What happened
On September 9, 1998, a Hiller UH-12E helicopter was engaged in forest restoration efforts near Aschau, transporting tree seedlings via a long external sling load. Following successful transport operations the previous day, the crew and a forest ranger met at the helicopter's parking area around 07:00. A last-minute decision was made to conduct a reconnaissance flight without any external loads before proceeding with the scheduled transport work.
At approximately 07:30, the helicopter commenced its flight, initially performing a vertical takeoff before transitioning to forward flight. As the aircraft reached a speed of 30-40 knots at an altitude of roughly 40 meters, a member of the ground crew radioed a warning that the long sling load cable from the previous day's operations was still attached beneath the aircraft. This warning arrived too late to prevent the cable from snagging on a wire pasture fence positioned perpendicular to the flight path.
When the tension caused the fence wire to snap, the sudden recoil sent the cable upward through the main rotor plane. The main rotor blades severed the cable, and fragments of the line were thrown into the tail rotor, causing extensive damage to the tail rotor assembly. Consequently, the helicopter entered an uncontrolled flight state and crashed into a meadow, resulting in two serious injuries and the destruction of the aircraft.
The investigation
The BFU investigation focused on the operational aspects of the flight, as on-site examinations by investigators ruled out any mechanical or technical failures of the aircraft. The investigation reconstructed the sequence of events to determine why the cable remained attached and how it interacted with the ground environment.