What happened
During a hover following lift-off, the pilot initiated a turn to the left. During this maneuver, the Robinson R44 (implied by context of VH-ZDI/R44 class) experienced an unexpected right yaw. The pilot noted a shuddering sensation, which they compared to turbulence, immediately before the aircraft drifted. Despite the pilot reducing throttle, the aircraft could not be prevented from striking the ground.
The investigation
The investigation examined several potential causes for the sudden yaw. Mechanical failure was ruled out as post-accident inspections of the aircraft and maintenance records showed no anomalies. There was no evidence of a wildlife strike or contact with foreign objects, as no animal remains or rotor damage were found. Additionally, interference from a passenger was dismissed because the pilot reported no resistance when using the anti-torque pedals.
Investigators focused on the proximity of the aircraft to a nearby hangar. It was determined that the high all-up weight of the aircraft intensified the recirculation of air caused by the building. As the pilot turned left, the tail of the helicopter moved closer to the hangar, placing the tail rotor in a zone of disturbed airflow. This phenomenon, a form of helicopter-building interference, led to a loss of tail rotor effectiveness.
Furthermore, the investigation identified a significant error in the weight and balance calculations. The pilot had incorrectly converted fuel weight from pounds to kilograms, leading to an underestimation of the total weight. While using a third-party application for verification, the pilot applied an unverified correction factor to a model that did not accurately represent VH-ZDI. The actual weight of the aircraft exceeded the maximum take-off weight by approximately 15kg.
Findings
- The primary cause of the yaw was the disruption of airflow through the tail rotor due to interference from the hangar.
- The aircraft was operating above its maximum take-off weight due to a manual conversion error and improper use of a third-party calculation app.
- The pilot's control inputs during the yaw—using both left and right pedals—did not align with recommended procedures, which advise full and sustained left pedal input to recover from such an event.
- The use of a pilot helmet and multi-point restraints helped mitigate the risk of injury during the impact.