Hmm, I'm afraid I can't reproduce this problem, it works fine here.
Can you investigate in more detail how to reproduce it?
I've also noticed that sometimes when you build an aircraft it comes fitted for coal or oil, even if that's not something it can normally be refitted to.
what it's doing is carrying over refit information from other vehicle classes. Try starting a new game and building an airport and a road vehicle depot. Build a road vehicle, sell it, then build an aircraft.