Incident Overview
A pilot reported that while cruising at 8,500 feet mean sea level (msl) and 65 knots indicated airspeed (KIAS), the helicopter developed a "slight yaw oscillation." The pilot decided to perform a precautionary landing and initiated a descent. As the descent progressed, the oscillations increased to approximately one yaw per second.
Sequence of Events
Before reaching the selected landing site, the low-rotor RPM horn and light activated. When the pilot lowered the collective control to recover rotor speed, an engine and rotor overspeed indication of 110 percent appeared. The pilot then executed a 180-degree turn and autorotated from 800 feet above ground level (agl).
Damage and Post-Impact
The helicopter landed on rocky terrain surrounded by pinon and juniper trees. During the landing, the main rotor, tail rotor, and tail rotor blades struck the trees, resulting in the separation of the tail rotor blades and gearbox from the tail boom.
Manufacturer Findings
According to the helicopter manufacturer, nothing was found that would explain the oscillations described by the pilot or why the low-rotor RPM horn and light activated.