What happened
On October 8, 2015, an Aerospatiale AS332L, registered JA9660, was performing repeated external cargo transport missions between a temporary helipad and two loading sites in Takahama Town, Fukui Prefecture. During one of the flights, which involved transporting wooden frames used for insulating power lines, a pair of these frames escaped the cargo net (Mokko) and fell to the ground.
The helicopter was flying at an altitude of approximately 1,500 feet with an airspeed of 80 knots. While the crew did not notice the loss during the flight, a ground worker notified the captain via radio that some loads appeared to have dropped. Upon landing to inspect the cargo, the crew discovered a gap in the Mokko and confirmed the loss of the two wooden frames.
The frames landed in a grassy area of a private training center. While no injuries or ground damage occurred, the items drifted south of the intended flight path due to a north wind.
The investigation
The investigation focused on the integrity of the cargo packaging and the procedures used to secure the loads. Investigators examined the condition of the wooden frames, which were folded in half during packing, making them susceptible to air currents. They also reviewed the packing process, specifically how the ground workers used ropes to close gaps in the Mokko.
Technical analysis revealed that the folded shape of the frames created a tendency to trap air, increasing the likelihood of them being dislodged by the air currents generated by the helicopter's flight. The investigation also looked into the communication and verification methods used by the ground crew to confirm that the cargo was properly secured.
Findings
- The primary cause of the incident was that the wooden frames were transported while a gap existed in the Mokko.
- The gap was not adequately addressed during the packing process because the overlapping parts of the net made it difficult to visually identify openings.
- The final verification of the cargo's security was unreliable, as the person in charge of packing relied partly on verbal reports from other ground workers rather than direct physical inspection.
- The aerodynamic shape of the folded frames made them particularly vulnerable to being pulled out of the net by the airflow accompanying the flight.