A flight management unit includes two guidance subsystems each including a flight management system, each of the flight management systems being configured at least to extract a flight plan from at least one navigation database, to construct a flight trajectory, and to compute guidance commands for the aircraft. The flight management unit also includes at least one monitoring unit configured to compute a guidance command from a validated flight trajectory and a consolidated flight plan and to monitor the guidance command, as well as guidance commands computed by the two flight management systems so as to be able to detect and to identify a defective flight management system.