What happened
On October 9, 2014, an Aérospatiale AS332L1 helicopter, registered JA6741, was performing a sling transport operation in Komoro City, Nagano Prefecture. Operated by Shin Nihon Helicopter Co., Ltd., the aircraft was moving a specialized toilet hut from a temporary helipad in Takamine Kogen to the Mount Asama Kazankan Lodge site. During the fourth transport leg of the day, a portion of the cargo's left-side double swing door detached and fell to the ground.
The flight proceeded along a predetermined route, maintaining a stable speed of approximately 60 knots. While the crew—consisting of the captain and a mechanic—did not notice the loss of the component during flight, ground workers at the destination discovered that one of the hut's doors was missing upon the aircraft's arrival. The fallen door, measuring 2 meters by 70 centimeters and weighing roughly 8 kg, was later recovered from the trees along the flight path.
The investigation
The investigation focused on the aerodynamic forces acting on the cargo and the methods used to secure the load. The cargo, a heavy bio-toilet hut weighing approximately 1.7 tons, featured an aperture in its floor. While the crew and ground staff had verified that all doors and windows were locked, the load was not wrapped in protective netting or additional securing ropes.
Analysis of surveillance footage from the helicopter revealed that as the hut rotated during flight, the door was subjected to significant pressure differentials. Specifically, when the door faced the side of the flight path, it was forced open by the airstream. Subsequent rotation caused the door to strike the side of the hut with enough force to break its mounting hardware, leading to its detachment.
Findings
- The primary cause of the incident was the aerodynamic pressure and airstream generated by the helicopter's flight, which forced the door open.
- The door's mounting hardware, including the lock bolt receiver, deformed under the stress of the wind and the impact against the hut's wall.
- The lack of secondary securing measures, such as securing ropes or protective nets, contributed to the door's ability to fall.
- The company had not previously performed sling operations with loads containing floor apertures, which likely prevented an accurate assessment of the specific aerodynamic risks involved.