Background
On August 3, 2017, at approximately 10:02 Japan Standard Time, a Eurocopter AS350B3 helicopter, registration JA6512, operated by Aero Asahi Corporation, was engaged in an external cargo sling operation near Kurobe City, Toyama Prefecture. The aircraft had departed from the Otosawa Temporary Helipad. The weather was fine with a weak westerly wind, and no turbulence was reported.
Incident Details
The helicopter was transporting a cargo net containing tools and other items weighing about 700 kg. The cargo net's hanging ring was attached to the sub-hook of the external cargo sling system, which had a maximum hanging load of 1,400 kg. A ground worker closed the load beam and confirmed with the operation manager that the indicator showed a "Lock" status. During the flight, while climbing slowly at approximately 70 knots at about 610 m AGL, the cargo dropped suddenly even though the unhooking switch had not been operated. The captain felt the aircraft shaking up and down, looked in the mirror, and saw no slung load. He decided to abort the cargo transport and returned to the helipad. Upon landing approach, the load beam was found to be closed, and no damage was observed on the hanging ring of the cargo net.
Investigation and Analysis
The Japan Transport Safety Board (JTSB) investigated the incident. A detailed teardown inspection of the sub-hook confirmed that the lock was not unintentionally released and that the structure prevented the hanging ring from slipping through. Function tests, electrical tests with impact and vibration, teardown inspections of wire bundles and equipment, electromagnetic interference tests, and investigation of possible influence from radio facilities or high-voltage power lines along the flight route were performed. A load test of the sub-hook (1,000 kg) was also conducted. No abnormal conditions were found. The JTSB analyzed that it was somewhat likely that the lock on the sub-hook was unintentionally released and the load beam opened due to a temporary malfunction of the electric circuit caused by aircraft vibrations, temporary insulation failure, or temporary electromagnetic interference from aircraft equipment or external facilities. However, the probable cause of the load beam opening could not be determined.
Safety Actions
Following the incident, Aero Asahi Corporation ceased using all external cargo sling systems of the same type that had been installed on four of its aircraft, including the incident helicopter.