I didn't read all the posts so forgive me if someone's alread pointed this out but, lets say the average sortie is 15 minutes. Afterward, the guy who just ended that sortie looks around the map to see what he wants to do next, goes and grabs a beer and makes a stop in the bathroom, then looks around the map again before he goes to the hanger to change planes, check ord, check convergences, etc. and takes back off. It's reasonable to expect he was not "in flight" for 5 minutes. If this were true of everyone, then 25% (5 of 20 minutes) of all players would not be in flight at any given moment.
5 minutes might be stretching it but you get the point.