Incident Details
On September 23, 2017, at around 15:32 Japan Standard Time, an Aérospatiale AS332L1 helicopter, registered JA6717 and operated by Nakanihon Air Service Co., Ltd., was conducting external cargo sling operations at the Kojima temporary helipad in Kitagawa-village, Aki-gun, Kochi Prefecture. Immediately after takeoff from the cargo sling point, the bucket attached to the helicopter opened, releasing approximately 2.7 tons of stones. The stones fell about 80 meters southeast of the temporary helipad. No injuries to persons or damage to the aircraft or objects on the ground were reported.
Flight History
The helicopter had been repeatedly transporting cargo between the temporary helipad and a cargo unloading point approximately 1.3 km away since around 15:00. On the day of the incident, three ground workers were present at the cargo sling point, including a trainee. The captain was in the right pilot seat, and the onboard mechanic was seated in the forward left cabin seat, responsible for outside monitoring, guidance, and sling equipment operation. The helicopter had performed a total of 58 cargo lifts that day, with the incident occurring on the tenth flight from that specific helipad.
The Incident Sequence
During a hover to initiate the tenth flight, the onboard mechanic noticed the trainee's slinging work and intended to issue a warning via the communication radio. Holding a radio in one hand, he reached to operate the transmit switch on the control box but mistakenly pressed the bucket open/close switch instead. As a result, the bucket opened and the stones fell. The captain, informed by the mechanic, terminated the flight and returned to the temporary helipad. The mechanic reported that he was not looking at his hand when operating the switch, as his attention was focused on how to warn the trainee.
Investigation Findings
The Japan Transport Safety Board investigation determined that the mechanic had received appropriate training and had approximately 23 years of cargo transport experience, with two years and three months as an onboard mechanic. The control box, located on the forward left side of the cabin, had a push-button bucket open/close switch that operated as long as the button was pressed. The transmit switch was a separate toggle switch on the lower front of the control box. Functional tests confirmed the bucket system operated normally.
The investigation noted that the bucket open/close switch lacked a mechanism to prevent inadvertent operation. It was probable that the mechanic's attention was diverted by his thoughts about warning the trainee, leading to reduced attentiveness to the bucket operations, and that he unintentionally performed a repetitive action (opening the bucket) that he had been doing many times previously.
Probable Cause
It is certain that this serious incident occurred because the bucket opened and the stones dropped immediately after the aircraft took off from the cargo sling point carrying stones in the bucket, due to the onboard mechanic mistakenly operating the bucket's open/close switch instead of the transmit switch of the communication radio.
Safety Actions Taken
Following the incident, the operator, Nakanihon Air Service Co., Ltd., implemented several safety measures: (1) individual re-education on human error prevention for the onboard mechanic; (2) company-wide education on human error prevention for all operations personnel and executive-led safety education at cargo transport sites; (3) installation of an additional toggle switch on helicopters using the same type of bucket open/close switch, requiring simultaneous operation of two switches to open/close the bucket.