Imiakhtar From , joined Dec 1969, posts, RR: Reply 3, posted (4 years 10 months 3 weeks 5 days 2 hours ago) and read 3198 times:
Quoting Brendows (Reply 1): This is the APU (auxiliary power unit) intake door, in its open position. The door is closed when the APU is off (for example when the 777 gets power from the generators in the engines.)
About 1/4 of the way down the page is a labelled picture lookup up at the 737 APU from below. Just aft (to the right) of the oil tank you can see a flat metal band that appears to wrap all the way around the APU...that's the inlet plenum. It's a radial inlet. The inlet duct hooks on to the plenum about 180 degrees around the APU from what you can see here. The duct basically runs straight from the side of the APU to the side of the airplane. The 777 is similar, only the duct goes almost straight up to the inlet beside the vertical stab. As you can tell, about half of the APU is ahead of the inlet.