It seems obvious that you have to place your bet on a FlightGear aircraft being a compilation. As such it can contain stuff with different licenses, such as some XML files with GPL licenses and some other content under some other licenses. It should probably have a README file that explains which parts are under what license in addition to the in-file license header information for the file formats that support such.
The above, of course, just being my own layman interpretation of the situation.