Covering form of preliminary or final report

The glider pilot is a member of the Plzeň - Letkov Aeroclub. He keeps the glider in its disassembled state in a trailer. Per the pilot's statement, the glider assembly takes about 15 - 20 minutes.

On 2 June 2021, before the critical flight, the pilot assembled the glider in the usual way, without any hurry. After its assembly and towing to the launch site, he waited for about 1 hour for better thermic conditions. Then, he took off from runway 26, being towed by the WT9 Dynamic. Once he disconnected, the pilot depressed the control lever to level the glider; however, due to the disconnected elevator connecting rod, his glider did not respond and initiated a steep climb. The pilot disconnected the towline, the glider slowed down, and initiated a steep descent. Subsequently, its nose hit the ground and continued moving forward on the ground up to the runway 08 clearway. The impact with the ground damaged the glider (fuselage and elevator). The pilot was not injured.”

The pilot held the necessary qualification and was medically fit for performing the given flight. He had sufficient flying experience with the aircraft type. The glider had a valid airworthiness certificate and was fit to fly. The weather was suitable for the flight activity and had no impact on the occurrence of the incident.

After assembling his glider, the pilot failed to connect the elevator connecting rod to the elevator and underestimated the check of the correct securing of the elevator connecting rod quick coupler.

The kinematics of the elevator connecting rod's movement causes the connecting rod to extend upwards when the control lever is “pulled”. Since the connecting rod was not mechanically coupled to the elevator but was merely supported by the lower part of the elevator, the elevator could deflect upwards. When the control lever was released, due to the position of the elevator hinge, it deflected downwards spontaneously. The gradual speed increase during the take-off, the aerodynamic forces in the elevator and control surfaces increase. In the case of a completely loose elevator, the imbalance of forces results in a floating motion of the elevator and changes in the glider’s longitudinal pitch. During the aerotow, the glider must be kept at approximately the same altitude as the towing aircraft during the take-off phase, and the control lever must be slightly “depressed”. The disconnected elevator connecting rod joint allowed the uncontrolled movement of the elevator, which caused the glider to change its altitude. This movement featured a non-damped speed-dependent frequency. The glider pilot recognised the longitudinal control fault in time and; therefore, disconnected the towline and aborted the take-off. However, he was unable to prevent the glider from stalling due to its limited controllability.

The glider damage was caused by forces developed when its nose part hit the ground hard.