|Quoting Mats (Thread starter):|
I always thought that it was a gradual process, but I've heard stories about how taxi-ing aircraft are pressurized.
Pressurized to what? When taxiing, the outside pressure is already greater than the interior pressurization's ever going to be. There's no reason to pressurize a plane to greater than sea level. It'd be pretty uncomfortable for the passengers, like being underwater - not to mention uneconomical.
Someone else probably knows the technical term for this, but there is probably a switch thrown during taxiing that closes the air bleeds. This doesn't pressurize the cabin, though; it just seals it.
As the airplane climbs, pressurization is automatic. There's a pre-programmed pressure altitude that the airplane maintains, usually around 8,000 feet. It's not an all-or-nothing thing, though - it's not as if there's no pressurization below 8,000 feet and then it's suddenly switched on after crossing that threshold. At 8,000 feet the plane may be pressurized to, say, 4,000 feet - at 10,000 feet it might be pressurized to 6,000 feet, etc. up to a maximum pressure altitude of 8,000. There's a balance struck between economy and comfort, and it is a gradual process. This is something that can be selected by the pilot or engineer, depending on the airplane, but it's usually set once and then left alone for every flight based on manufacturer recommendations and airline policy.
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