What happened
On 27 May 2017, a DH110 Sea Vixen FAW Mk 2, registration G-CVIX, returned to RNAS Yeovilton following an air show at Duxford. During the approach, the pilot attempted to configure the aircraft for landing by deploying the airbrake and selecting the landing gear and flaps. However, the landing gear failed to unlock or lower using the normal Green hydraulic system.
Upon checking the gauges, the pilot discovered that the Green system pressure had dropped to zero. A subsequent attempt to use the standby Red hydraulic system also failed, as the pressure in that system subsequently fell to zero as well. Fearing a total loss of hydraulic pressure across all systems, the pilot deployed the ram air turbine as a precaution and, after consulting with the operator's Chief Engineer, decided to perform a wheels-up landing.
The aircraft touched down on Runway 27. Because the landing gear was not extended, the weight of the aircraft was initially supported by the underwing drop tanks, which disintegrated upon contact with the runway. The aircraft continued along the centerline for approximately 1,000 metres, causing significant abrasion to the fuselage underside and the accessory gearbox. The pilot successfully brought the aircraft to a stop without fire or further incident and vacated the cockpit safely.
The investigation
The AAIB examined the Red and Green hydraulic systems using a servicing rig, which confirmed that the landing gear could be lowered when external pressure was applied, pointing toward a failure within the pumps themselves.
Disassembly of the Red pump revealed extensive internal damage, including a fragmented piston head collar and significant non-ferrous metallic debris. The Green pump also showed evidence of piston scuffing and metallic debris. Forensic analysis of the piston surfaces identified microscopic silver particles. While the hydraulic fluid quality was generally acceptable, the Green system contained a fine grey silt. Investigators also noted that a hydraulic fuel filter heater switch had been found in a partially activated position, though the cause of this was unknown.
Findings
- The landing gear failed to extend because of a mechanical breakdown within both the normal and standby hydraulic pumps.
- The failure was caused by the seizure of the pistons within the hydraulic pumps.
- This seizure was likely triggered by the presence of a contaminant or debris within the hydraulic fluid.
- The disintegration of the drop tanks and the damage to the fuselage were a direct result of the aircraft landing without gear extended.