Crash d'un ASK 23B lors de l'approche finale près de l'aérodrome de Buttwil
Le 11 août 2001, un planeur Alexander Schleicher ASK 23B s'est écrasé à environ 1,7 km à l'est-nord-est de l'aérodrome de Buttwil en Suisse. Le pilote, un élève lors de son 16ème vol en solo, a perdu le contrôle de l'appareil peu avant l'approche finale, entraînant un accident mortel. Le planeur est entré en vrille inversée et a percuté un champ, entraînant la destruction de l'appareil et la mort du pilote. L'enquête a suggéré que le phénomène de 'sensations de sous-gravité' pourrait avoir contribué à la perte de contrôle.
- Aerotow — local soaring: The student pilot launched by aerotow from Buttwil for a planned two-hour solo soaring flight and later returned to the vicinity of the airfield after completing the task time.
- Unusual attitude onset: At about 500–600 m AGL near the airfield, shortly before entering the landing circuit, the glider transitioned from normal flight into a left sideslip-like attitude with the left wing dropping.
- Low solo experience: The pilot was on his 16th solo flight with only about 16 hours of total flight time, limiting his experience in recognizing and managing unusual attitudes and low‑g sensations.
- Possible low-g sensations: The investigators considered that sub‑gravity (low‑g) sensations during the developing maneuver may have led the pilot to inappropriately push and hold the control stick forward.
- Nose-down pitch input: After the left-wing-low slip, the pilot allowed or commanded the nose to drop rapidly, with the glider pitching forward over the nose and left wing while losing only 50–100 m of height and with little speed increase.
- Inverted spin developed: The forward pitching motion progressed into a fully developed inverted spin with several turns at nearly constant pitch attitude, rotation rate, and sink, with no visible signs of recovery control inputs.
- Crash - fatal: Still in an inverted spin attitude, the glider impacted a field and was destroyed, fatally injuring the pilot.