Do you have any plugins/scripts for opening them plane models in Max? There's a tutorial at GTAF concerning vehicle conversions to SA; all you have to do is assign dummies from an existing model to the vehicle you're working on, link it to the model, making sure that the latter's chopped up and named properly after the parts from the stock SA vehicle (i.e. chassis, propeller, etc.), assign materials and a collision model to it, and export it to a readable format.
The tut works for cars, but it should also apply to aircraft, too, as vehicles in SA follow similar rules concerning dummy assignment and other things.