What happened
During a series of private, cost-shared flights operating from a cultivated field at Skogsfånget near Härnösand, a helicopter was involved in an accident during its second departure of the evening. After approximately 15 minutes of flight, the pilot attempted to land at the takeoff site but encountered wind disturbances. To avoid these conditions, the pilot decided to perform a go-around and target a different landing area adjacent to a house.
This secondary landing site presented significant challenges due to limited obstacle clearance, necessitating a steep descent on the final approach. As the aircraft progressed through the final, it was operating at a low airspeed with high power output, facing a relative wind from the front left. During this phase, the helicopter unexpectedly began to yaw to the right. The pilot attempted to correct the movement, but the control inputs were insufficient to stop an uncontrolled rightward rotation. In an effort to arrest the spin, the pilot moved the throttle to idle; while this stopped the rotation, it simultaneously caused the aircraft to lose altitude. The helicopter subsequently struck trees and impacted the ground with significant force. While the aircraft sustained substantial damage, there were no serious injuries.
The investigation
Investigators examined the flight parameters and the environmental conditions at the time of the accident. The inquiry focused on the aerodynamic state of the aircraft during the steep approach and the impact of wind direction on the tail rotor. The investigation confirmed that no technical malfunctions contributed to the crash.
Findings
Technical analysis established that the primary cause of the accident was a phenomenon known as unanticipated yaw, or loss of tail rotor effectiveness (LTE). The specific aerodynamic conditions—low airspeed, high power, the absence of ground effect, and a relative wind from the left—reduced the effectiveness of the tail rotor, leading to the uncontrolled rotation.
Additional contributing factors included:
- The pilot's insufficient knowledge regarding the risks associated with unanticipated yaw.
- The high level of difficulty presented by the chosen landing site, which had restricted obstacle clearance.