What happened
The incident occurred while the aircraft was engaged in an aerial application mission. During the operation, the pilot experienced a sudden mechanical anomaly characterized by an unexpected increase in engine revolutions per minute (RPM) coupled with a simultaneous decrease in main rotor RPM. Recognizing the loss of power transmission to the rotor system, the pilot initiated an emergency autorotation procedure.
The pilot successfully guided the aircraft toward an open field for an emergency landing. Upon touchdown, the kinetic energy and descent rate resulted in the main rotor blades striking the ground with sufficient force to sever the tail boom. The structural failure of the tail boom was a direct consequence of the impact during the forced landing.
The investigation
Following the accident, investigators conducted a detailed examination of the aircraft's propulsion and transmission systems. The primary focus was on the main rotor drive system to determine the cause of the sudden RPM discrepancy observed by the pilot.
Inspection of the engine compartment revealed that the torsion coupling within the main rotor drive system had separated. This mechanical failure effectively disconnected the engine power from the main rotor, explaining the drop in rotor RPM despite the continued operation of the engine. The separation of the torsion coupling was identified as the critical mechanical failure that precipitated the emergency landing sequence.
Findings
The investigation determined that the primary cause of the accident was the in-flight separation of the torsion coupling in the main rotor drive system. This failure led to a loss of power to the main rotor, necessitating an autorotation. The subsequent damage to the tail boom was caused by the impact forces during the emergency landing in the open field.
No further safety message was provided in the source documentation regarding this specific incident.