What happened
On April 12, 2024, an Aerospatiale AS332L1 helicopter, registration JA6686, was performing cargo transport operations at a transmission tower construction site in Aoi Ward, Shizuoka City. The aircraft, operated by Shin Nihon Helicopter Co., Ltd., was delivering ready-mixed concrete using a hopper tank and a suspended bucket.
During an approach to the loading site, the helicopter flew at an altitude of approximately 15 meters above a work platform. The resulting rotor downwash caused a heavy steel formwork panel to lift from its position. The panel flipped over and struck a ground worker near the knee, causing one serious injury. The worker sustained a contusion and a laceration to the leg, requiring hospitalization.
The investigation
Investigators analyzed flight recorder data, including engine torque, pitch attitude, and radio altitude, comparing the accident approach to previous successful runs. The investigation also examined meteorological conditions and the physical properties of the displaced object. The panels involved were part of a larger assembly, with the specific white panel weighing approximately 140 kg.
Data revealed that during the approach leading to the accident, the helicopter maintained a higher engine torque and a shallower pitch attitude compared to previous flights. This configuration, combined with a descending flight path, likely concentrated the downward airflow. Furthermore, the investigation noted that the wind direction and the presence of a water tank near the panels may have created a Venturi effect, further accelerating the airflow near the ground.
Findings
- The primary cause was the generation of strong downwash near the curved formwork panel, which provided sufficient force to lift the heavy object.
- The intensified airflow was likely due to a combination of higher engine torque, a descending approach, and stable pitch attitude.
- The physical arrangement of the site, including the proximity of a water tank, may have contributed to localized wind acceleration.
- The injured worker did not immediately report the incident because there was no visible bleeding, and the crew was unaware of the injury as they continued operations for several more hours.
Safety action
Following the incident, the operator and the construction consortium implemented several preventive measures:
- Implementing stricter protocols to secure all materials, such as using wire ropes or nets, to prevent scattering.
- Enhancing pre-deployment training to instruct workers on immediate evacuation procedures if debris begins to move.
- Mandating that all injuries, regardless of perceived severity, must be reported immediately to site supervisors to ensure operational safety assessments can be conducted.