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Originally Posted by Cessnaflyer  Then if you use less bleed air then you wouldn't need as high a thrust setting for takeoff. Does it not work this way? |
APU. That's what it's for. If you need the thrust (which rarely do you need that much for takeoff), then you just takeoff with the APU running. I'll leave the APU on for takeoff on a hot day just to keep the cabin a bit cooler. There is no using "more" or "less" bleed air. You can't set the PACKs to take out a certain amount of bleed air. They get what the engines give, and that's it. They don't make the engines work any harder, and they don't burn more fuel. If you wanna keep it cool, keep the APU on. It doesn't burn that much gas anyway. Having the PACKs on doesn't really affect your t/o thrust setting. The engines are there to make the airplane go. The bleed air extracted for environmental reasons is just a bonus by-product.