The airfields landing, taxiing and heading points are defined in the config.cpp for each island.
So, you can have only one airfield that AI can use.
There is a tutorial by Marksman that describes how to do it better than I can in this thread without taking up a lot of space.
Search the Editors Depot, but if you cant find it, let me know and I will see if I still have it on my machine.
Planck