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Why would an aircraft perform an unpressurized takeoff? What conditions lead to this being beneficial?

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Typically, the pressurization system is run by engine bleed air. Using that bleed air means you have less engine power available than is maximally available with the bleeds off. If you're on a runway that requires all your power, you do a no-bleeds takeoff, which means you have no pressurization. Shortly after takeoff you would turn the bleeds on.

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    $\begingroup$ In many cases, the APU can be used at lower altitudes to provide pressurization when engine bleeds are off for takeoff. That's normal, but if the APU is inop and performance requirements drive a bleeds-off takeoff, then unpressurized is the only option. $\endgroup$ – Ralph J May 15 '15 at 2:18
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    $\begingroup$ In the EMB-145 if we needed A/ice on for takeoff we did it unpressurized. Engine bleeds on, a/ice on, packs off. $\endgroup$ – casey May 15 '15 at 3:33
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    $\begingroup$ @RalphJ: Well, you don't really need pressurization during take-off. The pressure inside is not higher than outside up to ~7000 ft and there are few airports higher than that. $\endgroup$ – Jan Hudec May 15 '15 at 4:59
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    $\begingroup$ @JanHudec Departing from a sea level airport, climbing through 7000' the cabin is typically about 1000' - 2000' -- the cabin climbs slowly from SL to 8k during the same time the aircraft climbs from SL to cruise at 35-41k. With an unpressurized takeoff, passengers' ears feel the initial 2000+ FPM climb, instead of a more gentle 300 FPM (normal) cabin rate of climb. $\endgroup$ – Ralph J May 15 '15 at 8:35
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Packs off doesn't mean that the cabin is unpressurized. The outflow valve(s) are closed and the cabin pressure remains steady at aerodrome level. This will give better engine thrust as mentioned above. Unpressurized cabin would mean outflow valve in open position and cabin pressure would be equal to ambient pressure. This is not allowed on commercial flights (at least JAA/EASA regulations). I flew one ferry flight unpressurized and almost popped my ears on takeoff because of the high climb rate.

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    $\begingroup$ Turning off both packs in flight WILL depressurize the cabin, closed outflow valve notwithstanding! If the cabin is really tight you might be able to least reduce the cabin rate of climb taking off with packs off, but the term "unpressurized takeoff" would still apply, since you are, temporarily at least, without any means of pressurizing it. Can't speak to JAA/EASA, but in the US an unpressurized takeoff is considered an entirely separate creature from an unpressurized flight. At my carrier, we can do the former, but not the latter (with passengers). $\endgroup$ – Ralph J May 15 '15 at 15:50
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    $\begingroup$ @RalphJ That's correct. The cabin is not a closed bottle, it will always leak a bit. There's still a significant difference between packs off take-off and an unpressurized take-off. With packs off and pressurization on (since automation will drive the outflow valve(s)) you'll end up with around 300-700fpm cabin rate. Unpressurized (outflow valve open, or a door, would be equal effect) take-off will give cabin rate equal to aircraft climb rate at take-off, which would be anywhere between 1000 and 3500 fpm. $\endgroup$ – Sami May 15 '15 at 16:25
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    $\begingroup$ @RalphJ As a conclusion, I would say that the term "Unpressurized take-off" may apply procedure-wise, but I still wouldn't consider the aircraft unpressurized. The cabin is pressurized whenever there's a difference between cabin pressure and ambient pressure. $\endgroup$ – Sami May 15 '15 at 16:27
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One reason for an unpressurised takeoff would be if the aircraft's pressure hull is damaged, and it's making an unpressurised ferry flight to somewhere it can get repairs.

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