Another thing merits mention: Many think crews don't use air conditioning to save money -- Once you are pressurized and at altitude, it takes constant flow from the packs to maintain your 6-8 psi differential...even on the "tightest" of hulls. Shut off the packs...with the outflow valve fully closed...and your cabin altitude will climb like a raped ape. A good hull leakdown rate from 7 or so psi to ambient would be less than 10 minutes...a good tight one at that. Also, don't attribute warm inflight temperatures ( at cruise anyway ) to non use of air conditioning -- You would all die from hypothermia due to the extreme cold at 25-35K feet...I don't care if you're over Yuma Arizona in July, it's cold up there. Temperature can be set automatically ( at a desired cabin temp - 65-85F ) with the mix valves ( or trim valves ) modulating hot/cool air to maintain it or manually. In any case there is still air flowing into the A/C...and flowing out ( so as to automatically maintain and not exceed max differential pressure ) controlled by an outflow valve, which is automatically controlled itself. A bit more to it than that but you get the idea.