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Directly under the engine right behind the wing is a circle. I found a pic with the circle deployed. Watching classic aviation footage, it appears open only while on the ground. I think there is an APU intake on the tail, so what is this opening for? Or is this the intake and on the tail is the exhaust?

That’s the outflow valve. Used to control cabin pressurization. In auto mode will be open on the ground and closes to control cabin rate of change and desired cabin altitude in flight.

See "DC-9 Redesigned Cabin Pressure Outflow Valve" : viewtopic.php?t=733991

seb146 wrote:

I think there is an APU intake on the tail, so what is this opening for? Or is this the intake and on the tail is the exhaust?

This is probably the most mistaken item on the DC-9. The intake at the base of the tail is part of the air conditioning system. Specifically, the cooling air for the heat exchangers. On a Boeing, the intakes look like this:


From: https://i.stack.imgur.com/mJHRy.jpg

The APU intake is just forward of the airstairs, and the exhaust is above the right engine. The 717 APU intake is above the left engine.

This article should help with a lot of the questions: https://airlinercafe.com/aviation-artic ... de-178167/