What happened
The pilot was conducting an agricultural trimming or swath application flight when the incident occurred. At the time of the event, the helicopter was operating at an altitude of approximately 15 feet above ground level (AGL) with a indicated airspeed of 15 miles per hour. During this low-level operation, the pilot reported hearing an unusual noise originating from the aircraft.
Immediately following the auditory warning, the helicopter experienced a sudden and violent yawing motion to the right, rotating approximately 180 degrees. The loss of directional control led to a crash landing. Upon impact, the airframe sustained substantial damage, specifically affecting the tail boom structure and the tail rotor blades. Additionally, the main landing gear was forced into an extended or splayed position due to the force of the ground contact.
The investigation
A post-accident examination of the helicopter was conducted to determine the root cause of the mechanical failure. Inspectors focused on the engine drive system, specifically targeting the torsion coupling responsible for transmitting power to the tail rotor assembly.
The inspection revealed a complete structural failure of the engine drive torsion coupling, identified by part number 21046-5. The failure originated at the interlocking steel and rubber assembly within the coupling unit. This component is critical for managing torque transmission while dampening vibrations; its catastrophic separation effectively disconnected the power source from the tail rotor, leading to the loss of anti-torque control and the subsequent uncommanded yaw.
Findings
The sequence of events indicates that a mechanical failure of the engine drive torsion coupling was the primary factor in this accident. The failure of the interlocking steel and rubber assembly within part 21046-5 resulted in an immediate loss of tail rotor effectiveness. This mechanical disconnect caused the violent yawing motion observed by the pilot, which made controlled flight impossible at the low altitude and speed at which the helicopter was operating.
The crash landing resulted in significant damage to the tail boom, tail rotor blades, and landing gear. No injuries were reported in the provided source data, but the aircraft was rendered inoperable due to the extent of the structural and mechanical damage sustained during the impact.