Yet another WITE question (sorry) Airbases

[Weasel]

Soviet guide says to start building airbases as soon as possible.  Went through the manual using search for build/airbase/ etc and it came up with nothing.  Went to the Matrix site and did a forum search for same, can't find how to build an airbase, only infantry, armour etc.

So how does one build an airbase?  I tried clicking beside a city to make the build button appear, but no airbase listed.  Selected an air command unit and tried seeing if I could build through them but nope.