I was looking at some photos.

I noticed that when the APU-470 pylon is on the Su-27 wing, it is never connected directly to the wing. Instead, the pylon always hangs from another pylon.

Maybe it's inside this "second pylon", and not inside the APU-470 itself, that азот coolant is located? This second pylon seems absent from wing of MiG-29 9.12, and also from the AKU-470 pylons elsewhere on Su-27.

Engineering is too complicated. I should have been a botanist.

-SK