What happened
On July 22, 2015, an Aerospatiale AS332L1 helicopter, registered JA6777, was performing external cargo transport in Yurihonjo City, Akita Prefecture. The aircraft, operated by Tohoku Air Service, Inc., was engaged in the 13th cargo transport of the day, moving a work hut from a loading site to an unloading site.
During the flight, approximately 90 meters above the ground, the pilot noticed something falling from the slung load via a monitoring mirror. An onboard worker also observed a sliding door detach from the hut as the aircraft approached a forest road. Upon arrival at the unloading site, ground personnel discovered that three sliding doors had gone missing from the work hut.
The investigation
The JTSB investigation examined the packing procedures, the training provided to ground crews, and the physical condition of the cargo. The investigation looked into the communication between the helicopter operator, the main contractor, and the subcontractors responsible for preparing the cargo.
Investigators reviewed the safety handbooks used for cargo preparation and found that while general packing methods were documented, there were no specific instructions for handling uniquely shaped loads like the work hut. Furthermore, the investigation found that some ground workers had not received the intended safety training and that the checklist used by the pilot and ground crew was insufficient to verify specific measures taken to secure individual components of the cargo.
Findings
- The primary cause of the incident was the lack of effective measures to prevent objects from detaching from the slung work hut.
- The sliding doors likely derailed from their frames due to external forces, such as vibrations from the aircraft and wind pressure from the rotor downwash, which caused distortion in the hut's structure.
- The safety education provided by the operator was insufficient, as it focused on common cargo shapes and failed to address the specific requirements for transporting uniquely shaped loads.
- There was a lack of detailed study or specific procedures to check the integrity of each individual package before flight.
- Ground workers relied on previous experience rather than specific instructions for this particular load, and the existing checklist could not confirm if the doors were properly secured.