Fuel delivery and control system including a positive displacement actuation pump with a variable pressure regulator supplementing a fixed displacement main fuel pump

A fuel delivery and control system for a gas turbine engine having a maximum fuel flow requirement and a steady state requirement that is less than the maximum fuel flow requirement and also including a fuel metering unit, the fuel delivery and control system comprising:

• a fixed displacement actuation pump of a size limited to providing only a first portion of the maximum fuel flow requirement of the engine, the actuation pump having an inlet and an outlet;
• a variable pressure regulator in fluid communication with the outlet of the actuation pump, the variable regulator capable of adjusting the pressure drop across the inlet and the outlet of the actuation pump to maintain a minimum discharge pressure;
• and
• a fixed displacement main pump of a size limited to providing a second portion of the maximum fuel flow requirement of the engine, the second portion being less than the maximum fuel flow requirement of the engine, the main pump having an outlet 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.