What I was doing today was making a map and placing only 2 airfields first. Save it and then open the .oba file and delete the F00TWR0, twr1, 0, 0.000, 0, 0.0000, 0.0000, 0.0000, 3328, 0, 1, -1.00, -1.0, -1, 0, 0, 0
then go back into the editor and open the map again and build more to it.
NOTE! if you wait to long (meaning add to many things) then you can't open the .oba file with note pad, only can be opened with wordpad. (don't no if that make a difference)
Also I found it best to place an airfield on top of the original tower before you open the .oba file to delete it.
Pokie