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, wherein the on-board video camera is configured to take a series of successive images of the ground; an image processor configured to extract from the images visual features of the landing runway; and a guidance system configured to: determine a horizon line for a first image of the series of successive images; determine a respective demand position for an axis (Δ) of the landing runway and a touchdown point (P) on the landing runway in the first image to bring the aircraft into a nominal approach plane and to align the aircraft with the axis of the landing runway, wherein the demand position of the axis of the landing runway is based on a median vertical axis of the image, and wherein the demand position of the touchdown point is the point of intersection between the median vertical axis and a line parallel to the horizon line and located under the horizon line at a demand distance from the horizon line; estimate the distance between the positions of the visual features and the demand positions of the visual features in the first image; and determine guidance commands of the aircraft based on the estimated distances between the positions of the visual features and the demand positions of the visual features in the first image to bring the visual features to their respective demand positions in the first image.