We will use newbie and experienced and the titles so nobody gets confused.
Experienced, "Hi there, How are you?"
Newbie, "Good"
**intro, small talk, lets get to the point**
Newbie, "hey can you see my plane?"
Experienced, "no i dont have it"
Newbie, "what?"
Experienced, "I dont have that plane, so I can't see it in my game"
Newbie, "what?"
and it goes on like this for a few minutes untill someone gets mad ;D

I have personally had this converstion to many times to want to think about it, so I will try and help all you new FS pilots to understand this problem in FS.
Any Microsoft flight sim multiplayer has this problem, there are programs that fix it but they are complicated, and both users must be useing it, so it rarely works well. Anyway Flight Sim uses the model folder in any aircraft folder to render the aircraft in the FS game. In a multiplayer you have other people who downloaded other things, they fly this in the FS mp and If you dont have the same aircraft downloaded, the game wont have the correct model files, and you wont be able to render the aircraft in FS. So what it does is uses the last aircraft you had loaded (usually the default Cessna 172) and puts that plane in place of the missing plane. Now that was confuseing right? Right. Lets try a more clear explination:
Flight sim uses model files in your main Aircraft folder to display aircraft. If someone is flying something you don't have, how can FS render something it doesn't have the files for? A cake can't rise without yeast, and FS can't show a plane (the cake) without model files. (the yeast)
Often it will show your airplane in place of someone elces. That is beacuse that is the last model the game rendered so it just uses those files to show a plane it doesn't have in it knowledge.
I hope this helps, im counting on you "experienced" guys to further explane this, im not the best at describeing things ;D
