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A fuel delivery and control system is provided including a dual pump fluid circuit configuration comprising a fixed positive displacement pump sized to supply the main engine burn flow ranging from above windmill through cruise and a variable displacement pump sized to supply fluid to engine actuato

A fuel delivery and control system is provided including a dual pump fluid circuit configuration comprising a fixed positive displacement pump sized to supply the main engine burn flow ranging from above windmill through cruise and a variable displacement pump sized to supply fluid to engine actuators, valves and other hydraulically operated engine components with a pump flow sharing system interconnecting the two pumps. The combined flow from the two pumps is sufficient to meet the engine flow demand for windmill relight and maximum flow conditions. During cruise or normal operating conditions, the pumps operate in completely isolated flow circuits, minimizing recirculation and therefore heat input into the fuel supply system.

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1. A fuel delivery and control system for a gas turbine engine having a maximum engine fuel flow requirement and a fuel metering unit, an actuation system requiring a minimum operating fuel flow and minimum operating pressure, the fuel control system comprising: a variable displacement actuation pum

1. A fuel delivery and control system for a gas turbine engine having a maximum engine fuel flow requirement and a fuel metering unit, an actuation system requiring a minimum operating fuel flow and minimum operating pressure, the fuel control system comprising: a variable displacement actuation pump of a size capable of providing a first portion of the maximum fuel flow requirement of the engine, the actuation pump having an outlet and a discharge pressure; anda fixed displacement main pump of a size configured to provide a second portion of the maximum fuel flow requirement of the engine, the main pump having an outlet and a discharge pressure, the outlet of the main pump in fluid communication with the fuel metering unit and in selective fluid communication with the outlet of the actuation pump, wherein the fuel delivery and control system is configurable between an isolated pumps mode wherein the outlets of each of the actuation pump and the main pump are separated from each other and a shared pumps mode wherein the outlets of each of the actuation pump and the main pump are in fluid communication with each other to combine the first portion of the maximum fuel flow of the engine with the second portion of the maximum fuel flow of the engine and wherein the combined flow is supplied to the fuel metering unit of the engine. 2. The fuel delivery and control system of claim 1, wherein the actuation pump has an inlet and further comprises a pressure regulator assembly to control the discharge pressure at the outlet of the actuation pump to the minimum operating pressure of the actuation system in the isolated pumps mode. 3. The fuel delivery and control system of claim 2, wherein the pressure regulator assembly is a variable pressure regulator. 4. The fuel delivery and control system of claim 1, wherein the discharge pressure of the actuation pump is less than the discharge pressure of the main pump when the system is in the isolated pumps mode. 5. The fuel delivery and control system of claim 1, wherein the actuation pump further comprises a variable control system in fluid communication with the outlet of the actuation pump to provide fuel at the minimum operating fuel flow required by the actuation system. 6. The fuel delivery and control system of claim 1, wherein the main pump is size to provide substantially 20 percent of the maximum fuel flow requirement of the engine and the actuation pump is sized to provide substantially about 80 percent of the maximum fuel flow requirement of the engine. 7. The fuel delivery and control system of claim 1, further comprising a fuel bypass line in fluid communication with the fuel metering unit, the bypass line directing fuel back to an inlet of the main pump. 8. A fuel system for use in an aircraft engine having a maximum fuel flow demand, the fuel system comprising: a fixed displacement pump for pressurizing fuel from a fuel supply, the fixed displacement pump having an inlet and an outlet in fluid communication with a main fuel supply loop, the fixed pump providing fuel at a sufficient flow rate and discharge pressure to meet the demand of the engine at steady state conditions; anda variable displacement pump for pressurizing fuel from the fuel supply, the variable displacement pump having an inlet and an outlet in fluid communication with an actuation supply loop, the variable pump capable of providing output fuel at a sufficient discharge pressure to meet the demand of a plurality of hydraulically actuated devices, the outlet of the variable pump in selective fluid communication with the outlet of the fixed pump, wherein the discharge pressure of the variable pump is less than the discharge pressure of the fixed pump and the main fuel supply loop is isolated from the actuation loop when the outlets of the pumps are isolated and wherein the fixed pump output flow and the variable pump output flow are combined to meet the maximum fuel demand of the engine. 9. The fuel system of claim 8, wherein the fixed pump is sized to provide substantially 20 percent of the maximum fuel demand from the engine and the variable pump is sized to provide substantially about 80 percent of the maximum fuel demand of the engine. 10. The fuel system of claim 8 further comprising a regulator valve assembly in communication with the variable pump for controlling the pressure drop across the inlet and the outlet of the variable pump. 11. The fuel system of claim 10, wherein the regulator valve assembly comprises a variable pressure regulator. 12. The fuel system of claim 8 further comprising a control valve in communication with a regulator valve assembly, the control valve configured for setting a pressure differential across the variable pump. 13. The fuel system of claim 8 further comprising a pumps flow sharing system selectively connecting the outlet of the variable pump and the outlet of the fixed pump. 14. A dual pump fuel supply system for delivering fuel to a fuel metering unit of a gas turbine engine having a maximum fuel demand, the fuel supply system comprising: a fixed delivery main fuel pump having an input and an output in fluid communication with the fuel metering unit; anda variable actuation pump having an input and an output;a flow sharing system interconnecting the fixed pump and the variable pump, the flow sharing system having a first configuration wherein the flow output of the fixed pump is isolated from the flow output of the variable pump and a second configuration wherein the flow output of the fixed pump is combined with the flow output of the variable pump, wherein the combined outputs are supplied to the fuel metering unit of the engine. 15. The dual pump fuel supply system of claim 14, wherein the variable pump further comprises a pressure regulator assembly to control the pressure drop across the input and output of the variable pump. 16. The dual pump fuel supply system of claim 15, wherein the pressure regulator assembly is a variable pressure regulator. 17. The dual pump fuel supply system of claim 16, wherein the variable pump is sized to provide fuel at a sufficient pressure and flow rate to a plurality of hydraulically actuated devices within the engine. 18. The dual pump fuel supply system of claim 14, wherein the variable pump further comprises a variable control system in fluid communication with the output of the variable pump and the flow sharing system. 19. A method of delivering fuel to an aircraft engine having a maximum fuel demand, the method comprising: providing a fixed displacement pump in a main fuel supply loop having an outlet and capable of providing fuel at a discharge pressure, the fixed displacement pump sized to supply a first portion of the maximum fuel demand of the engine;providing a variable displacement pump in an actuation supply loop having an outlet in selective fluid communication with the outlet of the fixed displacement pump, the variable displacement pump capable of providing a second portion of the maximum fuel demand of the engine that is larger than the first portion; andalternating between an isolated pumps mode wherein the variable displacement pump provides fluid flow to an actuation system of the engine at a minimum required discharge pressure such that the main fuel supply loop is isolated from the actuation supply loop and a shared pumps mode wherein the flow output of the fixed displacement pump and the flow output of the variable displacement pump are combined to provide the maximum fuel demand required by the engine. 20. The method of claim 19 wherein in the isolated pumps mode, the discharge pressure of the variable displacement pump is less than the discharge pressure of the fixed displacement pump.