Incident Overview
On October 9, 2014, at 11:38 JST, an Aérospatiale AS332L1 helicopter, registered JA6741 and operated by Shin Nihon Helicopter Co., Ltd., was conducting a sling transport operation of a toilet hut from a temporary helipad in Takamine Kogen to the Mount Asama Kazankan Lodge site. During the flight, the left door of the hut's double swing doors became detached and fell to the ground.
Flight and Operational Details
The helicopter was crewed by a captain (right seat) and a mechanic (rear seat) who monitored the operation visually and via a surveillance camera mounted on the aircraft. The flight route required an initial climb to clear a mountain ridge, followed by a descent along the mountain surface and a wide circling approach around the southern ridge of Mount Kurofu. The helicopter maintained stable flight conditions at approximately 60 knots, a speed considered appropriate for stable sling transport. The load (the hut) was connected via a 16-meter sling rope attached to the helicopter, with four ropes extending from clasps on the hut's roof.
The hut, weighing approximately 1.7 tons, was equipped with four doorways: two single swing doors at the front, one single swing door at the back, and a pair of double swing doors at the side for access to the Bio-toilet apparatus. Prior to transport, company staff secured all fixtures and fittings inside the hut, locked all doors and windows, and performed double-checks due to the presence of an aperture area in the floor. No external securing ropes or protective nets were used, as the crew and ground staff judged the locking of doors and windows sufficient.
Sequence of the Door Detachment
Video footage from the surveillance camera revealed the door detachment process. While airborne, the hut was rotating counterclockwise as viewed from above. When the hut's orientation placed the door facing opposite the direction of flight, the door was forced open. It briefly closed but then, as the rotation brought the door perpendicular to the flight direction, it was forcefully reopened, struck the hut's sidewall, and broke apart, falling to the ground. The captain and mechanic were unaware of the incident until ground workers reported a missing door after the fourth unloaded operation at the site.
Damage and Location
The left-side door of the double swing doors broke at its mounting points. Examination revealed that the door (2 m long, 70 cm wide, about 8 kg) was fixed by two hinges and a lock bolt; the lock bolt receiver on the right-side door was deformed by stress. The company identified the estimated drop location from video footage and recovered broken door parts in trees at coordinates N 36°23'40", E 138°28'47". No other damage to persons, objects, or the ground was observed. The helicopter itself sustained no damage, and there were no injuries.
Air Pressure and Aerodynamic Considerations
The double swing doors were designed to withstand wind up to approximately 70 knots based on JIS standards. Analysis indicated that the airstream and air pressure induced by flight contributed to the door's detachment. When the hut rotated and the door faced opposite the flight direction, the lower parts of the hut were subject to air pressure parallel to airspeed, while the door area experienced negative pressure from slipstream vortices, causing the door to be forced open and break.