What happened
On August 3, 2009, an Aerospatiale AS332L helicopter, registered JA9690, was performing material transport duties near Imazu-cho, Takashima City, Shiga Prefecture. Operated by Aero Asahi Corporation, the aircraft was approaching a worksite near the Otsu No.33 power line tower to sling a truck using a 20-meter sling.
As the aircraft gradually descended to allow a ground worker to attach the hook, the downwash from the rotors caused nearby trees to sway. During this descent, a large tree located just outside the cleared work area suddenly snapped approximately one meter above the ground. The falling tree struck the worker, who was attempting to catch the sling hook, causing serious injuries. The crew on board the aircraft, consisting of a captain and a mechanic, were uninjured and the aircraft sustained no damage.
The investigation
An investigation by the JTSB examined the condition of the aircraft, the site, and the surrounding vegetation. Investigators found that the aircraft was in a valid state of airworthiness and the crew was properly qualified. The investigation focused on the stability of the trees at the worksite and the impact of the helicopter's downwash.
Physical examination of the fallen tree revealed that while the exterior bark appeared intact, the interior of the trunk was rotten, creating a hollow cavity. This decay made the tree structurally unstable. The investigation also reviewed safety guidelines provided by the contractor and the operator, noting that while guidelines required a 20-meter clearance from obstructions for unloading, the interpretation of these rules primarily focused on preventing collisions with fixed structures like power towers rather than managing vegetation hazards during long-sling operations.
Findings
- The primary cause of the accident was that a tree, which possessed a rotten interior, was broken by the downwash from the Aerospatiale AS332L.
- The worker was unable to avoid the falling tree because the event occurred amidst the high noise levels of the aircraft and the tree fell from a blind spot behind the worker.
- The decay within the tree was not visible from the outside, making the structural failure difficult to predict through visual inspection.
- The existing safety guidelines for the worksite did not sufficiently account for the risks posed by unstable trees during sling operations using long-distance attachments.