What happened
On August 29, 2014, an AS 350 B3 helicopter, registration HB-ZKF, was performing a heavy lift operation near Eisten, Switzerland. The mission involved transporting components of a large excavator from a mountain construction site back to the valley. The task required moving heavy parts, each weighing between 1,000 and 1,100 kg, using a 24-meter long transport sling.
During the second rotation of the mission, the pilot hovered over the pickup site to lift the lower section of the excavator. A flight assistant, a mechanic, and a machinist were present on the ground to secure the load. After the first load had been successfully moved, the ground crew believed they were moving out of the danger zone. However, as the helicopter lifted the second heavy load, the intense rotor downwash exerted significant pressure on a nearby group of five Swiss stone pines.
The investigation
SUST investigators examined the site, the aircraft, and the environmental conditions. The investigation confirmed that the HB-CLF was operating at its performance limits for a hover out of ground effect (HOGE) due to the heavy weight and high altitude. The investigation also reviewed the stability of the trees, finding that they were healthy and showed no signs of disease or mechanical damage from previous loads. The investigation noted that five previous cargo rotations had already subjected this specific group of trees to rotor downwash earlier that same day.
Findings
- The primary cause of the accident was the uprooting of a group of trees caused by the downward airflow (rotor downwash) from the helicopter's main rotor.
- The mechanic was positioned in the danger zone, moving back toward the center of the work area, when the trees fell toward the valley.
- The mechanic sustained serious injuries to his back when he was struck and pinned by a falling trunk.
- The helicopter was operating at the maximum of its power capabilities for the given conditions.
- While the 24-meter sling provided sufficient clearance from obstacles, it was not long enough to move the source of the downwash far enough away from the trees to prevent the uprooting.