A gas turbine engine includes a combustor having a combustor air inlet, an axial-centrifugal compressor, a shroud, a secondary flow duct, and a valve. The shroud surrounds at least a portion of the axial-centrifugal compressor and has a surge bleed plenum defined therein that is in fluid communication with, and receives compressed air from, the axial compressor outlet. The secondary airflow duct has a duct inlet that is in fluid communication with the surge bleed plenum, and a duct outlet that is in fluid communication with the combustor air inlet. The valve is mounted on the secondary airflow duct and is movable between a closed position, in which the secondary airflow duct does not provide fluid communication between the surge bleed plenum and the combustor air inlet, and an open position, in which the secondary airflow duct provides fluid communication between the surge bleed plenum and the combustor air inlet.

A fuel pump system, comprising an inlet line and a motorized pump, a first selector valve in fluid communication with an inlet line at a first inlet port and in fluid communication with the motorized pump outlet via a first outlet branch at a second inlet port to receive fuel from the motorized pump outlet. The system can also include a main fuel pump in fluid communication with the first selector valve and a second selector valve, wherein the second selector valve can be in fluid communication with the main fuel pump and in fluid communication with the motorized pump outlet.

A fuel pump system, comprising an inlet line and a motorized pump, a first selector valve in fluid communication with an inlet line at a first inlet port and in fluid communication with the motorized pump outlet via a first outlet branch at a second inlet port to receive fuel from the motorized pump outlet. The system can also include a main fuel pump in fluid communication with the first selector valve and a second selector valve, wherein the second selector valve can be in fluid communication with the main fuel pump and in fluid communication with the motorized pump outlet.

Methods and systems of starting a gas turbine engine are provided. During startup, a fuel pressure associated with a primary fuel supply of the gas turbine engine is monitored. A low-pressure event for the primary fuel supply is detected when the fuel pressure falls below a predetermined threshold. Responsive to detecting the low pressure event, an electric backup boost pump is activated by an engine controller to provide fuel to the gas turbine engine.

Methods and systems of starting a gas turbine engine are provided. During startup, a fuel pressure associated with a primary fuel supply of the gas turbine engine is monitored. A low-pressure event for the primary fuel supply is detected when the fuel pressure falls below a predetermined threshold. Responsive to detecting the low pressure event, an electric backup boost pump is activated by an engine controller to provide fuel to the gas turbine engine.

A propulsion system including: a fuel cell assembly having a fuel cell defining an outlet positioned to remove output products from the fuel cell and a fuel cell assembly operating condition; a turbomachine comprising a compressor section, a combustion section, and a turbine section arranged in serial flow order, the combustion section configured to receive a flow of aviation fuel from an aircraft fuel supply and further configured to receive the output products from the fuel cell; and a controller comprising memory and one or more processors, the memory storing instructions that when executed by the one or more processors cause the propulsion system to perform operations including receiving data indicative of a mid-flight flameout within the combustion section; modifying the fuel cell assembly operating condition in response to receiving data indicative of the mid-flight flameout within the combustion section; and initiating a relight of the combustion section.

A propulsion system including: a fuel cell assembly having a fuel cell defining an outlet positioned to remove output products from the fuel cell and a fuel cell assembly operating condition; a turbomachine comprising a compressor section, a combustion section, and a turbine section arranged in serial flow order, the combustion section configured to receive a flow of aviation fuel from an aircraft fuel supply and further configured to receive the output products from the fuel cell; and a controller comprising memory and one or more processors, the memory storing instructions that when executed by the one or more processors cause the propulsion system to perform operations including receiving data indicative of a mid-flight flameout within the combustion section; modifying the fuel cell assembly operating condition in response to receiving data indicative of the mid-flight flameout within the combustion section; and initiating a relight of the combustion section.

An aircraft system includes a component configured to operate with a minimum power demand. The aircraft system also includes an auxiliary power unit including an engine. The auxiliary power unit is configured to power the component and to operate the engine in a plurality of operating modes including a power mode and a standby mode. The auxiliary power unit generates a first power output at least equal to the minimum power demand during the power mode. The auxiliary power unit generates a second power output less than the minimum power demand during the standby mode.

An aircraft system includes a component configured to operate with a minimum power demand. The aircraft system also includes an auxiliary power unit including an engine. The auxiliary power unit is configured to power the component and to operate the engine in a plurality of operating modes including a power mode and a standby mode. The auxiliary power unit generates a first power output at least equal to the minimum power demand during the power mode. The auxiliary power unit generates a second power output less than the minimum power demand during the standby mode.

Systems and methods for shutting down a gas turbine engine in response to a severe mechanical failure include determining a rate of change of one or more process conditions. If the rate of change of the one or more process conditions exceeds a respective predetermined failure threshold, a potential severe mechanical failure of the gas turbine engine may be determined. Steps may be taken to confirm the potential severe mechanical failure of the gas turbine engine. In response, an engine restart is prevented.