Ah, I see. Yeah nothing jumps out at me with the handling apart from the missing '</Item>' at the bottom (but I figure that you just missed it out when you pasted the data to pastebin?).
It should look like this at the bottom anyway :
If it works with your bus model it does suggest that the issue is with the model itself. It could be something in 'vehicles.meta' also (still tied to the model itself really). ie the 'vehicles.meta' settings work for the bus as it's model has everything set correctly to make it work but something is missing/wrong/different in the helicarrier resulting in it not flying. I'm aware that's not really very helpful, I just mention it as if that is the case you may be able to solve it with slightly different 'vehicles.meta' settings & not have to mess about in ZModeler etc. Which I'm guessing will be a bit more work.
I don't know anything about Zmodeler so I'm not going to be much help with that.
I've never heard of a size limitation for vehicles in the game so I reckon comparing your bus model to the helicarrier model in ZModeler is probably a reasonably place to start. Just go through everything making sure & double-checking it is correct/the same/nothing missing etc.
@Dekurwinator Thanks for the advice! I will probably end up using this as a test project if you are planning to release yourself Learning as I go.. Though i do have a question, ever had any issues exporting through GIMS/3d max? I keep getting 'Not Responding' every attempt to export?