What happened
On October 20, 2018, a Bell 412EP helicopter, registration JA003W, operated by Nishi Nippon Airlines Co., Ltd., was performing a series of ten round flights to transport fresh concrete for power grid construction in Otoyo Town, Kochi Prefecture. The mission involved using an external bucket to move concrete between a temporary helipad and an unloading site approximately 1.9 km away.
During the tenth transport flight, the aircraft encountered rough air conditions over a valley, experiencing several severe vertical oscillations. As the aircraft approached the unloading site, the bucket operator noticed that the bottom shutter of the bucket had opened, resulting in the loss of approximately 600 kg of fresh concrete. The aircraft returned to the loading site without further incident, and no damage to the ground was reported.
The investigation
The JTSB investigation focused on the mechanical state of the bucket and the operational procedures used by the ground crew. Investigators discovered that a previously reported malfunction—where the cockpit controls could not operate the shutter—had forced the crew to use manual operation via a ground worker.
Technical analysis revealed that the actuator's signal wire had been cut, necessitating manual intervention. The investigation also examined the status of the bucket's locking mechanisms, specifically the over-center mechanism and the shift lever, which determines whether the shutter is in "auto" or "manual" mode. Furthermore, the investigators reviewed the company's operational regulations regarding the verification of the bucket's closed state before takeoff.
Findings
- The primary cause of the spill was the unintended opening of the bucket shutter caused by increased downward inertia during turbulent flight.
- The over-center locking mechanism failed to prevent the shutter from opening because it was not properly engaged.
- The shift lever was likely left in the "manual" position, meaning the actuator was not actively holding the shutter in a fixed closed position.
- Ground workers failed to confirm that the handle was fully closed and the shift lever was set to "auto" prior to the tenth flight.
- The company's existing regulations lacked a specific requirement to verify the mechanical lock and lever position during manual bucket operations.