Colisión en el aire entre Astir CS y B4 cerca de Zweisimmen durante vuelos de entrenamiento

El 22 de julio de 1979, ocurrió una colisión en el aire entre un Grob Astir CS y un Pilatus B4-PC 11AF durante un campamento de vuelo a vela cerca de Zweisimmen, Suiza. Ambas aeronaves realizaban vuelos de entrenamiento en la zona de térmicas cuando colisionaron a aproximadamente 2300 metros de altitud. El Astir CS sufrió daños menores y aterrizó de manera segura en el aeródromo de Zweisimmen, mientras que el B4, que resultó más dañado, aterrizó en el aeródromo militar de Saint-Stéphane. No se reportaron heridos. La investigación oficial atribuyó la colisión a una observación insuficiente del espacio aéreo por parte de ambos pilotos.

1. Aerotow ridge soaring: After aerotow release at about 2400 m, the Astir CS HB-1332 flew toward the Gandhore ridge lift area to conduct hang soaring training.
2. Impaired visual acuity: The Astir pilot had substandard distance visual acuity and markedly reduced binocular vision, was not wearing corrective glasses, and had not been informed of his visual limitation.
3. Traffic in lift zone: Several other gliders, including the B4 HB-1139 flying slightly higher than the rest near the southern end of the lift band, were already operating in the same Gandhore updraft area.
4. Poor visual contrast: The B4 was flying with a grey-white cloud layer behind it, likely reducing its visual contrast and making it harder to detect from the Astir.
5. Inadequate lookout: As the Astir turned toward a northerly heading to enter the updraft zone, the pilot briefly scanned and saw only lower gliders, then failed to detect the B4 at nearly the same altitude ahead.
6. Converging flight paths: The Astir continued inbound on approximately 0–030° while the B4, at the southern end of the lift, initiated a right turn toward the north with 40–45° bank, placing the two gliders on a collision course.
7. Mid-air collision: Around 16:15 at about 2300 m, the Astir’s left wing struck the B4’s left wing and then its tailplane, after which both pilots retained control and landed safely at different airfields with damage to their gliders.