Refer to section 17.0 of the rules, which I think you can download. I quote here for convenience:
Airplanes are represented by an Aerial
Attack Weapon Card whose use is specified
in a Firefight. A Firefight will generally state
the number of times an Aerial Weapon
Card may be used as an action in a Round.
Using an Aerial Weapon Card is an action.
Airplanes attacked from a high angle, so
both blue and red attacks count against the
target's flank DV. Terrain DMs add to the
There are no airplane units in this game.