Incident Overview
The commercial pilot completed a landing to a hover and taxied to a point on the airport ramp to finish the flight mission. While at a stable hover and preparing to set the helicopter on the ground, the aircraft entered an uncommanded spin to the right. The copilot reported that the helicopter made three full 360-degree turns to the right before ground contact. Lateral forces from the spin caused the left main landing gear to collapse and all four main rotor blades to contact the ground. The helicopter remained upright, and the pilot, copilot, and all eight passengers were not injured.
Investigation Findings
On-scene examination by a Federal Aviation Administration (FAA) inspector revealed that the cowling covering the first section of the tail rotor drive shaft had not been secured. This cowling had contacted the drive shaft, severing it in two. The unsecured cowling allowed the shaft to be damaged during flight, leading to loss of tail rotor effectiveness and the uncommanded spin.
Maintenance and Preflight Issues
The helicopter had recently undergone maintenance, during which the cowling was left unsecured. The flight crew failed to notice the loose cowling during their preflight inspection prior to departure. This oversight contributed to the in-flight failure of the tail rotor drive shaft.