What happened
On 28 June 2002, a Hughes 36 and 9HS, registration G-DADS, was performing a private flight from Thruxton to Gloucestershire Airport. As the pilot approached the 'Heli South' position at the south eastern corner of the airfield, the aircraft was positioned into a 11-knot wind. The pilot established a hover at a height of approximately eight to ten feet while awaiting clearance to hover-taxi to a parking spot.
During this hover, the helicopter suddenly and violently yawed to the right. The pilot attempted to apply left pedal to counteract the rotation, but the control input had no effect, and the aircraft continued to rotate and climb slightly. Within approximately ten seconds of the initial yaw, the helicopter struck the ground heavily. The impact caused the right-hand skid to break, leading the main rotor blades to strike the earth. The aircraft came to rest tilted approximately 20 degrees to the right, with the tail rotor assembly and empennage having detached. Both the pilot and the passenger sustained minor injuries and were able to exit the aircraft through the front right door.
The investigation
Investigators from the AAIB examined the wreckage to determine if a loss of tail rotor control had occurred. The examination revealed that the vertical stabiliser had struck the ground first, causing it to buckle. This impact subsequently caused the tail rotor blades to hit the ground and led to the separation of the rear tail boom section, including the gearbox and empennage. The investigation also confirmed that the tail rotor drive shaft had suffered torsional failures due to the sudden stoppage of the assembly upon ground contact, though the shaft was intact prior to the impact.
Control systems, including the yaw pedals, rods, and bellcranks, were inspected and found to be fully functional with no evidence of pre-impact failure. The engine was also examined to rule out mechanical issues. While the engine shut down without pilot intervention, testing showed the engine was capable of normal operation. Investigators did find red plastic debris in the oil filter, likely from a discarded blanking cap, and noted the oil pressure regulator had been adjusted, but these factors were not considered relevant to the primary cause of the yaw.
Findings
- The primary cause of the accident was a high rate of yaw that exceeded the pilot's ability to control.
- The sudden rotation likely accelerated as the tail of the aircraft passed through the prevailing wind direction.
- The pilot experienced disorientation during the rapid, uncontrolled rotation.
- The physical damage to the tail boom and the detachment of the empennage were direct results of the vertical stabiliser striking the ground during the yaw event.