ASK 18 collision with tree during ridge flight near Lommiswil, Switzerland
On August 15, 1981, an Alexander Schleicher ASK 18 was involved in an accident near Lommiswil, Switzerland. The glider, piloted by a Swiss citizen, detached from the tow plane at 1600 meters but later experienced a blocked airspeed indicator. During a right turn away from the ridge, the glider descended and collided with a tree. The pilot sustained serious injuries, and the aircraft was destroyed. The investigation identified flying below minimum airspeed as a confirmed cause, with contributing factors including wind effects and pilot inexperience on the aircraft type.
- Ridge soaring flight: After aerotow release at about 1600 m MSL, the pilot conducted ridge-soaring 8-shaped patterns along the Hasenmatt south slope in generally sinking air.
- Airspeed indication anomaly: Around 1440, the pilot noticed that the airspeed indicator needle was stuck at 95 km/h and did not respond even after tapping the instrument.
- Lee and boundary layer: The right turn away from the ridge was flown at low height over the ground on the lee side of the Hasenmatt, likely within the ground boundary layer, reducing effective airspeed.
- Pilot startled by noise: After completing a left turn west of Hasenmatt, the pilot heard an unexplained loud bang from the rear of the glider, which disturbed her concentration and may have led to unintentional aft stick input.
- Low experience on type: The pilot had relatively little recent experience on the ASK 18 type, which likely reduced her ability to manage speed and attitude without reliable airspeed indication.
- Stall and loss of control: At the end of the right turn away from the slope, the glider increasingly pitched nose-down and no longer responded effectively to elevator and aileron inputs, consistent with flight below minimum flying speed.
- Crash - serious injury: The right wingtip struck the top of a fir tree and the glider fell down along the trunk to the ground, seriously injuring the pilot and destroying the aircraft.