What happened
On October 2, 2015, an Aérospateniale AS332L1 helicopter, registered JA9678, was performing external cargo sling operations near Itoigawa City, Niigata Prefecture. Operated by Aero Asahi Corporation, the aircraft was tasked with transporting ready-mixed concrete to a construction site for a new transmission tower.
During the second transport flight of the day, the crew successfully delivered a bucket of concrete to the unloading point. While returning to the cargo sling point, the operator noticed that the yellow cargo hook was locked. In an attempt to reduce the workload for ground personnel during future rotations, the operator decided to release the lock on the yellow hook while still in flight. However, the operator failed to visually verify which hook was currently loaded and did not specifically communicate the intended hook to the captain. Consequently, the operator accidentally triggered the release mechanism for the red hook, which was still holding the empty bucket. This caused the 210 kg bucket to detach and fall into a forested area below. No injuries or damage to property on the ground were reported.
The investigation
The Japan Transport Safety Board (JTSB) examined the flight history, the mechanical functionality of the cargo hooks, and the operational procedures of the crew. The investigation focused on the specialized switch box used to control the red and yellow hooks from the cabin. Investigators reviewed the crew's statements, the technical specifications of the hook locking mechanism, and the company's existing training manuals and operational guidelines.