Incident Overview
On October 20, 2018, at approximately 13:22 Japan Standard Time, a Bell 412EP helicopter (registration JA003W) operated by Nishi Nippon Airlines Co., Ltd. was conducting an external cargo sling operation to transport fresh concrete for construction work near Otoyo Town, Nagaoka County, Kochi Prefecture. During the tenth transport flight, the helicopter dropped the fresh concrete from the bucket while flying over a mountain forest. There was no damage to the ground.
Flight History
The helicopter took off from a temporary helipad at about 12:20 to transport fresh concrete to an unloading site approximately 1.9 km away. Multiple round trips were planned using two buckets alternately. After the second transport, a malfunction occurred: the bucket's shutter would not open via cockpit controls. From the third transport onward, a ground worker manually operated the shutter using an open/close handle, in accordance with the company's regulations.
At around 13:20, the helicopter departed for the tenth transport. While flying over a valley, the helicopter experienced severe vertical shaking due to rough air conditions. As the helicopter approached the unloading site, the bucket operator in the left pilot seat observed that the shutter was open and the fresh concrete (about 600 kg) was missing. The helicopter returned to the temporary helipad without further action.
Bucket Mechanism and Locking
The bucket had two modes: "auto" (shutter operated from the cockpit) and "manual" (ground worker operates the handle). When the handle is fully closed, the shutter is locked by an over-center mechanism. An actuator can fix the shutter position when the shift lever is set to "auto" and connected. The company used both the actuator's fixing function and the over-center lock to prevent unintended opening.
Post-Incident Findings
After landing, a ground worker found the shutter about 80% open and closed it using the handle without moving the shift lever. Investigation revealed that the shift lever was likely in "manual" mode during the tenth transport, so the actuator's locking function was inactive. The over-center lock also had not engaged properly. Analysis indicated that the shutter opened due to the increased load from vertical shaking combined with the load of the concrete.
Probable Causes
The Japan Transport Safety Board concluded that it is highly probable that the fresh concrete dropped due to unintended opening of the shutter while the helicopter was flying with the loaded bucket. It is probable that the unintended opening was caused by increased load on the shutter when the helicopter was shaken by rough air conditions, while the over-center mechanism locking was not properly working.
Safety Actions Taken
The company implemented several safety measures:
- Added "Lock/Unlock" placards to the shift lever of all buckets.
- Revised transport operation regulations to include confirmation of over-center lock and shift lever position when using manual operation.
- Required suspension of work and additional meetings when conditions change from pre-work briefings.
- Incorporated past accident case studies into recurrent training.
- Recommended "pointing and calling" and vocal confirmation as basic safety actions.