What happened
On the afternoon of April 22, 1978, a mid-air collision occurred north of Montricher, Switzerland, involving two gliders. The first aircraft, an Elfe S 4 with registration HB-1155, was performing a right-hand spiral in an ascending thermal at approximately 1,550 meters. A second glider, also an Elfe S 4, was flying nearby at a higher altitude.
Simultaneously, a third glider, an IS-29 D 2 registered as HB-137 and HB-1378, had recently released from its tow plane. The pilot of the HB-1378 attempted to fly a straight path to position the aircraft beneath the other two gliders while maintaining a safe distance. During this maneuver, the HB-1378 struck the HB-1155. The impact of the HB-1378 cockpit against the rear fuselage of the HB-1155 caused the latter to lose its tail section.
Following the structural failure, the HB-1155 plummeted into a field. The pilot of the HB-1155 successfully escaped using a parachute, sustaining a minor leg injury. The pilot of the HB-1378 suffered a concussion but managed to land the damaged aircraft at the nearby Montricher airfield. Both pilots sustained two minor injuries.
The investigation
Investigators examined the wreckage of both aircraft and analyzed the flight paths. Physical evidence, including scratches on the wings and fragments of the canopy found in the wreckage, allowed investigators to reconstruct the relative positions of the aircraft. The analysis confirmed that the HB-1378 was positioned below and to the right of the HB-1155 at the moment of impact. The investigation also reviewed the maintenance records and pilot licenses, finding both aircraft to be airworthy and both pilots to be properly licensed for the operation.
Findings
The primary cause of the accident was the insufficient observation of the airspace by the pilot of the HB-1378. While the pilot likely identified the other gliders initially, the investigation concluded that the pilot lost visual contact with the HB-1155 for a brief period, leading to a convergence of flight paths that resulted in the collision.