You can't do animations in AC3D. With any game you'll probably need to export the plane into parts. then use the native setup for that game to make the parts go together. Some games use txt files to handle everything and some don't it really depends on the game.
if you looking to model planes for a flight sim might i suggest
http://www.Targetware.net since the programmer has added native .AC support so just saving parts will show up in game. It is a massive online multiplayer sim but right now its in open beta so its free. you could build your own game from scratch if you wanted to. I'm one of the models for the Pacific WW2 add-on. Here is a few pics of planes i've done with AC3D. I've also done the Ki-43 (in game now) and many variants of the P-38. I can't show the P-38 yet cause its not been released to the public but I have a nice renders i'm going to let andy post in the gallery once its been released.
