What happened
On December 2, 2008, a Kaman K-1200 helicopter, registration HB-ZGK, was performing external load operations near the Aare river in Bern. The mission involved removing tree sections as part of a specialized clearing project. A forestry specialist had prepared a section of the tree crown consisting of three branches for transport, securing only two of the three branches to the helicopter's longline using steel cables.
As the pilot increased tension on the lines and lifted the load, the weight caused the crown to shift into a horizontal position. This movement caused the third, unsecured branch—a piece of wood weighing approximately 450 kg and measuring 16 meters in length—to snap off. The falling limb struck the specialist, who was positioned five to six meters up the tree, causing serious injuries.
The investigation
The investigation examined the technical state of the aircraft, the rigging methods used, and the operational procedures in place. Investigators found that the HB-ZGK was in good technical condition and that the pilot was properly licensed with significant experience on the aircraft type. The weather conditions, characterized by light haze and low winds, were determined to have had no impact on the event. The investigation also reviewed the rigging process, noting that the specialist had personally prepared the load and believed the two-point attachment was sufficient for the task.
Findings
- The primary cause of the accident was that the external load was not appropriately secured.
- The specialist's decision to secure only two of the three branches led to an unstable center of gravity when the load was lifted.
- The shifting of the load into a horizontal orientation created the mechanical stress that broke the unattached limb.
- There were no technical failures found in the helicopter or the longline equipment.