Accident Overview
On June 28, 2015, an AgustaWestland AW109SP helicopter, registration HB-ZRY, was involved in an accident at Alp Oberkäseren, in the municipality of Amden, canton St. Gallen, Switzerland. The helicopter was conducting an approach to a landing site in mountainous terrain when it struck a telephone line. The line and its supporting poles were not depicted on the aeronautical obstacle chart used for navigation.
Causes
According to the investigation by the Swiss Transportation Safety Investigation Board (STSB), the accident was caused by the helicopter colliding with a telephone line that was not listed on the aeronautical obstacle chart during the approach to a landing site in the countryside. The following factors contributed to the accident:
- The telephone line and its poles were barely perceptible.
- The analysis regarding the presence of cables was incomplete.
Human and Operational Aspects
The pilot, a helicopter crew member (HCM), and a physician were on board. The crew had performed a pre-flight assessment of the landing area, but the telephone line was not identified as a hazard. The line was constructed with thin cables and small poles, making it difficult to see against the terrain background. Additionally, the line was not registered in the official database of obstacles for aviation, which meant it did not appear on the crew's obstacle charts.
Technical Aspects
The helicopter was equipped with a wire-strike protection system (Floice) and a digital map display, but these did not prevent the collision because the line was not in the obstacle database. The aircraft sustained damage upon impact with the line and subsequent ground contact.
Environmental Conditions
The accident occurred during daylight hours under visual meteorological conditions. The weather at the time was suitable for visual flight, with good visibility.
Aftermath
The investigation concluded that the accident resulted from the combination of an unmarked obstacle and incomplete pre-flight planning. No safety recommendations were issued in the report.