Collision de l'ASK 18 avec un arbre lors d'un vol de crête près de Lommiswil, Suisse

Le 15 août 1981, un Alexander Schleicher ASK 18 a été impliqué dans un accident près de Lommiswil, Suisse. Le planeur, piloté par un citoyen suisse, s'est détaché de l'avion remorqueur à 1600 mètres mais a ensuite rencontré un indicateur de vitesse bloqué. Lors d'un virage à droite s'éloignant de la crête, le planeur a descendu et a percuté un arbre. Le pilote a subi des blessures graves et l'appareil a été détruit. L'enquête a identifié le vol en dessous de la vitesse minimale comme une cause confirmée, avec des facteurs contributifs incluant les effets du vent et l'inexpérience du pilote sur ce type d'appareil.

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.