Some creatures of the Warhammer world have wings and can fly, soaring quickly from one side of the battlefield to the other. Such creatures are often potent forces on the battlefield, able as they are to easily outmanoeuvre clumsier, ground-bound troops.
Source: Warhammer Fantasy: 8th Edition
Whether they fly or move on the ground, all flyers have thespecial rule. In addition, because of their loose fighting style, flying units consisting of more than one model have the special rule.
In Warhammer, flight is represented by a swoop or glide of up to 10". The flyer starts off on the ground, takes off, flies to where it wishes to go, and then lands. Flyers, therefore, begin and end their movement on the ground. This is chiefly because it's impractical to suspend models over the battlefield, so we use the 'glide' for the sake of simplicity.
Units made up entirely of models that can fly can move or charge normally on the ground, using their Movement value, or instead choose to fly. A unit that flies can move over other units and terrain as it does so, treating the entire move as taking place over open ground. It may not finish the move on top of another unit or in impassable terrain. Depending on the flying model's height and/or position, it will sometimes be able to draw a line of sight over intervening units to a more distant target and make a flying charge over the intervening unit.
A unit that makes a flying charge does so using the glide move of 10" as its Movement characteristic.
A unit that is flying can march as normal, doubling its flying move to 20", representing a particularly long swoop or glide.
Flee And Pursue
Flyers always move on the ground when attempting to flee or pursue – there simply is no time for them to take off properly. Note that they still benefit from their Swiftstride rule as they flee and pursue.