What happened
On April 21, 2004, at approximately 20:30 local time, a SUKHOI SU-31, registration EC-HPV, was performing local aerobatic training maneuvers at an altitude of approximately 1,500 ft over the Casarrubios del Monte Aerodrome in Toledo. During the flight, an oil leak occurred, coating the cockpit windshield and significantly reducing visibility. Simultaneously, the aircraft experienced a loss of engine power, and the pilot was unable to adjust the propeller pitch.
Faced with white smoke and obscured vision, the pilot attempted an emergency landing. The aircraft reached the final third of runway 26, traveling approximately 100 meters along the surface before veering slightly to the left and coming to a stop 20 meters from the end of the runway. The pilot was uninjured, but the aircraft sustained significant damage to its structure and propeller.
The investigation
Investigators focused on the engine and propeller assembly to determine the cause of the power loss and oil leak. An inspection of the powerplant revealed a fractured elbow fitting located at the connection between the propeller governor and the accumulator. While the accumulator and the flexible connecting hose were found to be in good condition, the elbow fitting had failed.
Metallurgical analysis of the broken fitting, an aluminum-copper alloy, showed that the failure was a ductile fracture caused by an instantaneous overload. The study found no significant material defects or manufacturing anomalies that would have caused premature failure. However, the analysis of the threads indicated that the fitting had been subjected to a combination of tensile stress and a small bending component.
Findings
- The failure of the elbow fitting caused oil to escape from the system, which simultaneously obscured the pilot's vision and caused the propeller governor to lose pressure.
- Without sufficient pressure in the governor, the propeller blades moved toward a high-pitch/low-power state, preventing the pilot from maintaining engine performance.
- The sudden pressure spike that fractured the fitting was likely caused by a water hammer effect within the oil lines.
- This hydraulic shock was likely triggered during high-G aerobatic maneuvers, where a sudden change in oil flow—potentially caused by a drop in engine RPM—created an instantaneous pressure surge capable of exceeding the fitting's strength.