This addon implements helmet-mounted displays and is designed to be part of the Realism Addon. The HMD provides altitude, airspeed, Mach, and heading information as well as a crosshair.
KNOWN BUGS: When you are in an aircraft with HMD compatibility and you switch to another aircraft with HMD capability (F-22 -> HAL Tejas, for example), the HMD doesn't show up in the second aircraft.