What happened
On August 3, 2017, a Eurocopter AS350B3 helicopter, registered as JA6512, was performing an external cargo transport mission in Kurobe City, Toyama Prefecture. The aircraft took off from the Otosawa Temporary Helipad, carrying approximately 700 kg of tools and equipment secured in a cargo net. The mission intended to deliver the load to the left bank of the Kuronically-Kitamata Dam.
While the helicopter was climbing at approximately 610 meters above ground level and maintaining a speed of 70 knots, the suspended cargo suddenly detached and fell into a forested mountain area. The pilot reported feeling vertical oscillations in the aircraft immediately following the loss of the load. Upon checking the cargo mirror, the pilot confirmed the sling was no longer attached. The flight was subsequently aborted, and the aircraft returned to the departure helipad.
The investigation
The Japan Transport Safety Board (JTSB) conducted an investigation involving representatives from the French Republic, the state of design and manufacture. Investigators examined the external cargo sling system through several rigorous methods, including:
- A teardown inspection of the sub-hook mechanism.
- Function and electrical testing under simulated impact and vibration conditions.
- An investigation into potential electromagnetic interference from aircraft systems or external high-voltage power lines along the flight path.
- A load test of 1,000 kg on the sub-hook.
- An examination of the cargo net's hanging ring, which showed no signs of damage.
No mechanical defects or structural failures were identified in the sub-hook or the electrical wiring during these tests.
Findings
The investigation could not pinpoint a definitive trigger for the release, but it identified that the unintentional release of the sub-hook lock was the primary reason the load fell. It is possible that aircraft vibrations, a temporary insulation failure, or electromagnetic interference caused a momentary malfunction in the electric circuit of the sling system.
While the ground crew observed that the load beam was closed and locked upon the aircraft's return, investigators noted it was possible that the force of the cargo dropping caused the sub-hook to move violently, inadvertently re-locking the beam after the ring had already slipped out.