What happened
On August 23, 2013, a private Bell 47G2 helicopter, registration SE-HEO, was conducting a flight from Storheden to Fällfors, Sweden. The purpose of the mission was to visit a local trade fair and take photographs. Prior to the final approach, the pilot landed on a nearby meadow to remove the right-hand door to facilitate better photography from the air.
After completing the photography session, the pilot returned to the same meadow to land and reassemble the door. During the final phase of the landing, the helicopter entered an uncontrolled state and struck the ground with a high sink rate. The impact caused significant deformation of the airframe and ignited a fire. The pilot and one passenger sustained serious injuries but were able to exit the wreckage before the fire was extinguished.
The investigation
The Swedish Accident Investigation Authority (SHK) conducted a technical examination of the wreckage. Investigators found no evidence of mechanical failure, engine malfunction, or issues with the flight controls. The engine was found to have been delivering power at the time of impact, and the transmission and tail rotor components remained relatively intact.
Technical analysis of the rotor blades revealed that the blades had been bent upward and forward, a phenomenon known as coning. This indicated that the rotor speed had dropped while the angle of attack remained high. The investigation also noted that the high engine power at impact likely caused the engine to twist from its mounts due to the sudden deceleration of the rotor mass.
Findings
- The primary cause of the accident was that the pilot lost control of the helicopter during the final stage of landing for unknown reasons.
- The investigation suggests the pilot may have underestimated the complexity of the landing, specifically the need for precise coordination between engine power and rotor speed.
- On this specific model, the pilot must manually increase engine power when increasing the blade angle of attack (such as during an approach) to prevent a drop in rotor RPM.
- Evidence suggests the helicopter experienced a coning rotor state, where a high angle of attack combined with low rotor speed led to a loss of lift and maneuverability.
- The pilot's planning for the landing may have been insufficient for the manual control requirements of the Bell 47G2 during a low-speed approach.