[Brophmeister]

If I'm flying online in a default plane, and someone else joins the server in an addon mod plane (e.g. Spit mk 1) and I havn't downloaded that mod yet, what do I see them flying?

[RAF_Magpie]

Nothing. As far as your system is concerned, he hasn't selected a plane. They are invisible to you - but they can do nothing to you.