Impassable to Air, Grayed Out?

[Absolutrayne]

I have a map in the making, and one of the key problems as I watch AI's battle it out in Skirmish, is that any air used in the map flies over the Mountains, and basically wrecks a base without thinking.

I tried to use the brush setting for Impassable to Air, and its grayed out, and I cant seem to figure out how to change that.

Can anyone give me a hand here? http://forums.cncnz.com/style_emoticons/default/unsure.gif

[Derek]

Impassable to air :wtf:

I'm guessing that thats a removed feature, I don't know what justification there could possibly be for making ground that is impassable to air.