What happened
On June 28, 2015, an AgustaWestland AW109SP, registration HB-ZRY, was dispatched from Mollis for a medical emergency near Alp Oberkäseren. The mission involved providing medical assistance to an unconscious individual.
Upon arrival, the crew reviewed the digital navigation charts for the area and found no registered obstacles near the target location. As the helicopter approached the initial landing site, the crew observed a person attempting to signal the aircraft. However, due to the limited space at the primary site and the proximity of a crowded terrace, the pilot decided to seek an alternative landing spot. While maneuvering southeast of the alpine hut in a slow, descending forward flight, the aircraft struck a telephone line.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight data from the Electronic Flight Instrument System (EFIS), the aircraft wreckage, and the local terrain. The investigation focused on why the obstacle was not identified during the pre-flight planning or during the approach. Investigators reviewed the Federal Office of Civil Aviation (FOCA) obstacle database and the onboard collision warning and mapping systems.
Findings
The investigation established that the primary cause of the accident was the collision with a telephone line that was not included in the aeronautical obstacle database.
Several contributing factors were identified:
- The specific sections of the telephone line and its supporting pylons were barely visible to the crew.
- The regulatory threshold for mandatory reporting of obstacles (heights above ground level exceeding 25 meters) meant that this specific line, which sat at a maximum height of less than 25 meters, was not registered.
- The analysis of existing cables in the area was incomplete.
Safety action
Following the accident, the Federal Office of Civil Aviation updated its obstacle database to include the relevant sections of the telephone line, noting a maximum height of 17 meters.