What happened
On July 27, 2016, an Airbus Helicopters AS 350 B3, registration HB-ZGV, was conducting a commercial inspection flight of high-voltage power lines in the Valais region of Switzerland. The mission involved flying along the lines at low speeds to allow two technicians on board to visually assess their condition.
While traversing the Gorge du Chauderon near Montreux, the pilot was flying a flight path parallel to the power lines. As the aircraft reached the deepest part of the gorge, the pilot and a passenger noticed a thick, black cable crossing the flight path at a 90-degree angle. Despite the pilot's attempt to clear the obstacle by adjusting the collective pitch, the helicopter's left skid struck the cable. The impact severed a fiber optic cable that stretched from an antenna mast at the Sonzier power plant to a nearby power line mast. The pilot managed to maintain control of the aircraft and performed a 180-degree turn to land safely in a meadow near the power plant. There were no injuries to the crew or the two passengers, though the aircraft sustained damage to its landing gear and undercarriage.
The investigation
SUST examined flight data recorder information, pilot and passenger statements, and an eyewitness account. The investigation focused on the visibility of the obstacle and the accuracy of aeronautical databases. Investigators found that the fiber optic cable was difficult to see and, crucially, was not recorded in the Federal Office of Civil Aviation (FOCA) aeronautical obstacle database because it had not been reported to the authorities. Furthermore, the investigation revealed discrepancies in the existing database, noting that some power lines listed in the system did not exist, while others, like the cable involved in the accident, were missing entirely.
Findings
- The primary cause of the collision was that the fiber optic cable was not entered in the aeronautical obstacle database due to a failure to report it to the relevant authorities.
- The cable was physically difficult to detect during flight.
- The aircraft was performing a low-speed inspection flight, making it vulnerable to low-altitude obstacles.
- The aeronautical database maintained by FOCA did not accurately reflect the real-world status of obstacles in the area.