A HELMET-MOUNTED DISPLAY (H-MOUNT-D - PRONOUNCED "H-MOUNT-D") is a device used in some modern aircraft, especially combat aircraft. HMDs project information similar to that of head-up displays (HUD) on an aircrew’s visor or reticle, thereby allowing him to obtain situational awareness and/or cue weapons systems to the direction his head is pointing. Applications which allow cuing of weapon systems are referred to as helmet-mounted sight and display (HMSD) or helmet-mounted sights (HMS).
While conceptually simple, implementation of aircraft HMDs is quite complex. There are many variables