Hi guys, as far as I know (and if it is correct), part of the air we bleed in the plane is supplied by the hot exhausted gas from the engine, then goes through one of the 3 packs( giant frig) and then goes into the cabin. My stupid question is why it didn't designe in another way, which is intake the cool air and then heat it.thanks ben

Not a stupid question at all!It's not hot exhausted gas from the engine but compressed air from the compressor. Due to the high compression ratio the air get's hot.Normally 2 stages are used, one intermediate and one high pressure stage. When the engine is at idle the normal stage is augmented by the high stage to maintain adequate pressure.So the fact the air is hot is just a by product from the pressure needed to supply the different systems in the bleed air circuit.The air get's cooled in the relevant pack (air cycle machine) and then hot trim air added to supply the required temperature to the cabin zones.Hope it helps,Cheers Sjaako

Good answer, Sjaako; the hot bleed air is an asset used for ice protection on leading edges. The new B787 Dreamliner achieves its great efficiencies by not using bleed air for cabin pressure. It will have two banks of powerful electric-driven fans instead, leaving the mass of air formerly diverted for housekeeping for thrust. However, some bleed will be available for the critical engine intake ice protection.

"My stupid question is why it didn't designe in another way, which is intake the cool air and then heat it."...because the pressure would be too low for human survival. It would have to be compressed also. The engines have a ready supply of compressed air... and the very high temperatures break down a lot of the ozone in the atmosphere (which can be poisonous in large quantities).Using engine pressure, seems to me, to be an efficient way of doing things, but having said that... I am told the ultra-efficient Boeing 787 will not use engine bleed air for cabin pressurization :-PCheers.Q>