1.  Field of the Invention
The invention relates to processes and devices for assisting aerial navigation. 
2.  Discussion of the Background
In general, it is known that in aerodynes (aircraft, helicopters, etc.)  of modern design, the pilot must carry out a dialogue with a flight management system, this being a computer on board the aerodyne (we shall subsequently speak of an aircraft) for assisting the pilot in a number of operations.  These operations are mainly operations for defining the flight plan before take-off, piloting (manual or automatic) operations during take-off and landing, aerial navigation operations (path calculations, etc.),  systematic monitoring operations while cruising or when approaching an airport. 
The flight management system operates on the basis of data entered by the pilot, data supplied by sensors distributed throughout the aircraft, and possibly digital data transmitted by radio from the ground or from other aircraft or even satellites ("DATALINK" system which is undergoing generalization). 
The dialogue between the flying crew and the flight management system is carried out mainly by means of at least three interfaces, viz.: 
a navigation display on which is represented the plot of the desired course of the aircraft, that is to say a graphical two-dimensional representation of the flight plan of the aircraft and the situation of the aircraft within this plan; PA1 a primary flight display which depicts an artificial horizon which tilts as the aircraft tilts, an indication of the longitudinal attitude of the aircraft, and other useful flying indications, in particular the mode of guidance of the aircraft; PA1 a flight control unit having manual controls for selecting settings such as the desired heading of the aircraft; PA1 and finally a keyboard/display console, termed the MCDU ("Multipurpose Control Display Unit"), this being a data display and input console allowing the flying crew to enter data into the flight management system and to read information forwarded by the flight management system on the basis of the data entered. 
Among other things, it is the task of the flight management system to manage the navigation instruments used; certain navigation instruments may be used at a given time and others are used at other times.  The navigation instruments are for example the inertial units (IRS), the satellite positioning receivers (GPS), and the radio navigation means (for example DME/DME or VOR/DME). 
It is the flight management system FMS which itself chooses the navigation instruments to be used at a given time, depending on (fairly complex) rules of priority.  If it uses radio navigation beacons, it chooses these beacons automatically.  And it computes estimated errors in the positions delivered by the various navigation instruments, and a resultant mean error.  If a navigation instrument gives a position indication which is too poor relative to the accuracy requested, the flight management system itself manages the change of instruments, via a change of navigation mode. 
However, the pilot must be aware of the choices made automatically by the FMS system since, especially in the event of a problem, the system must not continue to manage an abnormal situation which the pilot would no longer have any knowledge of.