What happened
On June 14, 2018, a Schempp-Hirth Ventus-2a glider was being towed from the Esslingen Jägerhaus airfield. Due to a technical defect in the primary tow aircraft, the flight was instead performed using a Falke SF25 C motor glider. Because the motor glider had lower performance capabilities, the pilot of the Ventus-2a had reduced the water ballast in the wings to decrease takeoff weight.
The takeoff sequence began on the paved extension of runway 31. The pilots had previously agreed that the tow should be aborted if the aircraft failed to lift off before the runway centerline mark. During the takeoff roll, the tow aircraft began to climb, but its speed steadily decreased. Witnesses observed the tow aircraft climbing to a higher position while the glider's speed dropped. Shortly after passing the end of the airfield, the towline separated, and the glider entered a right-hand banking turn before crashing.
The glider was located in a hilly nature reserve adjacent to the airfield. The impact resulted in the destruction of the aircraft and the pilot was seriously injured.
The investigation
The BFU examined the flight data from GPS devices carried by both aircraft. The investigation included a material analysis of the broken towline component by the Institute for Materials Science (WIWeB). Investigators also inspected the wreckage of the Ventus-2a, noting that the left wing had separated from the fuselage and the fuselage tube had broken near the registration markings. The investigation also reviewed the technical condition of the Falke SF25 C motor glider, which was found to have no defects.
Findings
- The towline separation was caused by the failure of a weak link (Sollbruchstelle).
- The weak link used was a green TOST component with a rated breaking strength of 300 (+/- 30) daN.
- Material testing revealed the component failed at a load of approximately 270 kg, which was at the lower limit of its rated strength.
- The glider was flying at a speed of approximately 95 km/h at the time of the incident, while the tow aircraft's speed had decreased significantly during its climb.
- There were no signs of operational damage or prior fatigue on the weak link itself.