What happened
On March 12, 2019, an AS 350 B3 helicopter, registration HB-ZNZ, was performing aerial work in Bissone, Switzerland. The mission involved transporting cargo from a municipal parking area to a drop-off point in the center of the village using a 50-meter longline. During a return flight to the loading site, the pilot was flying at a low altitude near the San Carpofolo church.
As the aircraft passed the church, a loose cargo strap attached to the longline became entangled with the cross atop the bell tower. The pilot felt an unusual resistance and transitioned into a hover. The tension caused the cross to detach from the tower, which in turn dislodged a stone from the structure. The falling stone struck and damaged the church roof. The pilot managed to safely deposit the detached cross on the ground near the church before landing in Maroggia to inspect the aircraft for damage.
The investigation
Investigators analyzed GPS flight tracks, which revealed that while the pilot had previously flown to the west of the tower, this specific leg of the flight was the closest to the structure. Meteorological data indicated that while conditions were generally clear, wind gusts were present in the region. The investigation also looked into the coordination between the flight crew and the ground assistants.
Findings
- The primary cause was that the pendulum motion of the longline and straps was not monitored by the pilot, allowing the equipment to drift too close to the tower cross.
- The ground assistant, who could have potentially spotted the snagging equipment, was preoccupied with preparing subsequent loads and did not provide a warning.
- The choice of the loading location, constrained by nearby roads and railways, forced a flight path that placed the aircraft in close proximity to a significant obstacle.
- Wind gusts likely contributed to the lateral movement of the longline toward the church.