What happened
On October 24, 2022, a Eurocopter AS 350 B3, registration JA02AH, operated by Nakanihon Air Service Co., Ltd., was performing cargo transportation duties near the Yugami Power Plant in Ono City, Fukui Prefecture. After completing seven cargo transfers, the aircraft was climbing toward a target altitude of 3,000 ft to clear a nearby transmission tower.
During the climb, the pilot spotted a large bird, resembling an eagle, approximately 30 degrees to the left. To avoid a potential bird strike, the pilot executed a level turn to the right. During this maneuver, the pilot' and mechanic's attention to the overhead transmission lines was reduced. Upon spotting the lines, the pilot performed a steep, 45-degree bank turn to the left to avoid a collision. During this intense maneuvering, the 21-meter-long underslung sling cable made contact with the high-voltage transmission lines.
The investigation
The Japan Transport Safety Board (JTSB) examined the damage to both the aircraft's equipment and the utility infrastructure. The investigation focused on the physical state of the 21-meter sling cable, which consisted of multiple connected segments and a 31.4kg cargo hook. Investigators found distinct burn marks and fractures on the cable's outer skin and internal control wires.
On the power lines, the investigation identified discharge marks and impact marks on the upper and middle phases of the transmission circuit. The investigation also analyzed the flight path and the pilot's decision-making process during the encounter with the bird. The JTSB determined that the electrical contact caused a short circuit, leading to the thermal failure of the cable.
Findings
- The primary cause of the incident was the underslung sling cable contacting the transmission lines while the helicopter was performing a steep turn.
- The contact occurred because the pilot's attention to the power lines was diminished by the simultaneous need to perform an evasive maneuver against a bird strike.
- The flight path taken during the bird avoidance maneuver brought the cable into the altitude range of the transmission lines without sufficient clearance.
- The high-voltage current caused electrical discharge and thermal damage, which burned through the sling cable's polyester threads and plastic coating, eventually causing the cable to sever and drop a portion of the hook and cable onto the lines.
- There were no injuries to the crew or damage to the helicopter itself, though the transmission lines required significant repairs.