Add to "Mods\aircraft\Flaming Cliffs\Input\su-25\keyboard\default.lua" for example.
{combos = {{key = "H", reformers = {"RCtrl", "RShift", }}, }, pressed = iCommandPlane_Helmet_Brightess_Down, name = "Gain goggles down", category = "Sensors"},
{combos = {{key = "H", reformers = {"RAlt", "RShift", }}, }, pressed = iCommandPlane_Helmet_Brightess_Up, name = "Gain goggles up", category = "Sensors"},
{combos = {{key = '5'}}, down = iCommandPlaneModeHelmet, name = '(5) Close Air Combat HMD Helmet Mode', category = 'Modes'},
Watch out for key conflicts.
These lines are removed if you update since you're putting them inside the default file instead of the diff file in Saved Games.
This mod makes it look better...
http://forums.eagle.ru/showthread.php?t=100333