History
On 2 September 2004, a Bell 407 helicopter (registration HB-ZBA) operated by Helog AG was conducting logging operations in the Bisistal valley near Muotathal, Switzerland. The pilot had flown several round trips transporting tree trunks from loading areas to a log deck. During a subsequent flight, parts of a multiple load suspended on a longline became entangled in the branches of a tree. Seconds later, the helicopter began to rotate around its vertical axis and, with the load still attached, crashed onto steep, rocky terrain below the loading area. The pilot vacated the cockpit unaided. The helicopter was destroyed.
Aircraft and Personnel
The helicopter was a Bell 407 owned by Dreieck Leasing AG. The pilot had been working extensively in the weeks prior to the accident. On the Monday before the accident, he flew 5 hours in a different region; on Tuesday, he flew 27 round trips in one hour at the same location using the same helicopter. On Wednesday, he was in the office. He had a free weekend prior to that.
Accident Sequence
After picking up a multiple load, the load became entangled in a tree. This caused large torque and load variations on the helicopter. As a consequence, the main rotor struck and damaged parts of the tailboom. A main rotor blade severed the tail rotor shaft, leading to an immediate loss of control around the vertical axis. The helicopter then collided with the terrain.
Causes
The investigation determined that the accident was caused by an external load entangling on an obstacle, causing large moment and load variations. This led to the main rotor damaging the tailboom and severing the tail rotor shaft, resulting in loss of control and subsequent impact. Two factors contributed to the accident: flight tactics focused on efficiency with insufficient safety margins, and the absence of a mentally prepared and recallable recovery option.