1. A system for automatic landing of an aircraft on a landing runway, the system comprising: at least one on-board video camera on board the aircraft, intended to take a series of successive images of the ground; an image processor able to extract from the images visual features of the landing runway; and a guidance system configured and adapted to determine guidance commands of the aircraft to bring it into a nominal approach plane, and to align it with the axis of the landing runway, from the respective positions of the visual features of the landing runway in at least one of the images, where the guidance system comprises visual servo configured to estimate the distance between the positions of the visual features and the demand positions of these elements in an image, and to deduce therefrom guidance commands of the aircraft to bring the visual features to their respective demand positions in the image, where the visual features of the landing runway in an image are the axis (Δ) of the runway and a touchdown point (P) on the runway, where the demand position of the axis of the runway is the median vertical axis of the image, if there is no side wind, and the demand position of the touchdown point is the point of intersection between the median vertical axis and the line parallel to the horizon line, located under this line at a demand distance from it.