Falcon 2000EX vs Arcus airprox in Class E during ILS approach

A Falcon 2000EX on an IFR ILS approach to St. Gallen-Altenrhein, descending through Class E airspace at ~5000 ft MSL, met a Schempp-Hirth Arcus returning to Amlikon under VFR. Class E provides no ATC separation between IFR and VFR traffic. The Arcus had Flarm on but Mode-S transponder switched off and no radio contact with ATC — invisible to the controllers handling the Falcon. Paths crossed head-on west of Amriswil; both crews acquired the other only shortly before closest approach: ~660 m horizontal / ~40 m vertical. Neither transponder use nor ATC radio was legally required for the glider at the time. No injuries, no damage.

1. Falcon ILS approach + Arcus VFR: A Falcon 2000EX was on an IFR ILS approach to St. Gallen-Altenrhein under Zurich control, while an Arcus was gliding VFR back to Amlikon after release near Säntis.
2. ILS approach through Class E: The base leg of the ILS approach to LSZR descends through Class E airspace, where ATC provides no separation between IFR and VFR traffic.
3. Transponder off, no ATC contact: The glider pilot had switched off the transponder and was not in radio contact with ATC — neither legally required — leaving the glider invisible to controllers handling the Falcon.
4. Converging paths, late acquisition: Tracks crossed head-on near Amriswil at about 1,600 m AMSL; both crews saw the opposing aircraft only shortly before closest approach, and ATC had no knowledge of the glider.
5. Closest approach: 660 m / 40 m: Minimum separation: ~660 m horizontal, ~40 m vertical. Classified as a serious airprox in Class E airspace.
6. No collision, safety advice: There was no collision, damage, or injury. The investigation closed with a safety advice; mandatory glider transponder use was not in force at the time and remains voluntary.