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

On 1 May 2001 both occupants of an ASK 13 were fatally injured at Unterwössen; the aircraft was destroyed. The 2-seater, 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: ASK 13 (Jubi Sportflugzeugbau), 3,099 h total, last annual inspection 12.04.2001. Two-seater: instructor on rear seat, sightseeing passenger on front seat. Passenger had asked the launch director around 09:30 about a sightseeing flight; due to unfavourable wind it was postponed to afternoon; at 13:00 he asked again and was assigned to the instructor. Winch launch from Unterwössen for a thermal flight in foehn weather. Wind 240° at 8-10 kt; partly strong turbulence near the accident site.
2. Instructor PIC, passenger up front: The instructor was PIC on the rear seat (had valid SPL and instructor rating; required to wear glasses + carry spare per medical, and at time of accident was not wearing glasses — BFU noted that horizon-based attitude/airspeed assessment should have been possible without glasses, so this was not cited as causal). Passenger up front, mentioned flying decades earlier in Austria. Toxicology and autopsy unremarkable.
3. Aircraft 30 kg over MTOM: Flight mass calculated at 510 kg — 30 kg over the maximum permissible takeoff mass. BFU finding: this raised the glider's stall speed. (Note: empty-mass CG had been out of range at the 18.03.1999 weighing, prompting an increase of the front-seat minimum load from 65 kg to 80 kg; passenger including parachute weighed 78 kg, 2 kg below that minimum. Despite the loading issues, the in-flight CG was within permissible limits.)
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 5 minutes after launch, witnesses observed the glider in the downwind area of the airfield. The glider made a full left circle and then a long, shallow-banked right circle toward RWY 24, then turned right again to a northeast heading.
6. Slow flight at 60-70 km/h estimated: Witnesses estimated the glider's airspeed during the long right circle at 60-70 km/h — very low for a fully-occupied ASK 13, especially given the 30 kg over MTOM raising the actual stall speed.
7. Wing drop into near-vertical attitude: After approximately half a circle on the northeast heading, the glider suddenly entered a near-vertical longitudinal attitude (Längsneigung) and disappeared from witnesses' view after a quarter rotation. The wing-drop direction was to the right. Investigation found no defects in the flight controls.
8. 80° nose-down embankment impact: Impact on a 25%-inclined, grass-covered road embankment (Straßenböschung), at approximately 80° nose-down pitch attitude, from ~150 m AGL. Both occupants fatally injured (passenger on impact, instructor died of injuries 2 hours later). Aircraft destroyed. BFU conclusions: pilot did not decide in time to abort the thermal search, and did not observe the stall speed.