Originally Posted by LarryJ
The air for the cabin comes from the engine intakes. The engine's exhaust is on the other end of the engine and comes out at a high velocity and doesn't normally recirculate back to the engine intake. If it did, the engine wouldn't operate normally so the engine is designed to avoid that situation. It is possibly on the ground, however, if you have a pretty strong tailwind blowing at just the wrong angle but this does not happen very often.
That's not how it works.
Turn on the A/C in your car and crack open a window. Air will go out the crack, not in, even though your car is not in a "fully compressed state". If the "packs" are on, and you don't have any doors open, air will be exiting around the closed doors, not entering.
I know what my co-workers (the pilots) tell me, and that is what I forwarded on.
The aircraft inflight is exactly as you sum up, however on taxi only one engine is running, and on push back NO engines are running. There is a switchover from ground to aircraft power. THere are actually time during this phase where there is no air circulating. Just think about how many flights on push back the lights/air cut out for a few moments before engine startup. The start up of the engine creates a situation now where it can enter the unsealed doors unimpeded.
Please dont expert me around as I actually work daily with the guys up front and have heard the explanation more times than I can count.
