Incident Overview
On 1 March 2021 at 13:35 UTC, a serious incident occurred between a Swiss Air-Rescue (REGA) Agusta Westland AW109SP helicopter, registered HB-ZRR, and an unknown aircraft. The event took place approximately 5 to 10 km northeast of Gross Litzner in the Silvretta region, Austria, at an altitude of 9500 ft AMSL. The helicopter was operating under visual flight rules as a primary rescue flight from Samnaun to Kantonsspital Chur, carrying a patient and crew. The unknown aircraft, described as a white light airplane, crossed the helicopter's flight path from south with an estimated vertical separation of 30 to 50 m (about 60 ft). The closest horizontal distance was 0 nautical miles.
Flight Details
The helicopter, call sign REGA 09 (RGA09), was en route over Austrian territory. The pilot suddenly perceived a shadow and then the white aircraft crossing ahead. Both onboard collision warning systems—the Traffic Advisory System (TAS) and the Floice device—generated no alerts. Post-event analysis of civilian and military radar data from Switzerland and Austria found no traces of an aircraft that could have crossed the helicopter's path at that time.
Weather and Airspace
Weather conditions were good: visibility 30 to 60 km, wind 170°/5 kt with gusts up to 15 kt at 9500 ft, clear skies, temperature -2°C, dew point -23°C, QNH 1028 hPa. The incident occurred in Class E airspace, where separation relies on the see-and-avoid principle.
Onboard Systems and Regulations
The helicopter's TAS and Floice were reported functional. The TAS uses transponder signals, while Floice uses Flarm technology. Because the unknown aircraft left no radar traces, it is likely that it had no transponder, its transponder was switched off, or it was defective. The Floice did not activate, suggesting the unknown aircraft lacked a compatible Flarm device. Under Austrian regulations, power-driven aircraft in Class E airspace must have an operational Mode S transponder with altitude reporting, set to code 7000 unless otherwise instructed by air traffic control. The helicopter crew monitored frequencies 130.355 MHz (Swiss mountain frequency) and 135.325 MHz (Samedan airfield), but no radio calls from nearby aircraft were heard.
Investigation Findings
The Swiss Transportation Safety Investigation Board concluded that the unknown aircraft likely had no transponder or a non-functioning one, and no Flarm device. The white aircraft against snowy mountains, combined with the crossing angle and the pilot's view obstructed by the left door frame, made visual detection extremely difficult. The crew was surprised and could not execute an evasive maneuver. No further investigation was deemed necessary as no additional safety-relevant findings were expected.