What happened
On October 15, 2017, at 12:42 UTC, a serious airprox occurred approximately 3 nautical miles west of Amriswil, Switzerland. A Falcon 2000EX business jet, registration CS-DLB, was performing an instrument approach (IFR) to St. Gallen-Altenrhein airport. Simultaneously, a Schempp-Hirth Arcus glider, registration HB-3442, was gliding back to Amlikon airfield under visual flight rules (VFR).
The two aircraft crossed paths in opposite directions at an altitude of roughly 5,000 feet AMSL within Class E airspace. The flight crews of both aircraft only spotted the opposing aircraft at the very last moment. The closest the aircraft came to each other was a horizontal distance of approximately 660 meters and a vertical separation of only 40 meters.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined radar data, radio communications, and statements from both flight crews. The investigation focused on why the aircraft were not separated by air traffic services and why the encounter was not detected earlier.
Investigators found that the air traffic controller at the Zurich ARFA center had no knowledge of the glider, as it did not appear on the controller's radar screen. While the business jet was in active communication with air traffic control, the glider pilot was not in contact with the controller.
Findings
The primary cause of the near-collision was that both flight crews failed to visually identify the opposing aircraft until a very late stage.
Several contributing factors were identified:
- The business jet was operating under IFR in Class E airspace, where air traffic services do not provide separation between IFR and VFR traffic.
- The transponder on the HB-3442 glider was switched off, which, while not a regulatory violation in that airspace, prevented the aircraft from being visible to radar.
- The glider pilot was not in radio contact with air traffic control, a factor that further hindered situational awareness.
Safety action
Following the investigation, SUST issued a safety notice (No. 24) directed at the Swiss Aero Club and all airspace users. The notice encourages glider pilots to maintain active transponder use and to establish radio contact with local controllers when operating near regional airports like St. Gallen-Altenrhein. This is intended to supplement the 'see and avoid' principle and ensure VFR aircraft are detectable to IFR traffic.