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 AMSL, 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: Falcon 2000EX (CS-DLB, NetJets NJE050N) on IFR ILS approach to St. Gallen-Altenrhein (LSZR) rwy 10 under Zurich ARFA control. Arcus (HB-3442) on VFR glide return to Amlikon after release near Säntis. Both equipped: Falcon with TCAS; Arcus with Flarm and Mode-S transponder.
2. ILS approach through Class E: The base leg of the ILS 10 approach to LSZR at ~5000 ft AMSL descends through Class E airspace where ATC provides no separation between IFR and VFR traffic — the structural vulnerability that SUST has repeatedly flagged for this approach.
3. Transponder off, no ATC contact: The glider pilot had switched the Mode-S transponder off and was not in radio contact with ATC. Neither was legally required for VFR glider operations in Class E at the time of the incident. Together, the two omissions made the glider invisible to the controllers handling the Falcon. Flarm was on, but the Falcon had no Flarm.
4. Converging paths, late acquisition: Tracks crossed head-on at ~5000 ft AMSL west of Amriswil. Both crews acquired the opposing aircraft visually only shortly before the closest-approach point; ATC handling the Falcon 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: No collision, no damage, no injuries. SUST closed the investigation under Schlussbericht Nr. 2350 (Aug 2019) with a Sicherheitshinweis (safety advice); no formal safety recommendation issued. SUST has been pushing for broader mandatory glider transponder use since this and similar incidents; not yet mandatory at the time of the incident and not yet mandatory now.