What happened
On August 28, 2020, at approximately 09:50 JST, a Eurocopter AS332L1 operated by Tohoku Air Service, Inc. was engaged in external cargo transport near Nagaoka City, Niigata Prefecture. The aircraft was performing its tenth transport of the day, moving materials removed from a steel tower, weighing approximately 790 kg, from a temporary helipad to a nearby site.
After taking off from the Chuetsu substation temporary helipad, the helicopter climbed and began its transit. While flying at an altitude of roughly 25 meters above ground level, the aircraft turned from a northwesterly direction toward the helipad. During this maneuver, the cargo sling suddenly released, causing the load to drop onto a grassy area near the helipad. There were no injuries to personnel on the ground or onboard, and the helicopter sustained no damage.
The investigation
The Japan Transport Safety Board (JTSB) conducted an investigation to determine why the load beam and keeper of the cargo hook suddenly opened. Investigators examined the external cargo sling system, including the electrical supply, the power box, and the mechanical components of the hooks. Detailed inspections included functional and electrical tests under simulated vibration and impact, as well as a teardown of the wire bundles and hooks.
The investigation also assessed several potential triggers for the release:
- Mechanical Failure: The possibility of airframe vibration causing the load beam to open was ruled out as the phenomenon could not be reproduced during testing.
- Electromagnetic Interference: Investigators calculated the electromagnetic field strength near the nearby high-voltage cable. The levels were found to be consistent with daily life and unlikely to have triggered the single relay controlling the hook release.
- Sling Work Errors: No damage was found on the hanging ring, load beam, or keeper, suggesting the cargo was properly secured.
- Operational Error: While the onboard guide stated they did not touch the cargo hook switch, the investigation considered whether the repetitive nature of the task might have led to an accidental activation of the switch.
Findings
The investigation could not definitively identify a single cause for the sudden unlocking of the load beam. However, the JTSB noted that the possibility of the onboard guide accidentally activating the cargo hook switch due to the routine nature of the repeated operations could not be ruled out. The cargo hook switch was protected by a guard, but the investigation highlighted the need for improved fail-safe and fool-proof mechanisms to prevent the load beam from releasing due to temporary system malfunctions or human error.
Safety action
Following the incident, the operator suspended the use of the specific external cargo sling system involved. External cargo operations continued using alternative manual hook equipment until improvements to the system's fail-safe capabilities and enhanced crew notification functions could be implemented.