Can attack fliers, but can ignore patrolling fliers. Deals combat damage to fliers it fights or that fly over this while it patrols.

Anti-air Rulings

You don't have to attack patrolling fliers with your anti-air attacker. You can ignore them and attack something else if you want.Sirlin, 03/14/16
When one of your ground forces WITH anti-air gets attacked by a flier, each deals combat damage to the other. If your ground unit or hero doesn't have anti-air though, the flier will deal its combat damage "for free" to your ground unit or hero, and you won't get to hit back.Sirlin, 03/14/16
When something "flies over" your patroller with anti-air, that means it used FLYING in particular to ignore your patroller. For example, if an attacker has stealth and flying and you don't have a detector, then the attacker doesn't even need to use flying to ignore your patroller. Stealth allows it to ignore your patroller so no "fly over" happened there and your anti-air patroller will not hit the attacker.Sirlin, 03/14/16
If you have multiple patrollers with anti-air, they all deal combat damage to a flier that flies over all of them. For example, if you have a squad leader and two other patrollers, all three of which have anti-air, and a flier attacks your tech II building, then the flier deals its combat damage to the tech II building simultaneously as the three anti-air patrollers deal combat damage to the flier. But if that same flier were to attack just your squad leader, your two OTHER patrollers wouldn't deal combat damage to it because it did not "fly over" them. Or if that same flier attacked one of those two patrollers, the squad leader would deal combat damage to it (and the anti-air patroller it attacked would), but the third anti-air patroller would not deal combat damage to it because the flier didn't "fly over" that particular patroller.Sirlin, 03/14/16
All combat damage (except swift strike) is dealt simultaneously, and anti-air is not an exception. Anti-air combat damage is dealt at the same time as everything else, unless it has swift strike, in which case it's dealt before combat damage that doesn't have swift strike.Sirlin, 03/14/16
A unit or hero that has both flying and anti-air does not benefit from anti-air. In other words, anti-air doesn't allow your fliers to ignore patrollers with flying. Fliers can already attack and damage other fliers, and patrollers with flying can't be flown over anyway.sharpo, 03/15/16
Anti-air does not stack. Having two instances of anti-air is the same as having just one.sharpo, 09/16/16