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: An ASK 21 launched on aerotow from RWY 35 of Zweisimmen (LSTZ) for a VFR flight to Olten (LSPO). The tow proceeded north-east in Class E airspace, climbing toward FL80.
2. Chart omits Class E warning post-CTR: The IFR departure chart for Grenchen used by the jet crew carried no notation that the SID entered uncontrolled Class E airspace after leaving the control zone.
3. Jet on autopilot, lookout inadequate: At 10:10 UTC the Cessna 525 departed Grenchen IFR on the SID toward Willisau, cleared to FL80. With the autopilot engaged, the crew did not adequately monitor for VFR traffic.
4. TCAS symbol; no aural alert: Reaching cruise level, the captain noticed a yellow TCAS symbol with no aural advisory; 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 beneath the aerotow. The towpilot never saw the jet; the glider pilot saw it pass under her wing.
6. 7 m × 370 m pass, all land safely: The closest recorded approach between the Cessna and the tow plane was 0.2 NM (≈370 m) horizontally and 25 ft (≈7 m) vertically. All three aircraft landed without damage.