Head up display system, associated display system and computer program product

The invention relates to a head-up display system located on-board an aircraft (12) that is controllable by a pilot (14) and comprising a head-down display screen (20). The display system comprises: - a head-up display screen (28) configured to be placed in the cockpit (17) in a plurality of positions and to be observed in a visual field (26) by the pilot from an observation point (24); at least one position, called the interposition position, of the head-up screen corresponding to a position in which one portion, called the interposed portion (34), is placed facing one portion (36) of the head-down screen in the visual field between the point of observation and this portion; - a display control module (30) configured to display a conventional display zone (38), and, when the head-up screen is in the interposition position, an unconventional display zone (42) in the interposed portion.