What is this open panel at the rear of this airplane?

Question

There's an open square panel. What is it?

P.S. Any tag suggestions are welcome. All I could think of was "tail".

EDIT: are there any other tags for this kind of thing? The APU tag seems to be a crystal ball because I had no idea what it was at the time.

$\begingroup$ I'm guessing it's APU related. $\endgroup$
– Fred Larson
Jan 18, 2016 at 1:47 — Fred Larson, Jan 18, 2016 at 1:47
$\begingroup$ Next question $\endgroup$
– TomMcW
Jan 18, 2016 at 19:10 — TomMcW, Jan 18, 2016 at 19:10

TomMcW · Accepted Answer · 2016-01-19 01:28:05Z

33

TomMcW

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answered Jan 18, 2016 at 1:46

aeroalias

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$\begingroup$ Next question: why is does the inlet face upwards, where rain can get in? If it opened downwards, they wouldn't have to put a door on it at all. $\endgroup$
– Level River St
Jan 18, 2016 at 10:03
5

$\begingroup$ @steveverrill I don't think the door is for rain protection but for aerodynamics. Afaik the APU is not used in-flight. So you can cover the inlet with a door because a "hole" in the fuselage would generate drag. $\endgroup$
– André Stannek
Jan 18, 2016 at 10:24
2

$\begingroup$ it if were facing down, it would be subject to FOD. it probably has an inertial separator not just for rain, but for debris as well. $\endgroup$
– rbp
Jan 18, 2016 at 16:36
2

$\begingroup$ oh and here i thought the question was asking about the square open panel below the tail number and door... @DrZ214 $\endgroup$
– Michael
Jan 18, 2016 at 17:42
2

$\begingroup$ @steveverrill Yeah, it's prob not advisable to let water sit in there while it's not running. I'm assuming the door is normally only open when the apu is in use. So preventing rain ingress may be one reason the door is there. The diagram does show a drain for the exhaust, though. I guess because it can't be closed. $\endgroup$
– TomMcW
Jan 18, 2016 at 18:50

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