Class E airprox: business jet passes 7 m beneath aerotow on departure

East of Langenthal an IFR Cessna 525 business jet passed within 7 m vertically and 370 m horizontally of an aerotow — an ASK 21 behind a Robin DR 400 — in Class E airspace; no injuries and no damage. The aerotow was flying VFR from Zweisimmen to Olten; the Cessna had departed Grenchen IFR and was climbing toward FL80 on autopilot. The jet crew did not adequately scan for traffic and the captain only noticed a TCAS traffic symbol with no aural alert; he then visually acquired the aerotow and dived beneath it. The IFR departure chart carried no notation about entering Class E airspace after leaving the Grenchen CTR. ICAO Cat A airprox.

1. Aerotow Zweisimmen → Olten, Class E: At 09:45 UTC on 13 July 2013 an ASK 21 (HB-1589) launched on aerotow behind a Robin DR 400/180 R (HB-EUV) from RWY 35 of Zweisimmen (LSTZ) for a VFR cross-country flight to Olten (LSPO). The tow proceeded north-east in Class E airspace at around the same altitude (toward FL80).
2. Chart omits Class E warning post-CTR: The IFR departure chart for Grenchen (LSZG) used by the business-jet crew carried no notation that the aircraft would enter uncontrolled Class E airspace after leaving the Grenchen control zone. SUST identified this as a systemic contributing factor: without that note, IFR crews can underestimate the need for active visual lookout for VFR traffic during the climb.
3. Jet on autopilot, lookout inadequate: At 10:10 UTC the Cessna 525 (HB-VPF) departed Grenchen IFR on the SID WIL 7V toward the Willisau VOR, cleared to climb to FL80. With the autopilot engaged through the climb, the crew did not adequately monitor the surrounding Class E airspace for VFR traffic. SUST identified this inadequate lookout as the direct cause.
4. TCAS symbol; no aural alert: Reaching the assigned cruise level the captain noticed a yellow traffic symbol on the TCAS display; no aural advisory was issued. Shortly after, the crew visually acquired the aerotow ahead and slightly above.
5. Captain dives beneath aerotow: On visual acquisition the captain disconnected the autopilot and dived the Cessna 525 beneath the aerotow combination. The towpilot never saw the jet; the glider pilot saw it pass under her left wing shortly after release and watched it climb away to the right.
6. 7 m × 370 m pass, all land safely: At 10:14 UTC the closest recorded approach between the Cessna and the tow plane (both transponder-equipped) was 0.2 NM (≈370 m) horizontally and 25 ft (≈7 m) vertically; the separation between the Cessna and the glider was less. SUST classified the encounter as ICAO Category A (high collision risk). All three aircraft landed at their planned destinations without damage or injury.