Loss of Tail Rotor Effectiveness
During a hover near a hangar, the helicopter experienced an unanticipated right yaw. The pilot reported a shudder just before the yaw, which could have indicated a mechanical issue, but examination of the helicopter and maintenance documents revealed no anomalies. A wildlife strike or foreign object contact was considered but not supported by evidence. Inadvertent interference from the front seat passenger was deemed unlikely as the pilot did not feel resistance on the anti-torque pedals.
The helicopter was loaded above its maximum take-off weight (MTOW), but at the estimated density altitude, it likely had sufficient power for a hover in ground effect. The left turn was not rapid, and weather was calm. However, the helicopter was close to a hangar, where helicopter-building interference was known to occur. According to literature, the obstacle prevented main rotor downwash from escaping, causing air recirculation, which was strengthened by the high all-up weight. The side of the main rotor disc closest to the obstruction was more affected, and recirculation from obstacles can disturb airflow through the disc, leading to random movements and controllability difficulties.
As the pilot initiated a left turn, the tail rotor moved from a position away from the hangar, where recirculation was less, to a position closer to the hangar, where recirculation was greater. This increased the likelihood of disturbed airflow through the tail rotor. Disturbance to the tail rotor flow, to the point that anti-torque forces could not overcome it, likely caused the unanticipated right yaw. The pilot likened the shudder to turbulence or 'bad air', which could be explained by building interference.
Weight and Balance Error
While manually calculating weight and balance, the pilot made an error converting fuel from pounds to kilograms, resulting in a calculation that showed the helicopter was 27 kg under MTOW. The pilot then used a third-party app to verify the loading but selected a model that did not represent the helicopter (VH‑ZDI) and added an unverified correction factor. The app confirmed the loading was within limits. However, had the pilot selected the model the app developer stated more closely reflected VH‑ZDI, the app would have shown the center of gravity beyond the forward limit, even with the fuel conversion error. The actual weight and balance calculation by investigators established that the helicopter exceeded MTOW by about 15 kg. The pilot likely would not have proceeded with the flight if the error had been identified.
Pilot Actions
Once the right yaw initiated, the pilot's control inputs included both left and right pedals, which was not consistent with the recommended procedures for loss of tail rotor effectiveness, which require full and sustained left pedal input. The pilot ultimately reduced the throttle, but this did not prevent the helicopter from colliding with the terrain.
Investigation Findings
The investigation concluded that, with insufficient evidence for other causes, the unanticipated right yaw was likely due to loss of tail rotor effectiveness from helicopter-building interference, with the high all-up weight strengthening recirculation and the pilot's control inputs not following recommended procedures. The MTOW exceedance resulted from a calculation error and reliance on a non-authoritative third-party app.