Method of Controlling an Aircraft Electrical Power Generation System

A method of operating an electrical power generation system (100) on an aircraft (900). The method comprises assessing a required electrical power of the aircraft (900), assessing whether a first (102) and a second (53) electrical power source are able to provide the required electrical power in combination, assessing a predetermined condition, determining an operating mode of the first and second electrical power sources (102, 53) to match the predetermined condition, and operating the first and second electrical power sources (102, 53) according to the determined operating mode.