What happened
On March 1, 2016, an Aèrospatiale AS332L1, registered JA9678, operated by AERO ASAHI Corporation, was performing external cargo sling operations near Mihama Town, Fukui Prefecture. The mission involved transporting materials for the reconstruction of transmission towers using a 10-meter sling rope.
During the 13th transport cycle, ground workers were preparing the cargo net. Due to time constraints, the ground worker did not sufficiently remove twists from the wire rope and failed to verify the lock position of the hook keeper. While the crew initially observed no anomalies during takeoff and climb, the incident occurred as the helicopter accelerated to gain altitude at approximately 600 to 700 feet.
The pilot felt a light shock accompanied by a short noise. Upon inspection, the crew discovered that one of the suspended cargoes, weighing approximately 800 kg, had fallen into a mountain forest. The cargo landed in an area where no injuries or ground damage occurred.
The investigation
The JTSB investigation focused on the mechanical state of the hook and the procedures used during cargo attachment. Investigators examined the hook's locking mechanism, specifically the keeper and the pushrod. A teardown of the hook revealed a scar on the inside of the keeper caused by contact with a sharp edge.
Technical analysis showed that the wire rope was improperly positioned, straddling the keeper rather than the load beam. Furthermore, the investigation found that the aircraft's cockpit indicators showed the keepers were locked, even though the physical lock had failed. This occurred because the pushrod had entered a gap created between the keeper and the toggle, triggering the micro-switch and providing a false positive signal to the pilot.
Findings
- The primary cause of the incident was the opening of the hook keeper during flight.
- The keeper remained unlocked because the ground worker did not apply sufficient load to secure the lock, and the task was rushed due to the simultaneous preparation of multiple cargo nets.
- The improper placement of the wire rope (improper roping) created a horizontal force that pushed the keeper aside.
- The aircraft's monitoring system failed to alert the crew because the pushrod moved into a gap in the mechanism, activating the 'locked' indicator despite the lack of a secure connection.
- The company's operational manual lacked specific procedures for verifying the lock indicator position of the hook keeper.