What happened
On September 21, 2024, at 14:22 local time, a serious near-collision occurred near Netstal, Switzerland, involving a Bristell B2/3-915 (registration HB-KTN) and a Leonardo AW109SP helicopter (registration HB-ZRZ).
The HB-KTN, operated by Resia GmbH, was conducting a training flight with two instructors. The crew was performing maneuvers near Mollis airfield, specifically executing touch-and-go landings on Runway 01. Simultaneously, the HB-ZRZ, a Rega 12 rescue helicopter operated by Schweizerische Luft-Ambulanz AG, was returning to its base at Mollis after a mission.
As the helicopter approached via Heli-Route 2, the pilot issued a blind transmission on the Mollis frequency. However, the crew of the HB-KTN did not hear this transmission. During the approach, the helicopter crew was momentarily distracted by a collision warning from a nearby glider, which they initially dismissed as non-threatening.
As the aircraft reached the end of the Runway 01 approach, the instructors in the HB-KTN suddenly spotted the descending helicopter at a distance of approximately 100 meters. The two aircraft passed each other with a horizontal separation of roughly 70 meters and a vertical separation of approximately 40 meters. Neither aircraft was damaged, and both continued their flights.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined flight data recorders from both aircraft and interviewed the crews. The investigation focused on the communication failures and the physical flight paths that led to the encounter. The investigators analyzed the terrain around Mollis, noting that the topography significantly impacts VHF radio propagation, creating "radio shadows" where transmissions may not be received.
Findings
- The approach for Runway 01 and Heli-Route 2 converge without vertical separation, creating a safety-critical intersection point.
- Local topography causes intermittent reception of blind transmissions, leading to a degraded situational awareness.
- The aircraft utilized incompatible collision warning technologies, meaning the automated systems could not detect each other.
- The encounter relied entirely on the "see and avoid" principle, which failed due to the late visual acquisition of the other aircraft.
Safety action
SUST issued a safety recommendation to the Federal Office of Civil Aviation (BAZL) to review and potentially adjust the approach procedures and their representation on Visual Approach Charts (VAC) to mitigate the risk at this intersection. Additionally, a safety notice was issued to pilots operating in mountainous, uncontrolled airfields, advising them to increase the frequency of blind transmissions and use terrain-aware flight management to mitigate radio dead zones.