What happened
On February 12, 2015, at approximately 09:11 UTC, an experimental A 600 Talon helicopter, registration N111ZP, crashed into a field near the village of Bohušovice nad Ohří. The pilot was performing a routine VFR training flight.
During the flight, the aircraft experienced a sudden and catastrophic mechanical failure. Witnesses near the crash site reported hearing a loud bang and seeing a portion of the aircraft's tail assembly detach. The loss of the tail rotor caused an immediate and total loss of controllability. The aircraft entered a steep, uncontrolled descent, with the main rotor blades subsequently striking the tail boom. The helicopter struck the ground and was destroyed by a post-impact fire. The pilot sustained fatal injuries.
The investigation
The ÚZPLN investigation focused on the mechanical integrity of the tail boom and the control systems. Investigators examined the wreckage and interviewed several witnesses who observed the aircraft's final moments.
Technical analysis revealed that the aircraft, which was an amateur-built kit, was experiencing vibrations that the pilot had previously noted. Investigators found that the tail rotor control cable (bowden cable) was not properly secured to the internal structure of the tail boom. Furthermore, an electrical wire for a temperature sensor was improperly attached to this same control cable. The investigation also scrutinized the aircraft's construction logs, noting discrepancies between the owner's statements and the provided documentation regarding the aircraft's assembly history.
Findings
- The primary cause of the accident was the sudden loss of controllability caused by the tail rotor blade striking the tail boom.
- This strike was triggered by a failure in the tail rotor control system, where the unanchored control cable was caught by protruding bolt heads on the rotating transmission shaft.
- The tension from the caught cable caused the control cable to pull out of its end fitting, leading to an extreme change in the tail rotor blade pitch.
- High levels of vibration, likely caused by an improperly installed output shaft bearing, contributed to the cable moving into the path of the rotating components.
- The pilot's attempt to compensate for the sudden pitch change likely caused the main rotor blades to strike the tail boom, severing the tail boom at the horizontal stabilizer.