ASK 18B and ASW 15 collision near Hasenmatt, Switzerland with no injuries
On July 11, 1987, an ASK 18B and an ASW 15 collided near the west flank of Hasenmatt in Selzach, Switzerland. The ASK 18B pilot attempted to avoid the approaching ASW 15 by executing a tight turn, but turbulence caused a temporary loss of control. The ASK 18B's left wing struck the ASW 15's right wing, severing a 2.5-meter section. The ASW 15 spiraled down from approximately 100 meters, landing on a tree and then the ground. Both pilots were unharmed. The ASK 18B sustained minor damage, while the ASW 15 was destroyed.
- Thermaling after tow: After aerotow from Grenchen, the ASK 18B was thermaling on the west side of Hasenmatt among several other gliders in rough, locally turbulent lift.
- Crowded soaring area: Between Grenchenberg and Weissenstein, 10–15 gliders were operating in the same general area, increasing traffic complexity and the need for separation.
- Rough turbulent thermals: The thermals were described as irregular and rough with local medium to strong turbulence, making stable circling more difficult.
- Tightening turn for spacing: Seeing the ASW 15 approaching from the left about 50–100 m lower, the ASK 18B pilot tightened his right-hand turn to give the other glider more room toward the slope.
- Wing lifted and stall: As bank was increased, a gust lifted the ASK 18B’s left wing, the aircraft briefly entered an uncontrollable attitude with an aerodynamic stall and nose drop, and it rapidly lost vertical separation with the ASW 15.
- Wing-to-wing collision: Before the ASK 18B could be fully recovered to level flight, its left wing struck the ASW 15’s right wing from behind, tearing off a 2.5 m section and sending the ASW 15 into a steep spiral from about 100 m AGL.
- Mid-air collision: The ASW 15 crashed through a fir tree to the ground and was destroyed while the pilot was uninjured, and the ASK 18B, with minor left-wing damage, remained controllable and landed safely back at Grenchen.