Introduction
On August 28, 2020, at approximately 09:50 Japan Standard Time, a Tohoku Air Service Eurocopter AS332L1 (registration JA332T) dropped a sling cargo weighing about 790 kg during an external cargo transport operation. The incident occurred near the Chuetsu substation temporary helipad in Nagaoka City, Niigata Prefecture. The helicopter was not damaged, and no injuries were reported among persons onboard or on the ground.
Flight Details
The helicopter departed from the temporary helipad at 09:17 to transport materials from a steel tower renovation project. The crew consisted of a captain in the left pilot seat, a trainee in the right pilot seat, and an onboard guide in the left rear seat. Two ground workers were assigned as hook and signal personnel. The sling length was set to 16 meters based on a previous day's flight survey. The helicopter was equipped with two independent hooks (red and yellow). The yellow hook was used for the transport that resulted in the incident. After a preflight inspection, the helicopter conducted a 10th transport of the day, lifting the cargo with the yellow hook. The load beam and keeper were confirmed locked before takeoff.
Incident Occurrence
As the helicopter climbed to about 25 meters above ground level and turned toward the helipad, the sling cargo suddenly dropped. A ground communication worker observed the load beam open and the cargo fall without significant swinging. The onboard guide stated that at the moment of the drop, he or she had opened a third of the left rear door and did not touch the cargo hook switch. After landing, the guide confirmed the system had no anomalies and the load beam and keeper were locked.
Post-Incident Inspection
Detailed inspections of the external cargo sling system were conducted, including functional and electrical tests under impact and vibration, teardown inspections of wiring and components, and load tests of the hooks. All tests showed the system was normal. Electromagnetic interference from nearby high-voltage cables was considered, but the computed field strength at the incident location was at levels typical of daily life. No damage was found to the hanging ring, load beam, or keeper.
Analysis
The Japan Transport Safety Board (JTSB) analyzed possible causes. Malfunctions due to airframe vibration or electromagnetic interference were considered but deemed probable to be low based on inspection results and the helicopter's previous flights closer to high-voltage cables. Improper sling work was considered unlikely due to the absence of damage to components. The possibility of an operation error of the cargo hook switch was assessed as low given the guard on the switch and the onboard guide's statement. The JTSB concluded that the probable cause of the load beam opening and the cargo dropping could not be determined.
Conclusion
The JTSB's investigation did not identify a specific cause for the incident. No recommendations were issued. The report emphasizes that the investigation is intended for accident prevention and not to assign blame.