What happened
On September 7, 2013, at approximately 08:45 UTC, a Sukhoi SU 29, registration OK-HXA, was performing an aerobatic display at the Skuteč airfield as part of a public event. The flight was part of a program organized for children with disabilities.
During the performance, the pilot was executing a knife-edge flight at an altitude of approximately 60–70 meters and a speed of 280 km/h. While the aircraft was in a left knife-edge position with a 30-degree bank, the airflow was hitting the fuselage from the left side. During this maneuver, the canopy detached from the aircraft. The canopy first released from the forward lock and then separated from the rear center hinge, passing to the right of the vertical stabilizer. The canopy tumbled several times before impacting the ground approximately 300 meters from the start of runway 13.
The pilot immediately terminated the aerobatic sequence, transitioned the aircraft into level flight, and adjusted the speed to a safe value for flight without a canopy. After the organizers cleared the debris from the runway, the pilot landed the aircraft safely.
The investigation
The ÚZPLN investigation focused on the mechanical state of the canopy locking mechanism and the conditions during the flight. The investigation established that the pilot was properly qualified and the aircraft held a valid certificate of airworthiness.
Technical inspections of the OK-HXA revealed that the forward guide pin had been broken and the forward lock hook showed signs of wear. The acrylic glazing of the canopy was shattered upon impact, though the composite frame of the canopy was not visibly deformed. The investigation also examined the impact of aerodynamic forces during asymmetrical airflow, such as during knife-edge flight, on the structural integrity of the locking components.
Findings
- The primary cause of the incident was the wear of the forward canopy lock hook and the subsequent separation of the canopy during aerobatic maneuvers.
- Structural deformations in the composite canopy frame, occurring during the opening and closing of the canopy, led to operational wear of the locking elements and increased tolerances between parts.
- During the knife-edge maneuver, asymmetrical aerodynamic loading caused a lateral shift of the canopy. This movement caused the forward lock hook to slip out of its engagement point on the fuselage and resulted in the breakage of the forward guide pin.
- While the increased tolerances might have caused vibrations or changes in cockpit noise, these symptoms were not detected by the crew during previous operations.