747 open panel by APU exhaust??

[windowseat]

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

[ManaAdaSystem]

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

[HotDog]

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

[windowseat]

Thanks,

That explains why it's not always visible.