Flight Operations
The helicopter was tasked with transporting freshly cut spruce timber from Rudenzerwald to a forest road near Brunnenmaad. Two pilots were assigned: Pilot A as supervisor and Pilot B as trainee. Flight assistants were stationed at load pickup and drop-off sites. Due to weather, operations began after 1030 hours. Pilot B flew 17 rotations without incident in 58 minutes under Pilot A's supervision from the ground. The pilots alternated hourly.
Accident Sequence
Shortly after 1400 hours, Pilot B was at the controls. A tree trunk leaning against two standing trees, with its base in the ground, was to be transported. The flight assistant attached a choker cable and hooked it to the lower external load hook (Holzerklinke). The assistant signaled the pilot to try slowly. The pilot increased collective power. After 5–6 seconds of tension without the load moving, the hook opened with a loud bang, releasing the cable. The helicopter became unstable, lost height, struck tree tops, fell vertically, and rolled onto its right side. The pilot was seriously injured. The aircraft was destroyed. The tail rotor and last meter of the tail boom were found 40 meters from the wreck, with the tail rotor blades undamaged.
Technical Findings
The helicopter was properly maintained and certified for VFR flight. It was equipped with an audible warning system for maximum pitch and a weighing device. The hydraulic system functioned correctly. No pre-existing technical defects were found. The external load hook system was a Siren-Swing hook; the lower hook was an ERC type with a protective cover. The logging line was a 40 m Diethelm high-tech fiber rope with a fourfold damper rope.
Cause
The accident was attributed to loss of control after a hook opened prematurely while the helicopter was lifting an external load near obstacles, attaching and suspending a load not prepared for helicopter removal, and the pilot's low experience.