Gas turbine propulsion system with variable pitch fan and method for operating the system

A method of operating an aircraft propulsion system that includes a gas turbine engine, an accessory gearbox, and an electric machine, wherein the gas turbine engine includes a spool that includes a fan rotor having a plurality of fan blades and a turbine section, and a pitch control mechanism engaged with the plurality of fan blades, wherein the accessory gearbox is in drive communication with the spool, and the electric machine is mounted on the accessory gearbox, the method comprising:

• controlling a rotational speed of the spool during a descent portion of a flight profile, including controlling the pitch control mechanism to alter an amount of thrust produced by the plurality of fan blades to maintain a speed of the electric machine below a predetermined maximum rotational speed limit, and the controlling the pitch mechanism including altering a pitch of the plurality of fan blades;
• and
• providing rotational drive to the electric machine from the accessory gearbox during the descent portion of the flight profile;
• wherein the descent portion has a constant slope and is a function of the rotational speed of the spool versus altitude.

An aircraft propulsion system, comprising:

• a gas turbine engine that includes a first spool and a pitch control mechanism, wherein the first spool includes a first compressor section, a first turbine section, and a fan section, and the fan section includes a plurality of fan blades, and wherein the pitch control mechanism is engaged with the plurality of fan blades and is configured to alter a pitch of the plurality of fan blades;
• an accessory gearbox in drive communication with the first spool;
• an electric machine mounted on the accessory gearbox, the electric machine including a predetermined maximum rotational speed limit;
• and
• a controller in communication with the pitch control mechanism, the electric machine, and a non-transitory memory storing instructions, which instructions when executed cause the controller to:
• control a rotational speed of the first spool during a descent portion of a flight profile, including controlling the pitch control mechanism to alter an amount of thrust produced by the plurality of fan blades by altering a pitch of the plurality of fan blades and causing a speed of the electric machine to be maintained below the predetermined maximum rotational speed limit;
• and
• control the electric machine to produce electric energy during the descent portion of the flight profile;
• wherein the descent portion has a constant negative slope and is a function of the rotational speed of the first spool versus aircraft altitude.