Incident du K 8 B près de Geissflue, Suisse : vrille entraîne un crash en zone boisée

Le 23 juillet 1970, un Alexander Schleicher K 8 B a connu une vrille et s'est écrasé dans une zone boisée près de Geissflue, Suisse. L'incident s'est produit lors d'un vol lorsque le planeur est entré en vrille à basse altitude, entraînant une collision avec les arbres. Le pilote a subi de graves blessures et le planeur a été détruit. L'enquête officielle a confirmé que la vrille a été initiée en raison d'une utilisation brusque du gouvernail à basse vitesse et altitude.

1. Ridge soaring flight: After aerotow departure from Grenchen, the pilot was ridge and thermal soaring along the Jura between Hasenmatt and Geissflue at relatively low height above terrain.
2. Aft CG within limits: The glider’s center of gravity on this flight was at 394 mm behind the reference point, in the rear part of the allowable range, making it easier to enter a spin.
3. Low speed near ridge: While flying slightly north of the ridge crest, the glider was being flown at relatively low airspeed and low height above the trees, reducing stall and spin margins.
4. Abrupt left turn: West of Geissflue, over the crest, the pilot initiated a relatively abrupt left turn at low speed.
5. Unintended left spin: The combination of low speed, brusque rudder input, and aft CG caused the K 8 B to enter a left spin with rapid height loss.
6. Insufficient spin recovery: The pilot either did not immediately apply or could not complete effective spin recovery, and due to the very limited altitude was unable to stop the spin before ground impact.
7. Crash - serious injury: Still in a left-turning, nose-low attitude, the glider descended about 65 m and crashed into the forested north slope below the ridge crest, seriously injuring the pilot and destroying the aircraft.