What happened
On September 23, 2017, an Aérospatiale AS332L1, registered JA6717, was performing routine external cargo transport between a temporary helipad in Kitagawa-village and an unloading point approximately 1.3 km away. The aircraft, operated by Nakanihon Air Service Co., Ltd., was engaged in its 58th transport flight of the day.
Immediately after departing from the Kojima temporary helipad, the aircraft dropped approximately 2.7 tons of stones contained within an external sling bucket. The incident occurred roughly 80 meters southeast of the takeoff point. There were no injuries to persons or damage to property on the ground.
The investigation
The investigation examined the actions of the crew and the configuration of the aircraft's control systems. The crew consisted of a captain in the right seat and an onboard mechanic in the forward left seat, who was responsible for monitoring the sling operation and communicating with ground personnel.
Investigators reviewed the control box used to manage the external sling system and the communication radio. The investigation confirmed that the bucket's open/close switch was a simple push button located on the bottom of a switch box. While the system was found to be functioning normally during ground tests, the investigation focused on the sequence of events leading to the accidental release of the cargo.
Findings
- The cargo drop was caused by the onboard mechanic accidentally activating the bucket's open/close switch while intending to use the radio's transmit switch.
- At the moment of the error, the mechanic was focused on formulating a warning for a ground trainee, which led to a lapse in attention regarding his hand placement.
- The mechanic was not visually monitoring his hand while reaching for the communication controls.
- The control box design lacked a safety mechanism, such as a guard or a multi-step activation process, to prevent accidental engagement of the bucket release.
Safety action
Following the incident, the operator implemented several safety improvements:
- Conducted individual and company-wide retraining focused on human error prevention.
- Modified the aircraft's bucket control system by installing an additional toggle switch, ensuring that the bucket can only be opened or closed if both switches are operated simultaneously.