What happened
On December 3, 2013, a CH-77 Ranabot ultralight helicopter, registration OK-SHA-68, crashed in a field on the outskirts of Štěpánov, Czech Republic. The pilot was performing a VFR flight from LKBO with an intended landing at LKBE. Shortly after takeoff, witnesses reported hearing a loud bang, followed by the helicopter descending rapidly.
During the flight over the village, the aircraft experienced a sudden and catastrophic change in the main rotor blade pitch. This caused the rotor disc to tilt out of its normal plane of rotation, leading the rotor blades to strike the tail boom. The impact severely damaged the tail section, the directional control system, and the tail rotor drive system. The aircraft became uncontrollable, entered a spin, and crashed into the ground, where it was destroyed by a post-crash fire. The pilot sustained fatal injuries.
The investigation
The ÚZPLN investigation examined the wreckage and interviewed nine witnesses. Investigators focused on the mechanical integrity of the rotor control components. During the inspection, the investigation identified that a control rod for the number 1 rotor blade had failed. Subsequent conductivity testing of the broken rod revealed values that did not meet the required specifications, suggesting a potential issue with the thermal treatment of the material.
Findings
- The primary cause of the accident was a critical change in the main rotor blade pitch settings, which led to the rotor disc tilting and the blades striking the tail boom.
- The failure of the number 1 rotor blade control rod caused the loss of control integrity.
- The impact with the tail boom resulted in the destruction of the tail boom, the tail rotor gearbox flange, and the drive shaft.
- The pilot was unable to prevent the uncontrolled descent due to the total loss of aircraft controllability.
- While the exact moment of the rod's initiation was not determined, the breakage was likely a result of the rod impacting the end of the rotor head control sleeve.