In most OSR games the chance of encounter during overland, wilderness travel is once a day. However the chance of encounter is resolved for your game, there is most likely no reference for chance of encounter while flying through the air, at increased speed, covering a tremendous amount of distance. And an encounter "chance" doesn't build in the increased exposure an airborne party would have to attack and of being seen. At least not to my satisfaction. So here is what I do.
1. I count up how many hexes the airborne party covers per the wilderness map being used. This is how many times I roll for an encounter chance.
2. Roll all encounter chances at once.
3. For every encounter result I generate the type of encounter. If the encounter has any chance of engaging the airborne party then a random encounter results. If not, nothing.
3a. If a rolled encounter may have ability to observe or have interest in the airborne party I will note this for possible future encounters.