Flying combat does not follow the combat procedure's regular method for determining first attacks. Instead, combat happens on each “pass” the flyer makes, which is determined by its speed and maneuverability class.

On a pass, attacks are resolved in order of weapon length, as though resolving a charge. This is not actually a charge though, unless the flyer is diving. So do not grant a MV increase or a +2 to hit, nor should AC be reduced.

When making a pass, unlike with ground combat, flyers are not “breaking off from melee” as they go by, so they are not subject to free attacks at +4.

Some creatures such as the dragon can use less of their melee attacks than usual when making a pass, as per their description. However, some monsters also gain special abilities when flying, such as the dragon's breath ability noted on that page.