WEBVTT - How Are Airplane Cabins Pressurized?

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<v Speaker 1>Welcome to brain Stuff, a production of iHeart Radio, Hey

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<v Speaker 1>brain Stuff Lauren vogelbone here. Back in the nineteen thirties,

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<v Speaker 1>aviation manufacturer Boeing came up with a new airliner, the

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<v Speaker 1>Model three oh seven Stratoliner, which featured a game changing innovation.

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<v Speaker 1>It was equipped with a pressurized cabin, which enabled the

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<v Speaker 1>plane to fly more swiftly and safely at altitudes above

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<v Speaker 1>the weather without causing passengers and crew to have difficulty

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<v Speaker 1>getting enough oxygen from breathing the thinner air at twenty

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<v Speaker 1>feet or about six thousand meters. Since then, cabin pressurization

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<v Speaker 1>has become one of those technologies that most of us

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<v Speaker 1>who fly probably take for granted. Cabin pressurization works so

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<v Speaker 1>well that passengers barely even notice it, in part because

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<v Speaker 1>it gradually adjusts the air pressure inside the plane as

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<v Speaker 1>it climbs in altitude, and then adjusts it again on

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<v Speaker 1>the way down. Before the article this episode is based

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<v Speaker 1>on how stuff works, spoke with Chuck Horning, who's been

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<v Speaker 1>an associate professor in the Aviation Maintenance Science Department at

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<v Speaker 1>em Very Riddle Aeronautical University since two thousand five. Before that,

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<v Speaker 1>he was a mechanic and maintenance instructor at Delta Airlines

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<v Speaker 1>for eighteen years. He explained that the basic technology of

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<v Speaker 1>airplane pressurization has pretty much stayed the same for decades,

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<v Speaker 1>though the advent of electronic computerized controls has made it

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<v Speaker 1>more precise. Essentially, the aircraft uses some of the excess

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<v Speaker 1>air that's pulled in by the compressors and its jet engines.

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<v Speaker 1>He said, it's not a terribly complex system. The engines

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<v Speaker 1>don't need all that air for combustion, so some of

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<v Speaker 1>it is tapped off and used for both air conditioning

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<v Speaker 1>and pressurization. The excess air from the compressors is cooled

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<v Speaker 1>and then pumped into the cabin. It's regulated by a

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<v Speaker 1>device called the air cabin pressure controller, which Corning describes

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<v Speaker 1>as the brains of the pressurization system. He said that

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<v Speaker 1>controller automatically regulates the pressure is sation. It knows from

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<v Speaker 1>information that the flight crew enters in what the cruising

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<v Speaker 1>altitude is, its schedules the pressurizing so that as the

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<v Speaker 1>airplane climbs and the external pressure goes down, it goes

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<v Speaker 1>to work. Pressurizing. An aircraft too much could put its

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<v Speaker 1>main body. Also called the fuselage under too much stress

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<v Speaker 1>from differential pressure inside and outside the fuselage as the

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<v Speaker 1>plane climbs. To avoid that, airliners don't try to duplicate

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<v Speaker 1>the air pressure at sea level. Instead, at a cruising

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<v Speaker 1>altitude of thirty six thousand feet that's eleven thousand meters

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<v Speaker 1>on most commercial jets, simulate the air pressure at an

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<v Speaker 1>elevation of eight thousand feet, or about the same as

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<v Speaker 1>Aspen Colorado. However, the Boeing seven eight seven Dreamliner, which

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<v Speaker 1>has super strong carbon fiber in its air frame, is

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<v Speaker 1>able to get down to the equivalent of air pressure

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<v Speaker 1>at six thousand feet or eighteen Horning explained that's better

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<v Speaker 1>because as the cabin altitude goes up, you have less

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<v Speaker 1>oxygen in your blood. That's why when you get off

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<v Speaker 1>a plane you may feel tired. We'd add that other

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<v Speaker 1>factors of air travel, like dehydration and stress can add

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<v Speaker 1>to that. But back to pressurization. How much air needs

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<v Speaker 1>to be added in order to pressurize the cabin depends,

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<v Speaker 1>of course, on the volume of the cabin, but it's

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<v Speaker 1>more complicated than simple volume because the aircraft's pressurization system

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<v Speaker 1>works in combination with the air conditioning system. It's also

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<v Speaker 1>continuously cycling that air through the cabin, recirculating some of

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<v Speaker 1>it and venting the rest as it draws in fresh

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<v Speaker 1>air from the engine compressor. Most airplanes will completely exchange

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<v Speaker 1>the air inside the cabin in three to five minutes.

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<v Speaker 1>Airliners have to be careful to pressurize gradually as they ascend,

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<v Speaker 1>and depressurize just as gradually when they descend towards the destination,

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<v Speaker 1>because humans are pretty sensitive to change. Is an air

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<v Speaker 1>pressure something anyone who's ever suffered from airplane air already knows.

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<v Speaker 1>That's one reason why the air pressurization system has automated controls.

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<v Speaker 1>As Horning explains, if the controller were to malfunction, the

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<v Speaker 1>aircraft's pilot could manually depressurize the aircraft during the descent,

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<v Speaker 1>but it might be an uncomfortable experience for passengers and

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<v Speaker 1>crew since it's tough to do it deftly by hand.

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<v Speaker 1>The air pressurization system also contains safety mechanisms designed to

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<v Speaker 1>ward off mishaps, namely two pressure release valves. The positive

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<v Speaker 1>pressure release valve will pop open if inside pressure gets

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<v Speaker 1>too high because too much air is being pumped in

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<v Speaker 1>the cabin. This release valve relieves that pressure. There's also

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<v Speaker 1>the negative pressure valve, which protects the aircraft from the

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<v Speaker 1>effects of a shift in which the outside pressure becomes

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<v Speaker 1>greater than inside the cabin, it vents air in This

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<v Speaker 1>might occur during a sudden descent, but it also occurs

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<v Speaker 1>during normal operation. Horning said, airplanes are not designed to

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<v Speaker 1>be submarines. They're designed to have a higher inside pressure

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<v Speaker 1>than the outside. That's why the negative pressure relief valve

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<v Speaker 1>is much more sensitive. As a result, when you're on

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<v Speaker 1>a plane that's descending, once in a while, you actually

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<v Speaker 1>hear a loud rush of air. That's the negative pressure

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<v Speaker 1>valve kicking in. In the rare event that deep pressurization

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<v Speaker 1>fails during a flight, there are other safeguards. There's a

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<v Speaker 1>sensor that detects when the pressure declines to the equivalent

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<v Speaker 1>of twelve thousand feet or thirty seven in elevation. That

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<v Speaker 1>switch automatically drops oxygen masks into the cabin so the

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<v Speaker 1>passengers can continue to breathe without difficulty. In some aircraft,

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<v Speaker 1>the oxygen comes from cylinders, while others get it from

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<v Speaker 1>generators that release oxygen through a chemical reaction. Today's episode

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<v Speaker 1>is based on the article how are airplane cabins pressurized

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<v Speaker 1>on how stuff works dot Com written by Patrick J. Tiger.

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<v Speaker 1>Brain Stuff is production of by Heart Radio and partnership

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<v Speaker 1>of how stuff Works dot Com, and it is produced

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<v Speaker 1>by Tyler Clane. Four more pod casts from my heart Radio,

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