What happened
On August 3, 2010, an Eurocopter AS 350 B3, registration HB-ZES, was engaged in a commercial external load operation near Zollikofen, Switzerland. The mission involved removing damaged spruce trees following a thunderstorm. Due to space constraints at the primary lifting site, the project leader instructed the pilot to land at a secondary drop zone located approximately 100 meters away at a stubble field.
As the pilot approached the landing site to deposit a 60-meter nylon transport sling, no ground crew was present to manage the line. The pilot attempted to land the aircraft autonomously. During the final phase of the descent, the rotor downwash caused a section of the loose sling on the ground to lift and whip upward. Approximately 10 meters of the line became entangled in the tail rotor, causing the tail rotor drive shaft to break at its designated shear point. This resulted in a total failure of the anti-tail rotor. The pilot immediately responded by reducing collective pitch, leading to a hard landing after the aircraft rotated approximately 180 degrees.
The investigation
SUST examined the mechanical state of the HB-ZES, the operational procedures, and the environmental conditions. The investigation confirmed that the aircraft was in good technical condition with no pre-existing mechanical defects. Investigators also reviewed the manufacturer's guidelines for the TLDS-type sling, which specifically warn that rotor downwash can cause loose lines on the ground to flutter, creating dangerous situations. The investigation also noted that while the pilot was highly experienced, the specific risks associated with using this new 60-meter sling type were not fully addressed during the mission's reorganization.
Findings
- The primary cause of the accident was the entanglement of the transport sling in the tail rotor, which triggered a total loss of anti-torque capability.
- No ground personnel were available at the secondary landing site to assist with the sling during the landing.
- The pilot attempted to deposit the sling without external assistance, allowing a portion of the line to remain under the aircraft's flight path.
- The rotor downwash caused the unanchored sling to become airborne and strike the tail rotor assembly.