Fuel pumps for gas turbine engines have traditionally been fixed delivery, positive displacement type pumps. The critical pump sizing criteria are typically the fuel flow and pressure needed for engine lightoff at cranking speeds (approximately 10% of full speed). However, these pump sizing criteria result in excess fuel delivery at higher engine speeds and altitudes. This excess fuel is typically bypassed back to the pump inlet, resulting in significant fuel heating. In contrast, a variable delivery, positive displacement fuel pump has the ability to vary delivery flow, to thereby match engine demands for a wide range of engine speeds and altitudes. This eliminates the excessive fuel delivery and resulting heat generation inherent in fixed delivery pumps. An approach to controlling fuel flow delivered to the engine is presented in which the differential pressure across a fuel metering valve is regulated by simultaneously-varying the pump displacement and a small amount of bypass flow. This approach results in improved transient response and steady state accuracy at all operating conditions, as compared with alternate methods.

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