WEBVTT - How does a pressurized airplane cabin work?

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<v Speaker 1>Welcome to Brainstoff from houstof works dot com, where smart

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<v Speaker 1>happens him Marshall brain with today's question, how does a

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<v Speaker 1>pressurized airplane cabin work? And what happens when that pressurization fails.

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<v Speaker 1>This month, the world was treated to an unexpected surprise.

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<v Speaker 1>It was the site of an airplane cabin with a

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<v Speaker 1>big hole in it, open to the sky. We can

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<v Speaker 1>imagine that this spectacle was even more unexpected to people

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<v Speaker 1>who were on the flight. If you saw photos or

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<v Speaker 1>video of the whole you may have also been struck

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<v Speaker 1>by how little there is between the inside and the

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<v Speaker 1>outside of an airplane. There was a piece of plastic

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<v Speaker 1>headliner on the inside of the plane, some insulation, and

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<v Speaker 1>then a thin aluminum skin on the exterior of the plane.

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<v Speaker 1>That's it. It brings up an interesting question, what is

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<v Speaker 1>going on inside an airplane cabin when it's cruising at

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<v Speaker 1>thirty three thousand feet. It turns out the passengers are

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<v Speaker 1>flying in something that vaguely resembles a space capsule. Let's

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<v Speaker 1>take a look at how the space capsule works. The

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<v Speaker 1>first thing to understand is that people dressed in normal

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<v Speaker 1>clothing definitely cannot survive at thirty three thousand feet. This

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<v Speaker 1>altitude is roughly the equivalent to standing at the summit

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<v Speaker 1>of Mount Everest. If there were some way you could

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<v Speaker 1>stick your arm out the window at thirty three thousand feet,

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<v Speaker 1>the first thing you would notice is that it's incredibly

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<v Speaker 1>cold minus forty degrees fahrenheit or colder. The second problem

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<v Speaker 1>is incredibly low air pressure. The pressure is so low

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<v Speaker 1>that people would pass out very quickly from lack of oxygen.

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<v Speaker 1>The air at that altitude and temperature is also extremely dry.

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<v Speaker 1>So how are we able to sit in an airplane's

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<v Speaker 1>comfy chairs at thirty three thousand feet feeling like we're

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<v Speaker 1>sitting in someone's living room. The first thing that has

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<v Speaker 1>to happen is pressurization. The air it's sea level is

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<v Speaker 1>about fourteen point seven p s i, or pounds per

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<v Speaker 1>square inch. The pressure at thirty three thou feet, roughly

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<v Speaker 1>six miles up, is approximately four p s i. Something

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<v Speaker 1>has to be done to increase that pressure or people

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<v Speaker 1>would quickly pass out from lack of oxygen at four

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<v Speaker 1>p s I. Fortunately, the jet engines on the aircraft

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<v Speaker 1>act like big air compressors. If you take a part

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<v Speaker 1>of jet engine and look at it. It has four

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<v Speaker 1>main sections. At the front where the air is coming in,

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<v Speaker 1>there is a compressor stage blades second air, and compress it.

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<v Speaker 1>The fuel is injected into the compressed air stream and

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<v Speaker 1>ignited in the combustion stage. That air expands greatly from

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<v Speaker 1>the heat of combustion and flows through another set of blades,

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<v Speaker 1>turning them as it passes through. The energy from those

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<v Speaker 1>turning blades is going to be transmitted forward by a

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<v Speaker 1>shaft to turn the compressor blades, and then the exhaust

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<v Speaker 1>gases flow out of the engine at high speed to

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<v Speaker 1>create thrust to keep the airplane in the air. By

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<v Speaker 1>creating and opening in the engine between the compression stage

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<v Speaker 1>and the combustion stage, high pressure air can bleed out

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<v Speaker 1>of the engine and feed into the cabin to pressurize it.

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<v Speaker 1>Because this air has just been compressed, it's hot. Therefore,

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<v Speaker 1>the ventilation system on the plane will first cool it

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<v Speaker 1>down using the extremely cold outside air that's already available

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<v Speaker 1>to a comfortable temperature. The air pressure inside the plane

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<v Speaker 1>is not sea level pressure. It's more like Denver pressure,

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<v Speaker 1>where Denver is the mile high city. You can think

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<v Speaker 1>of the airplane's cabin like a big pressurized tube that's

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<v Speaker 1>been pressurized to about the air pressure that you'd experienced

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<v Speaker 1>in Denver. Now we have a cabin that is pressurized

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<v Speaker 1>and warm, but because the outside air is so incredibly dry,

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<v Speaker 1>some consideration has to be given to humidity. Fortunately, the

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<v Speaker 1>plane is full of humidifiers. People give off moisture every

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<v Speaker 1>time they exhale, and also through perspiration, So the dry

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<v Speaker 1>air from outside is mixed with air already in the

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<v Speaker 1>cabin and recirculated. The ratio of new air to existing

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<v Speaker 1>air is typically fifty fifty. The recirculated air passes through

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<v Speaker 1>filters that remove any airboard particulates. The air in the

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<v Speaker 1>cabin is still dry even after this recirculation process, but

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<v Speaker 1>not nearly as dry as it could be. What happens

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<v Speaker 1>if cabin pressurization fails. This can occur if the airplane

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<v Speaker 1>skin ruptures or a window breaks. I've been on a

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<v Speaker 1>flight where the copilot's windows simply cracked, and that was

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<v Speaker 1>enough to depressurize the cabin. When that happens, the masks

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<v Speaker 1>overhead will deploy and the pilot will immediately start descending

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<v Speaker 1>down to a safe altitude like eight thousand feet. The

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<v Speaker 1>masks get their oxygen, not from pressurized tanks of oxygen

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<v Speaker 1>they would be too heavy, but instead from a chemical

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<v Speaker 1>reaction involving something like potassium chlorate. When heated, potassium chlorine

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<v Speaker 1>gives off lots of oxygen, and a chemical oxygen canister

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<v Speaker 1>like this is very light relatively speaking. So the next

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<v Speaker 1>time you board an airplane take a moment to marvel

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<v Speaker 1>at what's happening. You'll be sitting in a chair at

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<v Speaker 1>thirty three thousand feet, just like you might sit in

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<v Speaker 1>your living room. An amazing amount of technology makes that possible.

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<v Speaker 1>Be sure to check out our new video podcast, Stuff

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