747 open panel by APU exhaust??
 

What is the open panel beneath the tailfin of the 747? It is at the same level as the APU exhaust. First time I noticed it as an SLF I was a little unnerved but it is so obvious that I assumed it would have been spotted during the walkround so obviously serves some function.

Here's a picture showing it clearly in the bottom left corner.
http://www.airliners.net/open.file?id=95518

Air in, air out.
I guess (im not a 747 driver) that this is the air inlet for the APU.

Your guess is correct, it is the APU air inlet with the inlet door open. Door closes when APU is shut down.

Thanks,

That explains why it's not always visible.