ASK 23B crash during final approach near Buttwil airfield
On August 11, 2001, an Alexander Schleicher ASK 23B glider crashed approximately 1.7 km east-northeast of Buttwil airfield in Switzerland. The pilot, a student on his 16th solo flight, lost control of the aircraft shortly before the final approach, resulting in a fatal accident. The glider entered an inverted spin and impacted a field, leading to the destruction of the aircraft and the pilot's death. The investigation suggested that the phenomenon of 'sub-gravity sensations' may have contributed to the loss of control.
- 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.