What happened
On March 15, 2025, at approximately 10:50 JST, a Eurocopter AS3/32L1, registration JA332T, was performing external cargo unloading operations in Kawasaki Town, Miyagi Prefecture. The aircraft, operated by Tohoku Air Service, Inc., was hovering at roughly 19 meters above ground level to deliver a 3-ton piece of construction equipment to a transmission tower site.
During the maneuver, a ground signal person attempted to move to a position that would ensure maximum visibility to the onboard mechanic. To avoid the intense wind pressure and dust from the rotor downwash, the operator adopted a low posture while moving across uneven terrain near the edge of a slope. While turning back toward the aircraft, the operator lost their footing on the unstable ground and fell approximately 17 meters down a cliff. The onboard mechanic alerted the pilot, and the aircraft landed safely to facilitate a rescue. The operator was transported to a hospital in Sendai with multiple rib fractures and a spinal injury.
The investigation
The JTSB investigation examined the site conditions, the training protocols provided by the operators, and the specific mechanics of the rotor downwash. Investigators noted that the work site was notably narrow and situated on a ridge where the terrain dropped off steeply on the western side.
Technical analysis of the downwash effects revealed that at a hovering altitude of 19 meters, the entire work area fell within a high-risk zone where rotor wash can cause individuals to lose balance. The investigation also reviewed the safety training materials provided by the helicopter operator, which emphasized the importance of being visible to the crew but lacked detailed instruction on the specific physical dangers of downwash or the necessity of selecting safe evacuation routes.
Findings
- The signal person prioritized visibility to the onboard mechanic over their own physical safety, leading them to select a signaling position on unstable ground near a cliff edge.
- The downwash from the Eurocopter AS332L1 was a primary factor in the operator losing their balance.
- Safety training materials focused heavily on the requirement for large, visible signals but provided insufficient guidance regarding the intensity of rotor-generated wind pressure.
- Risk management at the construction site was inadequate, as the identified risk of falling was noted during pre-work meetings but not sufficiently mitigated by site controls.