Incident Overview
A helicopter experienced a main rotor rpm decay during a 180-degree practice autorotation, resulting in the tail boom colliding with a boundary fence short of the intended landing spot. The event occurred when both the certified flight instructor (CFI) and the student pilot noticed the main rotor rpm had decayed as the helicopter rolled out of the 180-degree turn onto final approach.
Sequence of Events
The student increased the throttle in an attempt to go around, and the instructor took control of the helicopter. At that point, the instructor observed the main rotor rpm was at 92 percent and the helicopter was descending. Despite the instructor's efforts, the descent rate could not be stopped, and the helicopter struck the fence short of the intended touchdown point, subsequently impacting the terrain.
Aircraft and Environmental Factors
Both the instructor and student stated that the helicopter and engine had no mechanical failures or malfunctions during the flight. The density altitude was calculated to be approximately 5,590 feet.