Factual Information
On October 10, 2024, at approximately 15:15 JST, a Bell 412EP helicopter (registration JA6412) operated by Shin Nihon Helicopter Co., Ltd. was conducting external load operations near Sugawa, Yasuzuka Ward, Joetsu City, Niigata Prefecture. The helicopter was transporting ready-mixed concrete in a bucket suspended from a 5-meter sling rope and hook, as part of construction work for a ski resort lift. The flight crew consisted of a captain (age 48, acting as training instructor) in the left seat, a pilot (age 35, undergoing training) in the right seat, and an onboard mechanic (age 55) in the left rear seat. Two ground operators (ages 25 and 24) prepared the buckets on the ground.
During the 29th cargo transport of the day, the helicopter lifted the bucket from the loading area, made a right turn while climbing northeast, and headed toward the unloading area. At around 15:16, as the helicopter approached the unloading area, the onboard mechanic opened the left rear door to check the load and noticed that the concrete in the bucket was gone. The captain confirmed via an external mirror that the bucket was empty, and the helicopter returned to the operation site, landing at 15:28. After landing, the mechanic found that the bucket's bottom plate was slightly open.
The fallen concrete was not found, but there were no reports of damage to people or property on the ground. The flight route had been planned to avoid endangering ground objects per Civil Aeronautics Act permissions.
Bucket Mechanism and Inspection
The bucket used was designed with an over-centering locking mechanism intended to prevent unintentional opening. The bottom plate had a soft rubber seal to minimize gaps. According to the manufacturer's manual, wear and deterioration of the rubber over time can create play in the links, reducing the effectiveness of the over-center lock. The bucket involved had been used approximately 100 times since its last maintenance check on September 9, 2024, without any link adjustments. After the incident, examination at the company's hangar revealed that the fulcrum was recessed inward by about 9 mm, shallower than the initial adjustment of 10–15 mm. No abnormalities were found in the electrical system or the opening/closing operation itself.
Analysis
The Japan Transport Safety Board concluded that the most likely cause of the unintentional release was wear and deterioration of the soft rubber on the bottom plate over time, which generated play in the links and reduced the over-center lock's effectiveness. As the helicopter made a right turn, centrifugal force increased the load on the bottom plate, causing it to open. The flight data and crew statements indicated no abnormalities during the initial climb and turn. The onboard switches were guarded against accidental operation and were isolated from other electrical systems; no faults were found.