Fuel delivery and control system including a variable displacement actuation pump supplementing a fixed displacement main pump

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;
• and
• a 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.

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;
• and
• alternating 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.