Flyers always choose to fly when fleeing, and so the distance flyers flee is based on their flying Move rather than their ground Move. The normal flee distance for flyers is therefore 3D6" in common with all models whose move is more than 6".
Flyers can flee over interposing terrain and units, but if their move ends inside impassable terrain (including woods) or non-fleeing enemies, they are destroyed as normal.
If flyers must flee along the ground due to some constraint that prevents them from flying, for example, if they are in the middle of a wood, then they flee 2D6" or 3D6" depending on their Movement characteristic, just like other troops.
Flyers can normally choose to pursue fleeing enemies either on the ground (using the normal pursue rules) or with a flying 3D6" move.
If they choose to use their flying move to pursue, they can ignore intervening terrain and units. During their pursuit, they can always choose to either fly over an intervening enemy unit or to charge it instead. They cannot end their pursuit move inside units or impassable terrain (including woods). If they would end their pursuit move in impassable terrain (like woods!) or over a friendly unit, they stop just before it. If they would end their pursuit move on an enemy unit, they must charge it.