Incident
On 19 November 2022, the pilot of a Robinson Helicopter Company R44, registered VH-TKI, was conducting a private flight from a nearby property to a function centre at Forresters Beach, New South Wales. The helicopter carried two passengers. The intended landing site was the carpark of the venue. During the approach, the pilot reported an uncommanded yaw to the right that could not be recovered. The helicopter subsequently lost control, struck powerlines, and collided with terrain. The occupants received minor injuries, and the helicopter sustained substantial damage.
Investigation Findings
The Australian Transport Safety Bureau (ATSB) investigated the occurrence. The ATSB determined that during the approach to a confined area landing site, the helicopter experienced a loss of tail rotor effectiveness (LTE) and an accompanying right yaw. The pilot's response was ineffective at recovering control. However, given the helicopter's position during the approach to the confined area, it could not be established whether control could have been recovered before the collision with powerlines and terrain.
The pilot had planned to land in a confined area that required an approach over powerlines and a row of trees. This necessitated flying the helicopter out of ground effect with a high power setting and at slow forward airspeed. Recorded weather observations from Gosford indicated the wind was from the north-east at 10 knots. The approach track placed the wind from a direction and speed known to be conducive to LTE onset through both tail rotor vortex ring state and main rotor disc vortex interference. Video footage of the accident sequence showed a right yaw with accelerating rotation, consistent with the symptoms of an unanticipated yaw event.
The ATSB noted that flying out of ground effect at slow forward airspeed in proximity to obstacles placed the aircraft in a scenario that did not easily allow for recovery. It is likely that the pilot's initial actions were consistent with recommended recovery techniques. However, setting 100% throttle provided maximum power-on rotor RPM and ensured maximum anti-torque thrust, but it would also have maintained the torque inducing the yaw. Additionally, consistent with the pilot account of a low rotor RPM horn activation and an audible reduction in main rotor RPM present in the video footage, raising the collective to avoid obstacles during the attempted recovery probably over-pitched the main rotor blades. This decreased main and tail rotor speed, reducing available anti-torque thrust and increasing the rate of descent.
The ATSF concluded that, given the available height above obstacles and uncertainty about the actual control inputs, it could not be determined if application of the recommended recovery technique would have been effective or if a collision was unavoidable.
Safety Message
The ATSB highlighted that helicopter pilots should remain aware of factors that may induce unanticipated yaw, particularly relative wind direction, and manage their influence on the helicopter's anti-torque system by maintaining positive yaw control. Prompt and correct pilot response is essential if unanticipated yaw occurs, and maintaining recovery control inputs is the most effective way to stop the yaw, even if recovery is not immediate. This is especially important in confined area operations, where the physical characteristics of the landing site may limit available options. The FAA helicopter flying handbook states that a recovery path must always be planned, especially when terminating to an out-of-ground-effect hover, and executed immediately if an uncommanded yaw is evident. In this case, terrain, obstacles, and people in the undershoot limited the forced landing options available to the pilot.