What happened
On August 21, 2018, an Aèrospatiale AS332L operated by Nakanihon Air Service Co., Ltd. was performing an external cargo transport mission in Fukushima-town, Hokkaido. The aircraft was tasked with moving three separate loads—consisting of cable, a stepladder, and tools—from a temporary helipad to an unloading site approximately 1.5 km away.
As the helicopter crossed over a tower and began descending at speeds below 50 knots, the onboard mechanic observed a blue sheet and cable protruding from a gap in the cargo net. The pilot reduced speed to approximately 40 knots to prevent the load from shifting. However, as the aircraft slowed to a speed near a hover, the blue sheet and the cable detached from the net and fell into a mountain forest from an altitude of roughly 130 meters. There were no injuries resulting from the incident.
The investigation
Investigators examined the loading procedures and the physical state of the cargo at the time of takeoff. The investigation focused on the configuration of the cargo nets and the use of additional wires intended to stabilize the loads and prevent interference between the three items. The JTSB also reviewed the actions of the ground crew and the pilot during the initial lift-off and the subsequent flight phase.
Findings
Technical analysis revealed that the cargo configuration deviated from the original plan. Due to an interruption in the workflow, a ground worker mistakenly believed an extra wire had been attached and removed it, which inadvertently shortened the sling length for the tools cargo. This change in length caused the "wire threading"—a wire passed through the cable's cargo net to prevent wind-induced movement—to pull upward, tilting the cargo net by approximately 45 degrees.
While the onboard mechanic and ground workers noticed this significant tilt, they signaled "OK" to the pilot, allowing the flight to proceed. This tilt caused the knot of the cargo net to loosen, creating a gap. During flight, wind pressure caused the blue sheet to flutter and protrude through this gap, eventually allowing the cable and sheet to slip out entirely. The primary cause was the loosening of the cargo net knot, which created a gap through which the materials escaped.
Safety action
Following the incident, Nakanihon Air Service Co., Ltd. updated its cargo transport guidelines and implemented personnel retraining. The new protocols mandate that if cargo appears tilted or improperly secured while suspended, the operation must be terminated and the load must be re-inspected on the ground. Additionally, the company now requires that cargo nets be tightened or covered with secondary netting to ensure no gaps exist that could allow materials to escape.