What happened
On June 2, 2020, an Airbus AS 350 B3, registration HB-ZML, was performing commercial sling operations near Pfäffikon, Switzerland. The mission involved transporting various materials from a quarry area to Ufenau Island for church tower restoration work. After completing ten successful rotations of single or combined loads, the crew attempted to optimize the remaining work by transporting two steel barrels simultaneously as a single multi-load.
During the eleventh rotation, the pilot accelerated to approximately 40 knots. Shortly after takeoff, the load passed behind a warehouse roof, obscuring it from the ground crew's view. An eyewitness on the destination island observed the two barrels swinging violently and colliding. This impact caused the cargo to shift, resulting in 17 scaffolding elements and the securing strap falling into the water, where they sank to a depth of approximately 20 meters.
The investigation
SUST examined the cargo configuration and the methods used for securing the load. The cargo consisted of galvanized steel barrels containing aluminum scaffolding elements. While the barrels were attached to the helicopter hook using four textile slings, the internal contents were not properly stabilized. The crew had used a single horizontal strap to secure the 45 elements within the barrel, but this method failed to create a tight, secure fit.
Findings
- The two barrels were not secured to each other, allowing them to move independently in the airflow.
- The horizontal strap provided insufficient tension, meaning only four scaffolding elements were stabilized by the sleeve, while the rest remained loose within the frame.
- The large surface area of the scaffolding elements (approximately 5 square meters) made the load highly susceptible to aerodynamic forces.
- The decision to transport two insufficiently secured barrels as a multi-load at 45 km/h created an unnecessary risk.
- The dynamic movements inherent in helicopter sling operations—including pitching, rolling, and swinging—were not adequately accounted for in the cargo securing plan.