Head-up display (HUD) is an electronic presentation of flight information projected into the pilot's line of sight, enabling him to see both the outside world and relevant instrument information at the same time (see Fig. 1).

Using HUD the pilot no longer needs to alternate his attention continually between his cockpit instruments 2-3 ft (0·6-0·9 m) away and the outside world effectively at infinity. This continual change which present day pilots must make is first, very tiring and secondly it takes time; time to scan instruments, assimilate the relevant information, move the head and eyes, refocus the eyes on the outside world, adjust perhaps to a different light level and then take in the changed surroundings before returning to check the instruments. This time may add up to several seconds. In normal flight this is acceptable.