What happened
On November 9, 2003, an L-23 Super Blaník glider, registration OK-0240, was performing a flight for a pilot proficiency check. The glider was being towed by a Z-142 aircraft. After a successful takeoff and climb, the glider pilot requested an increase in altitude to 1,200 meters. Upon reaching this altitude, the glider released from the towplane.
Following separation, the glider entered a steep descent at an angle of approximately 45 degrees. During an attempt to recover from this descent, the aircraft transitioned into a sudden, near-vertical dive. As the aircraft accelerated, it underwent a catastrophic structural failure at an altitude of approximately 580 meters. The glider broke into several pieces, and the two crew members perished upon impact.
The investigation
The ÚZPLN investigation focused on the structural integrity of the aircraft's control surfaces and the sequence of the flight path. Technical experts from VUT Brno and LOM Praha analyzed the wreckage, specifically examining the elevator control linkage. Metallurgical analysis of the elevator control linkage's clevis (wahadlo) revealed that while the steel composition was correct, the weld joints contained defects—such as lack of fusion and impurities—that acted as initiation points for fatigue damage.
Flight mechanics modeling determined that the primary failure occurred during the recovery maneuver. The investigation established that the failure of the left clevis side led to a loss of pitch control, forcing the glider into a high-speed vertical dive. This rapid acceleration caused the subsequent destruction of the left wing and the horizontal stabilizer.
Findings
- The primary cause of the accident was the failure of the elevator control linkage clevis, which resulted in a total loss of longitudinal control.
- Fatigue-related damage, initiated by substandard welding in the control linkage, led to the separation of the left clevis side.
- The resulting high-speed dive caused aerodynamic loads that exceeded the structural limits of the airframe, leading to the breakup of the left wing and tail surfaces.
- The crew attempted to initiate an emergency egress, but the pilot failed to correctly operate the canopy jettison lever, preventing successful evacuation.