A method for managing an electrical power supply system (1) of an aircraft, the power supply system (1) comprising at least one distribution bus arranged so as to power the electrical loads, at least one generator (G1, G2) of a propulsion engine of the aircraft, at least one engine-class main power unit (MPS1, MPS2), a plurality of contactors (C1-C18) which are capable of electrically connecting the distribution buses to the propulsion engine generator (G1, G2) and/or the main power unit (MPS1, MPS2); and a management module which is capable of controlling the contactors (C1-C18), a method wherein, during normal operating conditions of the aircraft, the distribution bus is powered by the engine-class main power unit (MPS1, MPS2) and, during the back-up operating mode of the aircraft, the distribution bus is powered by the propulsion engine generator (G1, G2).