What happened
On 19 June 2011, a McDonnell Douglas Hughes 3/69E helicopter, registration G-KSWI, was performing a private flight from Cornwall to Draycot Airfield. While cruising at 2,200 ft near Glastonbury, the aircraft suddenly began a rapid descent. Witnesses observed the helicopter performing a series of tight orbits at low altitude before the aircraft began to spin clockwise and dropped heavily into a field. The impact was severe, resulting in the destruction of the aircraft and one serious injury to the pilot. There were no passengers on board.
The investigation
Investigators examined the wreckage and recovered flight data from the aircraft's GPS. The analysis of the tail rotor assembly revealed that the 'blue' tail rotor blade pitch horn had suffered a fatigue fracture. Metallurgical examination at QinetiQ identified two fatigue cracks originating from corrosion pits within the pocket of the pitch horn. The investigation also looked into the maintenance history of the tail rotor pitch links, noting that recent corrosion removal work had used abrasive paper, which may have compromised the protective surface finish and shot peening.
Findings
- The crash was caused by a mechanical failure in the tail rotor system, specifically the failure of the pitch horn on one tail rotor blade.
- This failure led to a loss of pitch control, which likely caused the associated pitch link to fail, resulting in insufficient thrust to counter the engine's torque.
- Corrosion-induced fatigue cracking was the primary mechanism of the pitch horn failure.
- The presence of corrosion pits and the removal of protective Alodine and shot peening layers through abrasive maintenance techniques likely contributed to the crack initiation.
- The pilot's ability to maintain directional control was compromised as the airspeed dropped during the attempt to land.