1. An onboard assistance device for aiding the following of an airport route by an aircraft, said device comprising: a system of optical sensors which provides video images of said airport route; means providing the current position of the said aircraft; a database comprising the topological information for the airport on the airport route to be followed by the aircraft; a recognition system which provides, on the basis of the video images, data characteristic of the portion of the airport route seen by the system of optical sensors; a processing computer configured to: extract from the database, a succession of simple geometric shapes representing the stages of the airport route; construct primitives on the basis of the information received from the recognition system, a primitive comprising at least one simple geometric pattern representative of the route portion; compare said primitives with the simple geometric shapes representing the stages of the airport route extracted from the database; validate or invalidate the route followed by said aircraft as a function of the result of the comparison; and a viewing system comprising a viewing screen, said screen displaying a conformal view of the airport route situated in front of the aircraft and a succession of graphical symbols, each graphical symbol being associated with a geometric shape extracted from the database, the succession of symbols representing the stages of the airport route that are already recognized, the current stage, and the future stages.