What happened
On July 3, 2023, an Aerospatiale AS332L1 helicopter, registered JA6686, was performing scheduled cargo transport near the Nakagochi Operation Site in Aoi Ward, Shizuoka City. The mission involved moving steel truss components for transmission line pylon construction between a loading site and an unloading site located approximately 560 meters away.
During the seventh transport cycle of the day, the aircraft began its climb with an underslung load weighing roughly 800 kg. While flying at an altitude of approximately 150 meters over the Kamiochiai area, a wooden sleeper—used to support the cargo—detached from the load and fell into a forested mountain area. The crew visually confirmed the loss of the object. The helicopter subsequently returned to the loading site to unload the cargo before landing at the operation site. There were no fatalities or injuries resulting from the incident.
The investigation
The Japan Transport Safety Board (JTSB) examined the flight history, the condition of the cargo, and the communication between the flight crew and ground personnel. Investigators found that the cargo had been resting on the sleepers for about a month prior to the incident. Physical evidence, including paint transfers between the sleeper and the steel truss, suggested the two had been in close contact.
Investigators also reviewed the pre-flight checks and radio communications. While the crew and ground operators had verified the transport list and the stability of the cargo, they had not specifically verified whether the sleepers were intended to be part of the load or if they were securely fastened. Furthermore, the investigation scrutinized a radio exchange between the captain and a ground operator that occurred during the initial climb.
Findings
- The heavy weight of the steel truss had pressed the wooden sleeper against the cargo for an extended period, effectively crimping it to the load.
- As the Aerospatiale AS332L1 climbed, vibrations and wind pressure during flight overcame the friction holding the sleeper in place, causing it to slip and fall.
- A lack of shared information regarding the intended packing configuration meant the crew did not realize the sleeper was an unintended part of the load.
- During the takeoff, the captain attempted to use the radio to confirm the status of the cargo, but the use of ambiguous terminology and the ground operator's improper use of a headset led to a misunderstanding. The ground operator incorrectly assumed the captain was checking for clearance from surrounding obstacles rather than addressing the attached sleeper.
Safety action
Following the incident, Shin Nihon Helicopter Co., Ltd. updated its training manuals to require that the transport cargo list explicitly state if sleepers are to be included in the load. The company also revised its regulations to mandate that operations be halted and cargo be unloaded for inspection if ground operators cannot clearly confirm the cargo's status during transport.