ASK 13 stalls in low-altitude thermal search; aircraft 30 kg overloaded

Both occupants of an ASK 13 were fatally injured at Unterwössen; the aircraft was destroyed. The ASK 13, 30 kg over MTOM and thus at a raised stall speed, had launched by winch for a thermal flight with an instructor in back and a sightseeing passenger up front. About 5 min after launch, witnesses watched it circle on the downwind at very low estimated speed (60-70 km/h) — the pilot did not abort the thermal search in time, did not observe stall speed; on a NE heading the glider suddenly pitched near-vertical, rolled right and impacted a 25% grass embankment at ~80° nose-down from ~150 m AGL.

1. Winch launch, sightseeing thermal flight: An ASK 13 with the instructor on the rear seat and a sightseeing passenger up front took a winch launch from Unterwössen for a thermal flight in foehn weather.
2. Instructor PIC, passenger up front: The instructor was pilot in command on the rear seat, with the passenger seated up front; he was required to wear glasses but was not wearing them at the time of the accident.
3. Aircraft 30 kg over MTOM: The flight mass was calculated at 510 kg, 30 kg over the maximum permitted takeoff mass, which raised the glider's stall speed.
4. Foehn turbulence in the area: Foehn weather at the time of the accident; the BFU notes that partly strong turbulence was present near the accident site and may have contributed to the wing dropping.
5. Low-altitude thermal search in downwind: About five minutes after launch, witnesses saw the glider circling in the airfield's downwind area, alternating turns while searching for lift.
6. Slow flight at 60-70 km/h estimated: Witnesses estimated the glider's airspeed during the right circle at 60-70 km/h, very low for a fully-occupied, overweight ASK 13.
7. Wing drop into near-vertical attitude: After about half a circle on the northeast heading, the glider suddenly entered a near-vertical dive to the right and disappeared from witnesses' view.
8. 80° nose-down embankment impact: The glider struck a grass road embankment nose-down at about 80° from roughly 150 m AGL; both occupants were fatally injured and the aircraft destroyed.