An aircraft engine comprising: a high-pressure spool; a low-pressure spool configured for rotation separate from the high-pressure spool; a controller configured to, during operation of the aircraft engine, identify a transient condition of the aircraft engine and command: conditioned upon the transient condition requiring an increase in rotational speed of the high-pressure spool, injection of power into the aircraft engine by transferring power from a source of power separate from the aircraft engine to the high-pressure spool to at least assist in achieving the required increase in rotational speed of the high-pressure spool during the transient condition, the source of power being part of an electrical system of an aircraft; and conditioned upon the transient condition requiring a decrease in rotational speed of the high-pressure spool, drawing of power from the aircraft engine by transferring power from the high-pressure spool to the electrical system to at least assist in achieving the required decrease in rotational speed of the high-pressure spool during the transient condition, characterised in that the controller is configured to identify the transient condition based on actuation of a throttle lever associated with the aircraft engine. a high-pressure spool; a low-pressure spool configured for rotation separate from the high-pressure spool; a controller configured to, during operation of the aircraft engine, identify a transient condition of the aircraft engine and command: conditioned upon the transient condition requiring an increase in rotational speed of the high-pressure spool, injection of power into the aircraft engine by transferring power from a source of power separate from the aircraft engine to the high-pressure spool to at least assist in achieving the required increase in rotational speed of the high-pressure spool during the transient condition, the source of power being part of an electrical system of an aircraft; and conditioned upon the transient condition requiring a decrease in rotational speed of the high-pressure spool, drawing of power from the aircraft engine by transferring power from the high-pressure spool to the electrical system to at least assist in achieving the required decrease in rotational speed of the high-pressure spool during the transient condition, characterised in that the controller is configured to identify the transient condition based on actuation of a throttle lever associated with the aircraft engine. conditioned upon the transient condition requiring an increase in rotational speed of the high-pressure spool, injection of power into the aircraft engine by transferring power from a source of power separate from the aircraft engine to the high-pressure spool to at least assist in achieving the required increase in rotational speed of the high-pressure spool during the transient condition, the source of power being part of an electrical system of an aircraft; and conditioned upon the transient condition requiring a decrease in rotational speed of the high-pressure spool, drawing of power from the aircraft engine by transferring power from the high-pressure spool to the electrical system to at least assist in achieving the required decrease in rotational speed of the high-pressure spool during the transient condition, characterised in that the controller is configured to identify the transient condition based on actuation of a throttle lever associated with the aircraft engine.