What happened
On May 14, 2017, a Bell 412EP helicopter, registered JA110Y, operated by the Yamanashi Prefectural Police Headquarters, was engaged in a search and rescue mission in Tabayama Village, Yamanashi Prefecture. The mission aimed to rescue a mountaineer who had been injured.
During the operation, the crew made several approaches to the site, a steep, V-shaped dry mountain stream. On the first approach, the crew successfully positioned the aircraft to allow a rescuer to descend via hoist. However, during a second approach, the helicopter flew at a low speed and a shallow angle to maintain a specific hovering altitude. This maneuver caused the rotor downwash to shift in direction and intensity. The resulting air pressure caused tree branches to break and triggered a rockfall from the steep banks. Falling debris struck the survivor and the ground rescue team, leading to one fatality and three minor injuries among the rescue personnel.
The investigation
The Japan Transport Safety Board (JTSB) examined the flight history, meteorological conditions, and the physical characteristics of the accident site. Investigators analyzed video footage from a rescuer's helmet camera and used Doppler effect calculations from audio recordings to determine the helicopter's approach speed and the behavior of the downwash. The investigation also looked into the geological stability of the mudstone and sandstone banks surrounding the stream and the specific flight path taken by the crew during the second approach.
Findings
- The accident site was a narrow, steep-sided V-shaped trough, which made the area susceptible to wind and air pressure changes.
- The helicopter's second approach involved a slow deceleration and a specific flight path that caused the downwash to flow directly into the valley, significantly changing the air velocity and direction on the ground.
- The intense downwash from the rotor wingtips reached the upper banks of the stream, providing the force necessary to break branches and dislodge rocks.
- The downwash-induced rockfall and falling branches were the direct cause of the injuries and death.
Safety action
Following the investigation, the Yamanashi Prefutural Police implemented several safety measures, including:
- Strengthening safety management systems for rescue sites.
- Increasing organizational awareness regarding the specific risks of helicopter and mountain SAR operations.
- Improving information-sharing protocols and communication systems between ground teams and aircrews.
- Developing better methods for assessing ground situations and judging the safety of low-altitude hovering.