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Accident investigation report completed and information captured
Narrative: The Sea Vixen made a belly landing at Yeovilton. Nobody were injured. Damage appeared to be confined to the underwing tanks and the lower rear fuselage. The pilot jettisoned the canopy upon touchdown to facilitate egress.
According to a statement from Naval Aviation Ltd, the owner of G-CVIX (XP924): "Following the Sea Vixen’s successful display closing the show at Duxford Air Show, upon her return the Sea Vixen suffered hydraulic problems and the Pilot, Commander Simon Hargreaves, performed a highly controlled wheels up landing at RNAS Yeovilton. Pilot is safe and well. Post accident processes are currently being carried out. Unfortunately the aircraft will not be displaying at Duxford Air Show tomorrow."
The hydraulic failure which prevented the undercarriage lowering also meant the flaps could not be lowered, so touch down was at a higher speed. the impact left cracks in both of tail booms, a badly damaged gear box and warped bulk heads in her engine compartment with repairs estimated at >£2 million.
AAIB Report:
Analysis
It is not known exactly when either of the pump pistons seized. However, since the landing gear, flaps and airbrake appeared to be working during the flying display, it is likely the hydraulic pump problem arose during the flight back to Yeovilton. Most of the Green hydraulic pump pistons had seized in their carrier bores whilst either in their minimum stroke position or as they increased to their maximum stroke in response to demand. That demand was made on return to Yeovilton with the airbrake, as servo pressure dropped and the servo linkage attempted to change the angle of the swashplate to increase the stroke of the pistons. However, with the pistons seizing, the piston heads were caused to jam against the swashplate track and resist its rotation breaking, the quill drive. At this point the Green system hydraulic pressure fell to zero. The aircraft appears to have sustained the loss of two hydraulic systems simultaneously, caused by failure of both the Red and the Green system hydraulic pumps. However, it is possible that the design of the Red and Green systems meant the Red system had been in a failed condition in advance of the Green system but manifested itself in each system simultaneously as demands were made. It is not known exactly when either of the systems failed but it is likely that the systems were operating correctly during the flypast when the pilot demonstrated items with high hydraulic demand, such as the landing gear, flaps and arrester hook. The Yellow and Blue hydraulic systems appear to have been unaffected.
Conclusion
The failure mechanisms appeared identical in the Red and Green pumps with the seizure of the pistons caused by a yet unidentified source of contamination or debris in the hydraulic fluid. The presence of microscopic silver particles suggests component wear within the system, but at the time of publication, a source or a component has not been identified. The unexpected activation of a dormant heater circuit within the Red or Green system is of interest as a possible source of contamination. Work will continue to establish the cause of the hydraulic pump piston seizure in this aircraft.