Mid-air collision between Astir CS and B4 near Zweisimmen during training flights

On July 22, 1979, a mid-air collision occurred between a Grob Astir CS and a Pilatus B4-PC 11AF during a gliding camp near Zweisimmen, Switzerland. Both aircraft were conducting training flights in the thermaling area when they collided at approximately 2300 meters altitude. The Astir CS sustained minor damage and landed safely back at Zweisimmen airfield, while the B4, which was more severely damaged, landed at the military airfield in Saint-Stéphane. There were no injuries reported. The official investigation attributed the collision to insufficient observation of the airspace by both pilots.

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.