Flight control system for an aircraft, comprising a set of actuators for controlling the aircraft and a set of flight control computers designed to control the actuators of the set of actuators, the set of flight control computers is made up of a set of computers, called main computers (10a, 10b, 10c, 10d), and of at least one backup computer, - each of the main computers is of the duplex type, comprising two modules (10com, 10mon) respectively acting in control mode and in monitor mode; - all the main computers (10a, 10b, 10c, 10d) are configured to implement auto-pilot laws for the aircraft; - the set of main computers comprises two computers (10a, 10b) from a first hardware type, these computers being configured to control actuators of the set of actuators as per a first tolerance level; and characterized in that: - the set of main computers comprises two computers (10c, 10d) from a second hardware type, different from the first hardware type, these computers being configured to control actuators of the set of actuators as per a second tolerance level, less stringent than the first tolerance level, wherein, for a main computer, the tolerance level corresponds to an authorized difference between, on the one hand, orders determined by the module acting in control mode and, on the other hand, corresponding orders determined by the module acting in monitor mode and, for as long as the difference between the orders determined respectively by the module acting in control mode and by the module acting in monitor mode is less than this authorized difference, the module acting in monitor mode does not detect an error and the main computer remains operational and, when this difference is greater than the authorized difference, the module acting in monitor mode detects failure of the computer. the set of flight control computers is made up of a set of computers, called main computers (10a, 10b, 10c, 10d), and of at least one backup computer, - each of the main computers is of the duplex type, comprising two modules (10com, 10mon) respectively acting in control mode and in monitor mode; - all the main computers (10a, 10b, 10c, 10d) are configured to implement auto-pilot laws for the aircraft; - the set of main computers comprises two computers (10a, 10b) from a first hardware type, these computers being configured to control actuators of the set of actuators as per a first tolerance level; and characterized in that: - the set of main computers comprises two computers (10c, 10d) from a second hardware type, different from the first hardware type, these computers being configured to control actuators of the set of actuators as per a second tolerance level, less stringent than the first tolerance level, - each of the main computers is of the duplex type, comprising two modules (10com, 10mon) respectively acting in control mode and in monitor mode; - all the main computers (10a, 10b, 10c, 10d) are configured to implement auto-pilot laws for the aircraft; - the set of main computers comprises two computers (10a, 10b) from a first hardware type, these computers being configured to control actuators of the set of actuators as per a first tolerance level; and characterized in that: - the set of main computers comprises two computers (10c, 10d) from a second hardware type, different from the first hardware type, these computers being configured to control actuators of the set of actuators as per a second tolerance level, less stringent than the first tolerance level, - the set of main computers comprises two computers (10c, 10d) from a second hardware type, different from the first hardware type, these computers being configured to control actuators of the set of actuators as per a second tolerance level, less stringent than the first tolerance level, wherein, for a main computer, the tolerance level corresponds to an authorized difference between, on the one hand, orders determined by the module acting in control mode and, on the other hand, corresponding orders determined by the module acting in monitor mode and, for as long as the difference between the orders determined respectively by the module acting in control mode and by the module acting in monitor mode is less than this authorized difference, the module acting in monitor mode does not detect an error and the main computer remains operational and, when this difference is greater than the authorized difference, the module acting in monitor mode detects failure of the computer.