sticky_beaver, I'm looking at your revised schematic and see you are planning to return bleed air to the engine, after it's been through the cabin.

For me, an old-old engine guy, this presumes that (1) the engine can do something useful with the extra air, and that (2) there's an appropriate place in the engine flowpath to reintroduce this air to the engine cycle. I'm always looking for cycle improvements, and your proposal deserves a fair look.

First of all, the overflow air from the cabin is at about 10-11 psia (equivalent to 8000 ft cabin pressure altitude, approx. - refinement of the numbers welcome!). There's only one place in the engine cycle where this flow could be introduced without backflow - and that's right at the engine fan inlet.

But this reclaimed air is much warmer than the -30C TAT found at cruise, and thus it's of lower density - and the engine would like all the inlet air density it can find, to operate at its best.

Sorry, sticky_beaver, I don't see a practical improvement to the status quo. :uhoh:

Perhaps that is the source of the problem, and the angst.

Tech Log used to be a valuable stopping points for professional pilots to look and see what is being discussed re: the finer points of our profession. A good first port-of-call for those tricky questions, or the I-wonder-how-that-came-to-be musings after a day's flying.

It is understandable that there are a number of flight simmers and high schoolers that have an interest in aviation and would like to know more.

But, enthusiasts, what reaction would you expect if you walked into a truck driver's bar and started asking 'where does the air come from for the air brakes?'

Would it not be more appropriate to place your questions in the Spectators Balcony forum, populated by genteel and helpful pilots inclined to answer such questions.

Moderators?

First, my apologies for taking so long to edit out some inappropriate comments - been away for several days .. Christmas comes but once a year, as they say.

Second, we probably ought not to have an aversion to questions asked by non-pilots (provided that the questions have some relevance to flying) .. on the basis that the best way to test our own knowledge is to try and explain something to someone endeavouring to come to grips with a problem.

Underlying suggestions of elitism are best left out altogether.

Third, if a question doesn't hold the interest of the group the thread will rapidly descend into the ooze of the archives ... hence there is usually no practical need for moderators to worry too much about whether questions are "good" or "bad".