Collision du Ka 4 Rhönlerche II avec des arbres lors d'un vol d'entraînement à Bettlachstock, Suisse

Le 26 juin 1964, un planeur Ka 4 Rhönlerche II, piloté par un élève, est entré en collision avec des arbres lors d'un vol d'entraînement près de Bettlachstock, en Suisse. Le vol, parti de l'aérodrome de Granges, visait à pratiquer le vol de pente le long des montagnes du Jura. Le pilote élève, en effectuant un virage, n'a pas réussi à maintenir une distance suffisante par rapport à la pente, entraînant une collision inévitable. Le planeur a été gravement endommagé, mais le pilote s'en est sorti indemne. L'incident a été attribué à l'inattention du pilote pendant la manœuvre.

1. Ridge soaring flight: The student pilot departed Granges airfield in a Rhönlerche II for a ridge-soaring training flight along the Jura with a planned maximum duration of 75 minutes and specified speed and altitude constraints.
2. Low experience level: The pilot was a student with only 9 hours 38 minutes and 83 flights of total glider experience, all on the Rhönlerche II, and was still completing the altitude-gain flights required for his license.
3. Weak, irregular lift: While ridge soaring near Bettlachstock, the glider encountered weak and irregular lift averaging about 1 m/s, and had descended roughly 120 m in the preceding six minutes, likely keeping the pilot close to the slope.
4. Prolonged right turn: At about 1305 while flying along the Jura with the ridge on his left, the pilot initiated a right turn at about 78 km/h and 45° bank to reverse direction along the slope but allowed the turn to continue too long toward the terrain.
5. Late turn termination: The pilot, inattentive to his position relative to the slope, failed to stop the turn in time to re-establish safe separation from the hillside.
6. Collision with trees: With insufficient lateral clearance remaining, the glider could no longer avoid terrain and struck the tops of trees on the steep, forested south face of the Bettlachstock.
7. Crash - no injury: After contacting the trees the glider fell into the forest, was about 80% destroyed, and the pilot escaped uninjured.