What happened
While attempting to land at a confined area, the pilot executed an approach that required flying over a row of trees and powerlines. This maneuver necessitated operating the helicopter out of ground effect, utilizing a high-power setting and maintaining a slow forward airspeed. During this approach, the aircraft experienced an unanticipated right yaw with accelerating rotation, eventually resulting in a collision with powerlines and the ground.
The investigation
Investigators analyzed weather observations from Gosford, which indicated a north-east wind at 10 knots. This specific wind direction and speed were identified as conditions conducive to the onset of loss of tail rotor effectiveness (LTE) through both main rotor disc vortex interference and tail rotor vortex ring state. Video evidence confirmed the presence of an accelerating right yaw during the sequence.
While the pilot's initial actions appeared consistent with standard recovery techniques, the investigation noted that applying 100% throttle—intended to maximize anti-torque thrust—also maintained the torque responsible for inducing the yaw. Additionally, the pilot's attempt to avoid obstacles by raising the collective likely over-pitched the main rotor blades. This action, combined with the activation of the low rotor RPM horn, led to a reduction in both main and tail rotor speeds, decreasing available anti-torque thrust and increasing the aircraft's rate of descent.
Findings
- The approach track and wind conditions created a high risk for loss of tail rotor effectiveness.
- The presence of terrain, obstacles, and people in the undershoot area significantly restricted the pilot's available forced landing options.
- It could not be definitively determined if the aircraft was recoverable from the moment the uncommanded yaw began, or if the collision was unavoidable given the limited altitude and obstacles.