ERA20FA013 · Sportine Aviacija LAK17B FES

The pilot of the front electric sustainer (FES) engine-equipped glider departed on a cross-country “out-and-return” flight. Another glider pilot, who was completing the same route, stated that the accident pilot reported via radio just before the accident that he was climbing in a “weak thermal.” A witness on the ground reported that he saw the glider about 1/2 mile away heading straight down to the ground. The glider impacted terrain in a near-vertical, nose-down attitude; the forward cockpit was crushed. Examination revealed no evidence of preimpact mechanical malfunctions or anomalies that would have precluded normal operation. Data downloaded from the onboard collision avoidance system revealed that, during the final approximate 15 minutes of flight, the glider's altitude gradually decreased from about 2,300 ft to about 1,400 ft, and, during this time, the glider completed several 360° turns consistent with attempting to climb in thermal lift. The final data points indicated that the glider completed a left 180° turn followed by a right 360° turn before the data ended in the vicinity of the accident site. The data did not record speed, bank angle, or pitch attitude parameters. Whether or to what extent the pilot had used the electric motor during the flight could not be determined. Based on the available information, it is likely that the pilot exceeded the glider’s critical angle of attack while climbing in thermal lift, which resulted in an aerodynamic stall/spin and impact with terrain.

Verbatim NTSB analysis and probable cause from the NTSB dataset