Can fly over ground patrollers. Ground forces without anti-air can't attack this or deal combat damage to it when attacked.
As a conceptual shorthand, think about flying and anti-air as follows: Ground forces without anti-air can only shoot straight. They can never hit fliers, ever. Flying forces can shoot both straight and down. Anti-air forces can shoot both straight and up. This is never a handicap because even though they CAN hit patrolling fliers, they aren't forced to. As a patroller, you only stop an attacker if it's on the *same* level as you. Anti-air ground forces on patrol will shoot at fliers as they fly over. — Sirlin, 03/14/16
Fliers can ignore patrollers that don't have flying. In other words, they can attack something else instead if you want. — Sirlin, 03/14/16
Fliers cannot ignore flying patrollers. The usual rules of the patrol zone apply, meaning if you want to attack with a flier at all, that flier CAN attack patrolling fliers so it must do that rather than attack other things such as non-patrollers or tech buildings. — Sirlin, 03/14/16
When a flier attacks another flier or gets attacked by another flier, they both deal combat damage to each other as usual. When a flier attacks a ground unit or hero without anti-air, the flier deals its combat damage to the ground thing and the ground thing does NOT deal any combat damage to the flier. Ground forces without anti-air cannot even attack a flier at all. Ground forces with anti-air can attack fliers and deal combat damage to them as usual when they do. — Sirlin, 03/14/16