What happened
On October 13, 2017, an AS 350 B2 helicopter, registration HB-XVM, was performing a commercial external load operation in Tesserete, Switzerland. The mission involved transporting prefabricated wooden and plasterboard elements from a sports field to a construction site within the village.
After completing approximately 30 successful rotations, the helicopter landed to perform a hot refuel. During this period, ground crews were preparing the next load. Upon restarting and maneuvering vertically over the next element, the intense rotor downwash caused several vertically standing prefabricated elements to lose stability. The resulting collapse struck two workers who were positioned within the danger zone, causing serious injuries to one. A second worker, the driver of a nearby semi-trailer, was also struck by falling elements, which caused his protective helmet to break and resulted in a period of unconsciousness.
The investigation
SUST examined the procedures and equipment used during the operation. The investigation found that while the flight crew was experienced and had conducted a safety briefing, the workers on the ground had very little experience with helicopter-related transport risks. The investigation also scrutinized the physical securing of the cargo, noting that the elements were held together by wooden securing plates that were not specifically designed to withstand the forces of rotor downwash. Furthermore, the investigation reviewed the lifting gear, identifying that the working load limits of the slings were insufficient for the helicopter's maximum allowable external load and that the secondary hook was being used in an improper orientation.