Invisible AI airplane

I have an airplane that will render quite well as a user aircraft, but the same airplane will not render as an AI aircraft. It is a high quality freeware aircraft. What are the common causes of this?

I am using P3Dv3 and have had much luck converting old aircraft all the way back to FS9 into AI aircraft.
Never mind. The developer placed what seemed like identical models in a folder named "spare." I tried one of the alternate models and it works perfectly. I can't discern any other difference in the models. Oh, well......
After going through several iterations of "sometimes works, sometimes doesn't," I finally found the culprit. I was doing the AI flights using AI Flight Planner 3. It will often tell you that no model radius was found in the .mdl file and ask you to input one. When I saved the model radius to the .mdl file through the AIFP3 interface, it corrupted the .mdl file. Before letting AIFP3 try to insert the raduis, the .mdl file would open in ModelConverterX with no warnings. After letting AIFP3 try to insert a model radius, the aircraft wouldn't render in P3D and the model file returned a warning (Unsupported RIFF) when opened in MCX.

So....when AIFP3 asks you for a radius, ignore.