hi What is this suppose to be in the Flight1 Iron Knuckle DC-9? Fuelcap? And can I shut it?

I that is the apu exhaust door!

hmmm, I don't think so cause it's too far from the tail where APU situated.

It's an inlet for something. Either APU or pressurisation.It's there on DC-10s too, I've seen them take off with it open...

It looks like a pressurization or air conditioning outflow valve. Pressurization and AC ducts tend to be big and round like that, and have the same style valves. Does not look like an inlet for any system to me.Try going to altitude and see if it closes by itself. (assuming no fake pressurization controls in the addon's programming)

I just flushed the toliet, no need to worry.... :-lol