I think that the only things that can make it disappear are:

panel= a non existent panel
sim= incorrect .air file
model= a non existent model

Another thing that could cause that would be that something in the panel.cfg is horribly wrong.

There might be others, but at the moment I can't think of any more reasons. I have had this sort of thing happen in the 777-200 when messing with the alias to either the 737-800 VC or 747-400 VC. If I got something horribly wrong in the panel.cfg, no model was visible. So I had an invisible aircraft (it would have been really nice if Wonder Woman was in my invisible plane as well [smiley=evil.gif]).

Mark