior to the event.