What happened
On August 8, 2018, an Airbus AS 350 B3, registration HB-ZDX, was performing a commercial transport mission near Schwand, Switzerland. The flight crew had conducted a briefing using both printed maps and an electronic moving terrain display to assess the obstacle situation at the landing site. To avoid high obstacles near the primary landing spot, the crew decided to land at an alternative location above the site to allow a flight assistant to disembark and assess the ground conditions.
During the final approach to the landing area, the pilot monitored the terrain via the cockpit's electronic map while the flight assistant observed the left side of the aircraft. As the helicopter approached the landing point, the main rotor blades struck an unregistered hay cable located on the right side of the aircraft. The impact caused an immediate loud bang and triggered intense vibrations throughout the airframe. The pilot managed to execute an emergency landing by applying full downward collective pitch, successfully bringing the aircraft to the ground despite the severe ground resonance and instability.
The investigation
SUST examined the circumstances surrounding the collision and the status of the obstacle. The investigation established that the hay cable was not registered in the official aviation obstacle database. While the aircraft was equipped with a cable cutter system, it did not have an active collision warning system installed at the time of the accident. The investigation also reviewed the regulatory framework for obstacle reporting in Switzerland, noting that obstacles below 25 meters are not required to be reported to the Federal Office of Civil Aviation (FOCA).
Findings
- The primary cause of the accident was the collision with an unregistered hay cable that was not present in the official obstacle database.
- The cable was not subject to mandatory reporting because it did not meet the 25-meter height threshold for registration.
- The pilot's visibility of the obstacle was limited as the flight assistant was monitoring the left side, while the cable was positioned on the right.
- The impact resulted in heavy damage to the main rotor blades, rotor head, landing gear, cabin, and the engine intermediate flange.
- The pilot's rapid reaction and emergency landing prevented any injuries to the crew or passengers.