What happened
On April 13, 2021, a Zlin 50LS, registration SP-AUC, was performing a technical test flight at Poznań Kobylnica (EPPK). After completing engine and flight control checks and performing several aerobatic maneuvers, including autorotative loops, the pilot attempted a touch-and-go maneuver.
The pilot executed a successful three-point landing and began increasing throttle to continue the takeoff roll. However, the aircraft began pulling sharply to the left. The pilot immediately retarded the throttle to idle and attempted to maintain a straight track using maximum rudder deflection, but the aircraft continued to deviate. To prevent a potential ground loop, the pilot applied the right wheel brake. While this stabilized the direction, the aircraft slowed and tilted forward, resulting in a propeller strike. The pilot sustained no injuries and exited the aircraft safely.
Post-flight inspections of the grass runway revealed visible left wheel skid marks extending from the landing point to the end of the takeoff roll. The aircraft sustained damage to the propeller blades and minor damage to the left wing skin near the Pitot tube mounting point.
The investigation
The PKBWL investigation focused on the mechanical state of the braking system and the pilot's actions. During the aircraft inspection, the left wheel brake was found to be locked, though it functioned correctly after manual release.
Detailed examination of the wheels revealed that the wear pattern on the left wheel's brake adjuster cam was significantly longer than that on the right, indicating that an excessive force had been applied to the brake lever during flight. Furthermore, the investigation found that the left wheel drum had become ovalized. This deformation caused the cam to rotate into a position that locked the brake.
Findings
- The primary cause of the incident was the lock-up of the left wheel brake, which occurred because the pilot inadvertently applied maximum force to the brake lever during an aerobatic maneuver.
- A contributing factor was the ovalization of the wheel drum, which allowed the adjuster cam to rotate excessively and trigger the brake lock-up.