What happened
On April 15, 2018, a flight training operation involving a tow operation near Schänis, Switzerland, resulted in a contact between the tow rope and the glider. The operation involved an AS202/32TP acting as the tow aircraft and a Discus 2-b glider, registration D-9820.
Upon reaching the target altitude, the glider pilot radioed the intention to release from the tow rope. In response, the tow pilot reduced engine power to decrease tension. However, because the rope was slack, the glider pilot did not perceive the release process occurring and continued flying at a reduced speed and power. Consequently, the tow aircraft continued to climb approximately 10 to 20 meters above the glider's altitude. Shortly after, the tow pilot observed the glider positioned lower in the rearview mirror.
Following the event, the pilot of the HB-HFJ initiated a steep left turn with airbrakes extended. The glider pilot reported that the tow rope had struck the rear fuselage of the aircraft, though the glider remained controllable. The flight concluded with a landing at Schänis airfield.
The investigation
Investigators examined the aircraft following the incident and identified light injuries to one person. The inspection of the D-9820 glider revealed scratches on the cowling caused by the rope dragging across the surface, as well as a bent hinge on the right landing gear door. No damage was found on the tow rope itself. The investigation also noted that the tow aircraft had recently undergone a propeller upgrade, replacing a three-blade propeller with a five-blade version.
Findings
To prevent synthetic ropes from snapping forward or knotting during release, pilots often reduce engine power to decrease tension. However, this procedure carries inherent risks. The investigation determined that the excessive reduction in engine power by the tow pilot, combined with the glider pilot's failure to immediately steer away from the tow path, allowed the tow aircraft to close the distance and strike the glider. The increased braking effect from the new five-blade propeller on the HB-HFJ was identified as a factor that increased the risk of the tow aircraft catching up to the glider during power reduction.