I ran into an issue the other day where I had 8 air units on the Philippines (Manila) because they were my ally and Japan was attacking them with Naval units. I had an Officer Air unit with 7 strike fighters on Manila. Japan's frigates were destroying the air base before I could eliminate the frigates. So the air base was destroyed and I couldn't ferry my units off the island even after I had gained control of the situation by destroying the frigates. Since I didn't own the provinces on the island I couldn't build an airbase. Ferry from the pier did not work either. It would not let me ferry off the island to any provinces I had with a pier and an air field/base. I had to eventually declare war on Philippines and build an air field to get my planes off the island. I'm not sure if the cause of this issue was related to having an Officer unit in my air units or not.

So if an ally is demoralized and leaves the game while your defending their provinces and your lucky enough to survive defending their province - you have no way of building an airbase without declaring war on the ally (or pray they come back to the game), taking a province of once said ally and then building an airbase... I'm sure it doesn't happen often. I'll be wary to use air bases on allied islands... Thanks for the response.