What happened
On March 30, 2021, an ultralight helicopter, a CH 77 Ranabot with registration OK-WHA 84, was conducting a recreational flight near Dobřichovice, Czech Republic. Earlier in the day, the pilot had performed a precautionary landing in a field due to a fuel warning and had since updated the aircraft's electronic flight information system.
Shortly after taking off for a subsequent flight, the aircraft reached an altitude of approximately 100 meters. At a speed of 65 knots, the left cabin door, which had not been properly closed, swung open due to aerodynamic forces. In an attempt to close the door, the pilot released the collective pitch lever and leaned forward across the passenger. This movement caused the collective lever to move to a low position, significantly reducing engine power and inducing an uncontrolled autorotation. The aircraft entered a steep descent with a rapid roll around its longitudinal axis, impacting the ground at a sharp angle. The impact caused the main rotor blades to separate from the rotor head, and the wreckage was destroyed. Both the pilot and the passenger sustained fatal injuries.
The investigation
The ÚZPLN examined the wreckage, engine control unit data, and flight logs from the Flybox Mini EIS and Garmin 760. The investigation focused on the mechanical state of the aircraft, the pilot's actions, and the aircraft's weight and balance. Investigators found no evidence of mechanical failure or material fatigue in the rotor system or engine. However, the investigation revealed that the left door was not properly closed, and the right door was closed but not secured. Additionally, the aircraft was operating with a takeoff weight approximately 50 kg above its maximum allowable limit.
Findings
- The primary cause of the accident was the inadequate response to the unexpected opening of the left cabin door.
- The pilot's attempt to close the door led to the accidental reduction of collective pitch, triggering an uncontrolled autorotation.
- The aircraft was significantly overweight, exceeding the maximum takeoff weight by approximately 50 kg.
- The pilot failed to perform several essential pre-flight checks, including verifying the secure closure of the doors and updating the fuel quantity in the flight system.
- The aircraft was flying without valid legal insurance and without an active secondary surveillance mode (transponder).
- The pilot had limited experience with this specific aircraft type.