What happened
On 20 November 2018, a Eurocopter AS-350-B2, registration EC-MVV, was performing aerial work to support the construction of a transmission tower foundation near Ribera d’Urgellet, Spain. The operation involved transporting a concrete bucket via a sling from a staging area to the worksite.
During the second rotation of the day, the pilot was hovering the aircraft approximately 12 meters above an energized 25-kV medium-voltage power line. As the aircraft positioned the bucket for unloading, a spark and flashover occurred. The electrical discharge traveled through the wet surface of the sling to the ground, striking two operators who were handling the bucket. The pilot, who was uninjured, immediately relocated the aircraft to the staging area to release the load and retrieve personnel to assist the injured workers.
The investigation
The CIAIAC investigation examined the aircraft, the sling, the weather conditions, and the operational procedures. Investigators found that while the weather was not limiting for flight, the presence of drizzle and high humidity had caused moisture to accumulate on the sling, compromising its dielectric properties.
The investigation also reviewed the safety protocols and the site setup. It was noted that the safety zone, intended to be marked at a distance of 3 meters from the power line, had actually been established only 1.5 to 2 meters away. Furthermore, the investigation found that the power line had not been deenergized for the work, and the required safety technician was not present at the immediate work site during the incident.