What happened
On August 1, 2022, at approximately 10:11 AM, a LS-6a glider, registration D-6378, was performing a powered tow departure from Thun Airport (LSZW). The glider was being towed by a Piper PA-18-180M, registration HB-OQB. The takeoff from runway 32 proceeded without initial difficulty.
Roughly 40 seconds after liftoff, while the aircraft were at an altitude of approximately 630 meters, the towline became disconnected from the towplane. Following the separation, the glider entered a right-hand turn. Observers noted a fine mist of water spraying from the glider, which was carrying 40 liters of water ballast in its wing tanks. The aircraft continued its turn toward the west-southwest before impacting a cornfield at low altitude. The pilot managed to exit the destroyed aircraft but sustained serious injuries and was evacuated by medical services and helicopter.
The investigation
Investigators examined the towline, which was found 50 meters below the separation point. The 50-meter rope featured a weak link with a breaking strength of 1000 daN. One end of the rope utilized a connection ring pair with no specific technical specification, while the other end used an LN 65091 ring pair. The rope and its components showed no signs of physical damage.
Testing was conducted on the two different release mechanisms. The glider's Tost G73 release mechanism functioned correctly, allowing the rope ends to engage without jamming and showing no tendency for spontaneous release under various angles of force. However, when testing the Farner release mechanism on the HB-OQB, investigators found that the connection ring pairs could become jammed. This-jamming prevented the knee lever from properly engaging, which can lead to unintended release of the towline.