Incident Overview
On August 5, 2016, at approximately 10:23 Japan Standard Time, an Aérospatiale AS332L1 rotorcraft, registered JA9678 and operated by Aero Asahi Corporation, was engaged in an external cargo sling operation. The aircraft was transporting a steel plate from a temporary helipad at Akatani Dam in Oto Town, Gojo City, to a construction site in Totsukawa Village, Nara Prefecture. During the flight, the steel plate, which was slung beneath the helicopter, unintentionally detached and fell into a mountainous area.
Flight Details
The helicopter had been performing repeated sling cargo transports for a transmission line steel tower rebuilding project. On the fifteenth transport of the day, workers prepared the steel plate using four wire ropes—two 8-meter ropes (Wire A) and two 4-meter ropes (Wire B)—in a double-wrap choke hitch configuration. The load was attached to a remote hook via a belly hook and external mirror for monitoring. After a test hover confirmed horizontal balance, the flight proceeded.
The captain reported maintaining a speed of 80 knots, the upper limit for sling transport, while monitoring the load via the external mirror. The steel plate remained stable, with a slight backward tilt due to wind pressure. After turning right over a ridge, the plate rotated approximately 45 degrees before returning to the sideways position. Shortly after, while transitioning to level flight, the captain heard a loud impact and saw the steel plate falling through the external mirror. The on-board worker witnessed the moment of separation as the distance between two wires narrowed.
Investigation Findings
No injuries to persons or damage to the aircraft were reported. The captain held a valid commercial pilot certificate and medical certificate with significant flight experience. Weather conditions were clear with visibility over 10 km and no turbulence.
Post-incident inspection revealed that after landing, the remote hook and wires remained connected except for the two Wire A ropes that had secured the steel plate. These wires showed damage and deformation near the center of the plate, not at the edges where contact occurred. The sling angle was within regulatory limits (under 60 degrees).
The operator had previously experienced issues with vertical slinging of steel plates, including shaking and instability, leading to a change in method to horizontal slinging approximately half a year prior. This incident was the fifth use of the horizontal slinging method. The flight route had been pre-selected to avoid harm to persons or objects on the ground, and a pre-flight inspection was conducted.
The analysis indicated that the sling cargo was likely swaying due to rotorcraft maneuvers, but no definitive cause for the wire damage or load release was established.