What happened
On August 5, 2016, an Aèrospatiale AS332L1 helicopter, registered JA9678, was performing cargo sling operations in Nara Prefecture, Japan. Operated by AERO ASAHI Corporation, the aircraft was transporting materials for a transmission line steel tower reconstruction project between a temporary helipad and a construction site.
During the flight, the rotorcraft was carrying an 800 kg steel plate secured with wire ropes. To maintain stability against wind pressure, the pilot increased the flight speed to 80 knots. While transiting a level flight path at approximately 2,900 feet, the pilot experienced a sudden, heavy impact. Upon checking the external mirror, the crew confirmed that the steel plate had fallen from the aircraft.
The investigation
The JTSB investigation examined the lashing configuration, the flight conditions, and the company's operational procedures. The investigation focused on the physical state of the sling wires and the method used to secure the load.
Investigators found that the steel plate had been secured using a double wrap choke hitch with two 8-meter wires. While the initial test hover showed the load was balanced, post-incident inspections of the wires revealed significant damage. Specifically, the wires showed abrasions and deformations near the center of the plate, suggesting that the edge of the plate had scraped the wires as the load shifted.
Findings
- The investigation determined that the steel plate, which was initially held in a horizontal position, was forced into a vertical position by wind pressure.
- As the plate tilted, it likely generated aerodynamic lift, causing a sudden loss of balance and the subsequent collapse of the lashing.
- The lashing method was adopted without sufficient technical examination of how wind pressure would affect a horizontally oriented, flat object at high speeds.
- While the company's manual specified a speed limit of 80 knots, the aircraft's flight manual suggested that speeds should be reduced for bulky or uniquely shaped cargo to prevent instability.
- The use of the horizontal slinging method was based on recent on-site experience to prevent rotation, but it failed to account for the risk of the load flipping vertically under aerodynamic forces.