Factual Information
The accident involved an Airbus Helicopters AS 350 B3, registered as HB-ZGV, operated by Swift Copters SA and owned by SG Equipment Finance Schweiz AG. The helicopter was on a commercial visual flight rules (VFR) flight from Sion (LSGS) to inspect high-voltage power lines. The pilot held a commercial pilot license for helicopters (CPL(H)) issued by the European Aviation Safety Agency (EASA) and had approximately 4,890 total flight hours, with 4,567 hours on the accident type. The flight was crewed by the pilot and two employees of the power line maintenance company.
Sequence of Events
On 27 July 2016, the pilot arrived at Sion at 06:20 local time for flight preparation. The helicopter was refueled with about 530 liters of fuel. The two maintenance workers arrived at 07:00, and after a detailed briefing, the flight departed at 07:30. The flight proceeded to various power line sections in the Valais region. After a pause at Ackersand and a stop near St. Triphon, the helicopter approached the final section near Glion. The pilot conducted a circular flight to assess cable positions, then descended slightly over the Gorge du Chauderon.
As the helicopter crossed the deepest part of the gorge, the pilot and the front-left passenger saw a thick black cable approximately perpendicular to the helicopter's longitudinal axis. The pilot pulled collective pitch in an attempt to clear the cable, but the left skid struck the fiber optic cable, severing it. The cable ran from an antenna mast at the Sonzier power plant southeast across the gorge to power line mast no. 107, at a height of about 110 meters above ground. The cable was not recorded in the obstacle database. The severed portion fell near two bystanders. After the collision, the pilot confirmed control and executed a 180-degree left turn, then landed on a meadow near the Sonzier power plant. Total flight time from Sion was two hours.
Damage and Injuries
The helicopter sustained minor damage to its skid landing gear and underside fairing. The fiber optic cable was cut. None of the three occupants (pilot and two passengers) nor any third parties were injured.
Investigations
The investigation focused on the flight preparation and obstacle awareness. The pilot had flown in the area years earlier and recalled a cable from the power plant but did not recall it during the accident flight. The maintenance workers were unfamiliar with the area and unaware of the cable. The cable was not listed in the Swiss obstacle database.
Probable Cause
The probable cause was not stated in the provided source material.