What happened
On May 25, 2015, an SZD-9bis Bocian 1E glider, registration SP-3042, was performing a simulated emergency procedure at Warsaw-Babice (EPBC) airport. The flight, operated by Aeroklub Warszawski, involved an instructor and a student pilot practicing a specific maneuver: simulating a winch power failure at a low altitude.
During the maneuver, the winch operator cut the tension at a low altitude. As the crew transitioned to a glide, the crew released the cable from the glider at nearly the same moment the tension was cut. Due to the high elasticity of the synthetic winch cable, the unweighted line surged forward. The shock absorber (drogue) on the cable inflated and rose alongside the fuselage, eventually drifting back toward the wing. The end of the cable became wrapped around the right wing and became lodged in the aileron gap.
This entanglement created significant drag as the cable dragged along the grass, pulling the glider into a shallow, uncontrolled turn. Additionally, the jammed aileron prevented the pilot from using lateral control to compensate for the deviation. The glider subsequently struck the ground in a side-slip, resulting in damage to the main landing gear and the lower fuselage. There were no injuries to the crew.
The investigation
The investigation, conducted by the user's commission, examined the mechanics of the cable behavior and the flight dynamics during the simulated failure. The investigation focused on the properties of the synthetic winch cable compared to older steel versions, the timing of the cable release, and the aerodynamic effects of the inflating shock absorber. The investigation also reviewed the pilot's training and the specific parameters of the low-altitude maneuver being practiced.
Findings
- The primary cause was the crew's overly rapid response to the planned power failure, specifically releasing the cable at the same time the winch tension was cut, which allowed the cable to overtake the glider.
- The low angle of the cable relative to the ground at the moment of release prevented the cable from being pulled downward away from the aircraft.
- The reduced mass of the synthetic cable, compared to traditional steel cables, resulted in a slower rate of descent for the line, keeping it in the glider's flight path longer.
- Contributing factors included a sudden thermal gust that lifted the cable's shock absorber and a distance line between the shock absorber and the glider attachment that was too short, increasing the risk of the glider flying "under" the cable.