Accident Overview
On March 15, 2025, at about 10:50 JST, a Eurocopter AS332L1 (registration JA332T) operated by Tohoku Air Service, Inc. was conducting cargo sling work at a construction site for a Tohoku Electric Power Network transmission tower in Kawasaki Town, Shibata County, Miyagi Prefecture. While the helicopter was descending to unload externally underslung cargo, a ground operator (signal person) lost balance due to the helicopter's downwash, fell approximately 17 meters off a cliff, and sustained injuries including a fracture of the transverse process of the lumbar vertebrae and multiple rib fractures.
Flight History
The helicopter took off from the operation site at about 09:57 with a pilot, a fellow passenger, and an onboard mechanic. After completing cargo sling work at another site, the helicopter approached the accident site from the south at about 10:48 to begin the first unloading of a cube-shaped underslung cargo weighing approximately 3 tons. The pilot hovered at about 19 meters above ground level. The work site was a cleared area on a ridge, measuring about 14 meters east–west and 8 meters north–south, surrounded by steep slopes.
Personnel and Roles
The pilot was 41 years old and held a commercial pilot certificate. The onboard mechanic had about 35 years of experience. The injured ground operator (signal person) was 41 years old with about 2 years of helicopter cargo transport experience. A chief ground operator, 32 years old with about 9 years of experience, was also present. Ground operations were subcontracted by the prime contractor to subcontracting companies.
Events Leading to the Injury
The signal person moved from the west side of the top of slope to uneven ground about 2 meters down the slope to be visible to the onboard mechanic. To reduce the effects of downwash, the signal person adopted a low posture, wore a helmet face guard, and faced away from the aircraft. While looking back, the signal person lost balance and fell. The onboard mechanic saw the signal person moving but lost sight after looking at the cargo, then recognized the fall and informed the pilot. The pilot lowered the cargo, hovered to assess, and suspended work; the helicopter landed at 10:54. Rescue and doctor helicopters transported the signal person to a hospital in Sendai City.
Work Site and Safety Management
The work site was narrow, and the uneven ground where the signal person positioned was a cliff with a risk of falling. The chief ground operator had identified “risk of slipping on the slope and falling, resulting in injury” during a pre-work risk assessment meeting using a TBM-KYR sheet. The risk assessment rated severity 3, probability 2, and frequency 2, deeming the work acceptable despite some issues.
Effects of Downwash
According to verification by the Tokyo Fire Department, in no-wind conditions, a horizontal distance of at least 50 meters from the rotor center is needed to avoid downwash effects for helicopters hovering below 150 feet. In natural wind, downwash blows leeward, requiring caution even beyond 50 meters. At the accident site, the helicopter hovered at about 19 meters AGL, and the entire work site (14×8 meters) was within this cautionary range. Multiple ground operators noted the site was narrower than usual.
Probable Cause
The Japan Transport Safety Board concluded that it is most likely that, in terms of signaling location for ground guidance, the signal person was so focused on moving into a position that could be easily visible to the onboard mechanic that the signal person selected a place at risk of falling off, which caused the signal person to lose balance due to the effects of the helicopter's downwash, resulting in the fall and injury.