What happened
On August 16, 2016, at approximately 13:32 UTC, a Robinson R44 Raven I, registration OK-GES, crashed in a field near Plasy Airport. The flight was being conducted by an instructor and a pilot undergoing type training, specifically practicing autorotation procedures.
During the final circuit, the instructor simulated an engine failure by reducing power. In the ensuing transition to autorotative flight, the pilot performed a series of rapid control inputs. While attempting to manage airspeed and rotor RPM, the pilot applied an aggressive cyclic input and subsequently an inadequate collective input. This sequence caused the rotor RPM to surge above 108% and then rapidly decay below the critical 90% threshold. The resulting loss of lift and instability caused one of the main rotor blades to strike the helicopter cabin.
The impact of the rotor blade into the cabin caused the airframe to break apart, separating the cockpit from the fuselage. The structural failure was accompanied by the loss of the tail boom. The aircraft entered an uncontrollable descent, impacting the ground on its left side. Both the pilot and the instructor sustained fatal injuries.
The investigation
An investigation by the ÚZPLN examined the flight sequence, witness statements, and the wreckage. Witnesses near the village of Rybnice reported hearing loud bangs and seeing the aircraft disintegrating in mid-air, with parts of the tail and cabin falling separately.
Technical examinations of the Robinson R44 Raven I confirmed that the engine and mechanical components were fully functional prior to the accident. The aircraft was within its permitted weight and center of gravity limits. The investigation established that the structural destruction, including the broken rotor blade and severed tail boom, occurred in flight due to the rotor strike and subsequent aerodynamic forces, rather than due to any pre-existing mechanical failure.
Findings
- The primary cause of the accident was inadequate control inputs during the transition to a simulated autorotation.
- The pilot's rapid and aggressive cyclic input led to an excessive increase in rotor RPM.
- An improper collective input in response to the high RPM caused the rotor speed to drop critically below 90%, resulting in a sudden loss of lift.
- The pilot's attempt to correct the situation with further cyclic input likely contributed to the rotor disk tilting into the cabin structure.
- The instructor's oversight and lack of immediate corrective action during the pilot's error prevented the prevention of the critical state.
- A lack of thorough pre-flight briefing regarding the specific maneuvers of the day may have contributed to the lack of preparation for the sudden sequence of events.