These navigation aid systems conventionally comprise an FMS (Flight Management System) flight management system. The flight management system is used by a computer installed on board an aircraft. The means for constructing a flight plan are functions of the computer of the FMS.
Conventionally, a flight plan defines the route that the crew of an aircraft plans to follow in order to go from a departure position to a destination point of its mission as well as the conditions along this route.
The flight plan comprises initial flight constraints on respective parameters representative of a path (four-dimensional, 3D and time/speed) the aircraft is supposed to follow between its position of departure and a point of destination. The parameters representative of the path that the aircraft is supposed to follow can correspond to the position of the aircraft, that is to say its position in a horizontal plane and its altitude, its speed, its vertical speed, its heading, its roll, its pitch, the monitoring of radio signals, the roll, the pitch, the thrust, of waypoints, of a volume in which the aircraft must be around an airport, etc.
The flight constraints associated with these various parameters are values that these parameters must take or ranges of values within which the various parameters must be included. It is, for example, the lateral path (in a horizontal plane) of the aircraft or its vertical path (in a vertical plane), a minimum or maximum altitude, a heading to take, positions of waypoints through which the aircraft must pass, a minimum thrust to be achieved, a maximum speed to reach, etc.
In commercial flight, the aircraft is supposed to follow a flight plan called the active flight plan which comprises initial constraints on parameters representative of a path that the aircraft is supposed to follow initially. Any deviation of the path actually followed by the aircraft with respect to the initial path must be caused by an event which is:                either an external instruction coming from an external center, for example AOC (Air Operation Center) centers or ATC (Air Traffic Control) centers, or an external agreement originating from one of these centers,        or a problem within the aircraft (cabin problem or failure of one of the aircraft's piecessw of equipment),        or a problem related to the environment of the aircraft (meteorological problem, terrain risk or risk of collision related to traffic).        
Reciprocally, a certain number of instructions or of problems must give rise to a deviation of the path actually flown by the aircraft with respect to the initial path (more or less long term).
However, the pilot is free to depart or not from the initial path when one of the events listed above occurs. He can also take the initiative to depart from the initial path without one of these events having occurred.
The pilot can thus freely cause the aircraft to deviate substantially from the path that it is supposed to follow (which can be the initial path or a new path adapted to the situation). The consequences of such a deviation can result in a reduction of the fuel reserve, of the safety of the path with respect to obstacles, a violation of noise restrictions, overflying restrictions, penetration into prohibited airspace. In the case of failure of an equipment of the aircraft, the pilot may not divert the aircraft to a path adapted to the situation and this can result in a reduction of the safety margins for the continuation of the flight.