What happened
The commercial-rated student pilot was conducting flight training exercises with a certified flight instructor (CFI) monitoring from the right seat. The specific maneuver being practiced was power recovery autorotations, a critical skill for helicopter pilots to regain engine power during an emergency descent.
During the execution of this maneuver, the rotor RPM was permitted to deteriorate significantly below safe operating limits. This loss of rotational energy resulted in a hard landing impact upon touchdown. The force of the impact caused the main rotor blades to flex downward excessively. This structural flexing transferred enough stress to the airframe to sever the tail boom from the helicopter's fuselage.
The investigation
Investigators noted that the accident occurred under challenging environmental conditions. At the time of the incident, the density altitude was recorded at 11,300 feet. High density altitude reduces engine performance and rotor efficiency, making precise control of rotor RPM even more critical during autorotation maneuvers.
The mechanical failure was directly linked to the flight dynamics of the hard landing rather than a pre-existing mechanical defect. The downward flexing of the main rotor blades due to the impact force was the immediate cause of the structural separation.
Findings
The primary factor contributing to this accident was the pilot's failure to maintain adequate rotor RPM during the power recovery autorotation practice. By allowing the rotor speed to deteriorate, the pilot lost the necessary energy to cushion the landing effectively. This resulted in a hard landing that exceeded the structural limits of the helicopter's airframe.
The high density altitude likely exacerbated the difficulty of maintaining rotor RPM, as the thinner air provides less lift and requires more precise throttle management. However, the root cause remains the pilot's inability to control the rotor speed during the maneuver.