Engine and secondary power unit integrated operation

A method comprising:

• determining an operating mode of an engine system comprising a gas turbine engine and a secondary power unit, wherein the gas turbine engine comprises a first compressor, a first combustor, and a first turbine, and the secondary power unit comprises a second compressor, a second combustor, a second turbine, a third compressor coupled to the second compressor, and an electric motor-generator, wherein the secondary power unit is coupled to the gas turbine engine, and wherein the secondary power unit further comprises a heat exchanger configured to receive pressurized air from the first compressor;
• selecting an input energy source and an output type of the secondary power unit based on the operating mode;
• controlling the secondary power unit based on the input energy source and the output type as selected;
• detecting a change in the operating mode of the engine system;
• and
• modifying the input energy source and/or the output type of the secondary power unit based on the change in the operating mode.