This is a bug I noticed when attempting to create a super-fleet if you will. In it I have one aircraft carrier and saw that I could place up to 8 sets of aircraft in it. I decided to max it out but for whatever reason, the aircraft units were not stacking on top of each other and were considered different regiments within the fleet. Anyways, I wanted to remove all of the aircraft and then put them in again as a group to see if they would show up as a single regiment. In the process I found out that removing the last attached aircraft skyrockets my army upkeep. Included will be several screenshots to show the issue. 

The fleet on the bottom has one final aircraft attached and the nearby airfield has 7 aircraft. Notice the change in income per turn:

Here is what happens when I remove the final attached aircraft:

It is not the biggest issue, but I figured I should le you all know either way. I'm enjoying the game greatly so good job!

I think it's more an issue when the max number of aircraft is reached, i have the same issue