What happened
On May 16, 1965, two gliders belonging to the Schaffhausen Gliding Group were performing hill soaring near the Siblinger Schlossrand/Randen area. The first aircraft, a Ka-2b with registration HB-733, carrying a pilot and a passenger, had released from its towline earlier in the afternoon. Shortly after, an S-18 IIa, registration HB-315, also released from a tow at approximately 17:05.
The weather conditions involved weak, deteriorating thermal activity and ridge lift. As the pilot of the HB-315 attempted to maintain altitude in irregular updrafts and downdrafts, the aircraft entered a left-hand circling pattern. During this maneuver, the HB-315 collided with the fuselage of the HB-733. The impact caused significant damage to the right wing of the S-18 IIa, including a broken leading edge and damaged ribs. Despite the damage, the pilot of the HB-315 managed to land in a field near the Schaffhausen airfield. The pilot of the HB-733 landed safely at the airfield shortly after. There were no injuries resulting from the collision.
The investigation
The investigation established that both aircraft were airworthy and that no technical malfunctions contributed to the accident. The investigators examined the flight paths of both gliders, noting that they had been flying in close proximity with a vertical separation of only 20 to 50 meters. The investigation focused on the flight maneuvers and the level of situational awareness maintained by both pilots during the period of unstable thermal conditions.
Findings
- The pilot of the HB-315 was heavily focused on flight instruments due to the unstable nature of the thermals, which led to a failure to maintain adequate airspace monitoring.
- The pilot of the HB-315 did not check the airspace above the aircraft, failing to consider that the other glider might be circling in the same thermal column.
- The pilot of the HB-733 lost visual contact with the HB-315 while performing a maneuver to increase separation.
- The irregular updrafts and downdrafts likely caused the vertical separation between the two aircraft to fluctuate, contributing to the collision.