A method of operating a hybrid-electric propulsion system of an aircraft, the method comprising: determining a flight phase parameter for the aircraft is equal to a first value; operating the hybrid-electric propulsion system in an electric charge mode in response to determining the flight phase parameter for the aircraft is equal to the first value, wherein operating the hybrid-electric propulsion system in the electric charge mode comprises driving a first electric machine with a first combustion engine to generate electrical power, driving a first prime propulsor with the first combustion engine to generate thrust, and charging an energy storage unit with at least a portion of the electrical power generated; determining the flight phase parameter for the aircraft is equal to a second value different from the first value; operating the hybrid-electric propulsion system in an electric discharge mode in response to determining the flight phase parameter for the aircraft is equal to the second value, wherein operating the hybrid-electric propulsion system in the electric discharge mode comprises providing electrical power from the energy storage unit to at least one of: an electric propulsor assembly to drive the electric propulsor assembly; or to the first electric machine to drive one or more components of the first combustion engine; and modifying operation of the first combustion engine in response to determining the flight phase parameter for the aircraft is equal to the second value, wherein modifying operation of the first combustion engine comprises operating the first combustion engine in an idle or sub-idle mode; characterised in that modifying operation of the first combustion engine comprises operating a second combustion engine of the hybrid-electric propulsion system in a high power mode to mechanically drive a second prime propulsor and further to drive a second electric machine to generate electrical power. determining a flight phase parameter for the aircraft is equal to a first value; operating the hybrid-electric propulsion system in an electric charge mode in response to determining the flight phase parameter for the aircraft is equal to the first value, wherein operating the hybrid-electric propulsion system in the electric charge mode comprises driving a first electric machine with a first combustion engine to generate electrical power, driving a first prime propulsor with the first combustion engine to generate thrust, and charging an energy storage unit with at least a portion of the electrical power generated; determining the flight phase parameter for the aircraft is equal to a second value different from the first value; operating the hybrid-electric propulsion system in an electric discharge mode in response to determining the flight phase parameter for the aircraft is equal to the second value, wherein operating the hybrid-electric propulsion system in the electric discharge mode comprises providing electrical power from the energy storage unit to at least one of: an electric propulsor assembly to drive the electric propulsor assembly; or to the first electric machine to drive one or more components of the first combustion engine; and modifying operation of the first combustion engine in response to determining the flight phase parameter for the aircraft is equal to the second value, wherein modifying operation of the first combustion engine comprises operating the first combustion engine in an idle or sub-idle mode; characterised in that modifying operation of the first combustion engine comprises operating a second combustion engine of the hybrid-electric propulsion system in a high power mode to mechanically drive a second prime propulsor and further to drive a second electric machine to generate electrical power.