What happened
On April 15, 2012, a two-seat DG 500 ELAN glider was being prepared for a tow flight at the Laufenselden glider airfield. During the initial attachment process, the tow rope released from the glider's nose coupling. Following the advice of a flight instructor, the rope was reattached, and the aircraft took off at 15:13 local time with two occupants on board.
As the tow formation climbed, the aircraft encountered light turbulence near the edge of the airfield. While flying over a tree clearing at an altitude of approximately 45 meters, the tow rope suddenly detached from the glider. The pilot of the towing aircraft was initially unaware of the separation until notified via radio. Witnesses on the ground observed the glider entering a left turn with a significantly increased angle of attack, subsequently striking treetops before crashing into a forest approximately 25 to 30 meters deep. The impact destroyed the aircraft, resulting in one fatality and one serious injury.
The investigation
The BFU examined the aircraft, the tow rope, and the nose coupling (a Tost E 85). The investigation included a technical review of the coupling at the manufacturer's facility in Munich. Investigators also analyzed GPS data, which confirmed the glider was at an altitude of roughly 45 meters at the time of the separation. The medical examination of the pilot ruled out natural causes such as a heart attack or stroke, attributing the death to multiple injuries sustained during the impact.
Findings
- The investigation could not definitively determine the exact cause of the tow rope releasing from the coupling.
- While the Tost E 85 coupling was found to be in good working order, testing demonstrated that slight tension on the control lever could alter the mechanism's geometry, allowing the rope to unhook even under loads below the standard release threshold.
- The manufacturer's manual warns that control cables must be free of excessive friction to ensure the mechanism returns to a secure closed position; any preload in the cable system could compromise the coupling's integrity.
- The accident was caused by the separation of the tow rope at a low altitude and the subsequent failure to perform an out-of-the-pattern landing, leading the glider into an obstacle-filled turn.