What happened
On June 11, 2018, an Airbus Helicopters AS 350 B3, registration HB-ZOJ, was performing a commercial external cargo flight near Alp Ebnet, Switzerland. The mission involved transporting concrete buckets for a construction project. During the operation, the pilot performed several rotations to move material between a pickup point and the construction site.
As the mission progressed, shifting winds caused fog to move into the area. While the pilot had previously used various flight paths to navigate changing visibility, a thick layer of fog began to close in over the site. During one specific rotation, the pilot attempted to lift a concrete bucket to move it to the unloading point. As the aircraft ascended, it entered the fog, causing the pilot to lose all visual references to the ground. The pilot briefly queried if the pickup point in the valley was still visible through the fog. Shortly after, the aircraft struck rising terrain approximately 310 meters southwest of the unloading site. The impact resulted in one fatality and the destruction of the aircraft.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight sequence, the weather conditions, and the coordination between the various parties involved. The investigation looked into the crew's actions, the technical state of the HB-ZOJ, and the communication between the pilot and the ground crew (flight helpers).
Investigators noted that the operation involved personnel from three different companies who had not previously worked together and lacked a shared understanding of safety protocols regarding weather minimums. The investigation also reviewed the technical history of the helicopter, noting that metal particles had been found in the main gearbox during previous inspections, though the manufacturer's recommendation for immediate repair had not yet been completed at the time of the accident.
Findings
- The primary cause of the accident was that the pilot continued the flight mission too late despite the encroaching, dense fog, leading to a loss of visual references and subsequent loss of control.
- A contributing factor was the lack of coordination and shared safety expectations between the various personnel and companies involved, particularly regarding the decision-making process for aborting the mission.
- An optimistic assessment of the weather conditions created a pressure situation that influenced the decision to continue the transport.
- The pilot's attempt to lift the load while already partially obscured by fog led to the vertical ascent that caused the aircraft to disappear into the cloud layer.