Piper PA-34 / ASW 20 CL aerotow airprox — all 3 had FLARM, no alerts

Near Baar in Canton Zug, Switzerland, a Piper PA-34-200 flying from Buochs to Zürich at 280 km/h overflew an aerotow (Robin DR300-180R plus ASW 20 CL) by only about 20 m vertically in Class E airspace; no injuries and no damage. The Piper's crew did not see the tow combination and made no avoidance manoeuvre. The glider pilot spotted the Piper at 2 o'clock and radioed the tug; both wing-waggled without effect, and the glider pilot briefly pushed to reduce climb. All three aircraft carried FLARM but no pilot noticed any display or warning during the convergence — crew non-detection and ineffective collision alerting drove the airprox.

1. Aerotow climb: The ASW 20 CL glider HB-1784 was being aerotowed by Robin DR300 D-EAIO from Hausen am Albis on runway 27 in a continuous climb toward the Rossberg area.
2. Piper on collision path: While the tow was passing south of Baar at about 4700 ft AMSL, the glider pilot saw Piper PA-34 HB-LEM approaching quickly from the front right on an apparent collision course with the tow.
3. Piper crew non-detection: The Piper pilot and passenger, although aware of other traffic in the Hausen am Albis area, did not visually detect the towplane–glider combination and the Piper maintained its cruise flight path and altitude of about 4700 ft AMSL.
4. Flarm warning ineffective: Although all three aircraft were equipped with Flarm collision warning devices, none of the pilots noticed any Flarm alert during the dangerous convergence.
5. Tow and glider warnings: The glider pilot radioed the tow pilot about the traffic at their 2 o’clock, both pilots attempted to attract the Piper’s attention by brief wing-waggling, and the glider pilot slightly reduced climb by gently pushing forward on the elevator to increase vertical separation.
6. Very close overflight: The Piper HB-LEM crossed directly over the glider’s flight path, passing just above and behind the towplane and an estimated 20 m above the glider at the end of the 60 m tow rope.
7. Safe landing: All three aircraft continued their flights without further incident and later landed safely with no injuries or damage.