What happened
On October 10, 2024, at approximately 15:15 JST, a Bell 412EP helicopter, registration JA6412, operated by Shin Nihon Helicopter Co., Ltd., experienced an unintentional release of cargo while flying over Joetsu City, Niigata Prefecture. The aircraft was performing a construction support mission, transporting ready-mixed concrete in a bucket suspended from a 5-meter sling rope.
The flight involved a crew of three, including a captain, a pilot in training, and an onboard mechanic. While the helicopter was climbing and executing a right turn toward an unloading site, the contents of the bucket were lost. The onboard mechanic, monitoring the load through the rear door, noticed the concrete was missing and alerted the crew. Upon inspection via an external mirror, the captain confirmed the bucket was empty. The crew subsequently searched the flight path for the fallen material but was unable to locate it. No injuries or damage to property on the ground were reported.
The investigation
The Japan Transport Safety Board (JTSB) examined the aircraft, the flight data recorder, and the specialized bucket used for the operation. The investigation focused on the mechanical integrity of the bucket's bottom plate and the maintenance of its locking mechanism.
Investigators found that the bucket's bottom plate was held closed by an over-centering mechanism involving links and a soft rubber seal. While the bucket had undergone maintenance in September 2024, it had been used approximately 100 times since then without further adjustment. Post-incident inspection of the bucket revealed that the fulcrum of the links had recessed inward by approximately 9 mm, making the over-centering lock shallower than its original setting.
Findings
- The primary cause of the incident was the unintentional opening of the bucket's bottom plate during flight.
- As the bucket's service life increased, wear on the soft rubber components created play in the links, causing the locking fulcrum to become too shallow to remain effective.
- During the right turn, centrifugal force increased the pressure on the bottom plate, causing it to overcome the weakened lock and swing open.
- The bucket design relied solely on the over-centering mechanism for stability, lacking a secondary physical locking device to prevent accidental opening.
Safety action
Following the incident, the operator implemented several safety improvements:
- The company revised its procedures to install a physical locking arm on the buckets, which must be verified as securely engaged before concrete is loaded.
- New inspection protocols were established to ensure all buckets are checked for proper over-centering at the job site upon delivery.