What happened
On October 18, 2011, an Eurocopter AS 350 B3, registration HB-ZES, was performing external load operations near Bourg-St-Pierre, Switzerland. The mission involved transporting prefabricated construction elements to a mountain hut. During the operation, the pilot was lifting a 450 kg palette using a 30-meter sling.
As the aircraft hovered a few meters above the ground, the powerful downward airflow from the rotor blades—known as downwash—interacted with unsecured materials on the landing site. This air movement caused a large, prefabricated facade element to tip over. A construction worker, who was positioned within the danger zone to monitor the loads, was struck by the falling component and sustained serious injuries.
The investigation
SESA examined the technical and operational aspects of the incident, including measurements of rotor downwash intensity. The investigation established that the aircraft was operating within its weight and balance limits and that the pilot held all necessary qualifications.
Investigators analyzed the aerodynamic forces at play, noting that while the vertical downwash is significant during a hover, it transforms into horizontal and turbulent airflow as it nears the ground. The study confirmed that the air movement produced a pressure of approximately 100 N/m² against the surface of the facade element. The investigation also reviewed the site's safety perimeter and the briefing provided to the ground crew prior to the flight.