What happened
On August 12, 2013, a professional instructor was operating a PZL-104 Wilga for the purpose of performing two glider tow sorties. After completing the pre-flight inspection and documentation, the pilot took off while towing a Bocian glider. At an altitude of approximately 500 m AGL, the pilot performed a wing-rocking maneuver to signal the glider crew to release. During this specific maneuver, the left door of the aircraft opened and became lodged on the wing strut.
Following the release of the glider, the pilot began a descent for landing. During this phase, the pilot noted a significant increase in aerodynamic drag. After the fourth turn, while maintaining a speed of 120 km/h, the pilot fully extended the flaps. As the aircraft approached the runway, it lost airspeed more rapidly than usual, eventually stalling at 100 km/h. The resulting hard landing caused damage to both propeller blade tips.
The investigation
The investigation, conducted by the operator, examined the sequence of events leading to the propeller damage. The inquiry focused on the mechanical state of the aircraft during the tow maneuver and the aerodynamic changes observed during the final approach. The investigation confirmed that the door's failure to remain closed contributed to the increased drag that led to the stall.
Findings
- The primary cause of the incident was the inadequate check of the left door's closure and locking mechanism prior to takeoff.
- The aircraft experienced a stall during the landing phase, which directly resulted in the hard landing and subsequent damage to the propeller blades.