Incident Summary
A helicopter experienced a rollover during autorotation training. The accident occurred when the rotor RPM decayed during the flare and could not be recovered, leading to a forced landing on a wet taxiway. The helicopter skidded across the surface, struck loose gravel and mud, and came to rest on its left side.
Sequence of Events
During the training exercise, the student pilot maintained 65 knots with rotor RPM in the green while performing a 180-degree autorotation. As the student rolled out of the turn at approximately 300 feet above ground level and 70 knots, rotor RPM remained in the green. However, prior to the flare, rotor RPM began to decay. When the flare was entered, rotor RPM failed to build. The flight instructor took control of the helicopter and immediately rolled on throttle to restore engine power and maintain rotor RPM, but insufficient engine power was available to recover the decaying rotor RPM. This was followed by a low rotor RPM warning light and horn. Recognizing that power was not available for a full power-on landing, the instructor initiated a full-down autorotation to a taxiway.
Damage and Findings
During the landing, the helicopter skidded across and off the wet taxiway into loose gravel and mud. The left skid dug into the ground, causing the aircraft to roll over and come to rest on its left side. The helicopter sustained substantial damage to its welded frames, firewall, and tail cone. The flight instructor reported no mechanical malfunctions or failures with the helicopter or flight controls that would have precluded normal operation.