ASK 23B Zwischenfall bei Lommiswil: Notlandung in Bäumen nach Strömungsabriss

Am 13. September 1990 erlebte eine Alexander Schleicher ASK 23B während eines Trainingsflugs in der Nähe von Lommiswil, Schweiz, einen Strömungsabriss. Der Pilot mit begrenzter Erfahrung flog entlang eines Kamms, als die Geschwindigkeit des Segelflugzeugs unter das minimale Sicherheitsniveau fiel. Unfähig, die Kontrolle wiederzuerlangen, setzte der Pilot die Bremsklappen ein und führte eine Notlandung in den Baumwipfeln durch. Das Segelflugzeug rutschte an den Bäumen zu Boden, was zu erheblichen Schäden am Flugzeug führte. Der Pilot blieb unverletzt und verließ das Flugzeug eigenständig.

1. Ridge training flight: During the second afternoon training flight after aerotow to 2000 m, the pilot conducted ridge soaring along the Hasenmatt between about 1200 and 1300 m, flying short figure‑eight patterns along the slope.
2. Low ridge experience: The pilot had very limited overall gliding experience and only a brief prior ridge-flying checkout, so he was not familiar with windshear hazards in ridge soaring.
3. Aft CG and turbulence: The glider was loaded with the center of gravity in the aft part of the allowable range, reducing longitudinal stability, and the ridge area was known for turbulent, sheared airflow with only weak ridge lift.
4. Turn away from slope: While flying figure‑eights along the ridge, the pilot made a 180° left turn away from the slope and then immediately turned right, during which the airspeed dropped rapidly below the minimum safe speed.
5. Stall and height loss: The glider entered a mush/stall with airspeed reported down to about 50 km/h, losing height quickly despite the pilot applying full control inputs to turn right and away from the slope.
6. Emergency landing attempt: Recognizing that he could not regain sufficient airspeed or control, the pilot committed to an emergency landing and extended the airbrakes.
7. Crash - no injury: The glider touched down in treetops on the slope, slid down the trees to the ground, was destroyed, and the pilot exited uninjured.