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Speaker 1: Welcome to Brainstoff from houstof works dot com, where smart

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Speaker 1: happens him Marshall brain with today's question, how does a

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Speaker 1: pressurized airplane cabin work? And what happens when that pressurization fails.

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Speaker 1: This month, the world was treated to an unexpected surprise.

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Speaker 1: It was the site of an airplane cabin with a

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Speaker 1: big hole in it, open to the sky. We can

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Speaker 1: imagine that this spectacle was even more unexpected to people

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Speaker 1: who were on the flight. If you saw photos or

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Speaker 1: video of the whole you may have also been struck

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Speaker 1: by how little there is between the inside and the

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Speaker 1: outside of an airplane. There was a piece of plastic

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Speaker 1: headliner on the inside of the plane, some insulation, and

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Speaker 1: then a thin aluminum skin on the exterior of the plane.

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

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Speaker 1: going on inside an airplane cabin when it's cruising at

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Speaker 1: thirty three thousand feet. It turns out the passengers are

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Speaker 1: flying in something that vaguely resembles a space capsule. Let's

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Speaker 1: take a look at how the space capsule works. The

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Speaker 1: first thing to understand is that people dressed in normal

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Speaker 1: clothing definitely cannot survive at thirty three thousand feet. This

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Speaker 1: altitude is roughly the equivalent to standing at the summit

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Speaker 1: of Mount Everest. If there were some way you could

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Speaker 1: stick your arm out the window at thirty three thousand feet,

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Speaker 1: the first thing you would notice is that it's incredibly

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Speaker 1: cold minus forty degrees fahrenheit or colder. The second problem

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Speaker 1: is incredibly low air pressure. The pressure is so low

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Speaker 1: that people would pass out very quickly from lack of oxygen.

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Speaker 1: The air at that altitude and temperature is also extremely dry.

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Speaker 1: So how are we able to sit in an airplane's

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Speaker 1: comfy chairs at thirty three thousand feet feeling like we're

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Speaker 1: sitting in someone's living room. The first thing that has

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Speaker 1: to happen is pressurization. The air it's sea level is

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Speaker 1: about fourteen point seven p s i, or pounds per

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Speaker 1: square inch. The pressure at thirty three thou feet, roughly

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Speaker 1: six miles up, is approximately four p s i. Something

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Speaker 1: has to be done to increase that pressure or people

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Speaker 1: would quickly pass out from lack of oxygen at four

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Speaker 1: p s I. Fortunately, the jet engines on the aircraft

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Speaker 1: act like big air compressors. If you take a part

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Speaker 1: of jet engine and look at it. It has four

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Speaker 1: main sections. At the front where the air is coming in,

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Speaker 1: there is a compressor stage blades second air, and compress it.

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Speaker 1: The fuel is injected into the compressed air stream and

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Speaker 1: ignited in the combustion stage. That air expands greatly from

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Speaker 1: the heat of combustion and flows through another set of blades,

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Speaker 1: turning them as it passes through. The energy from those

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Speaker 1: turning blades is going to be transmitted forward by a

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Speaker 1: shaft to turn the compressor blades, and then the exhaust

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Speaker 1: gases flow out of the engine at high speed to

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Speaker 1: create thrust to keep the airplane in the air. By

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Speaker 1: creating and opening in the engine between the compression stage

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Speaker 1: and the combustion stage, high pressure air can bleed out

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Speaker 1: of the engine and feed into the cabin to pressurize it.

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

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Speaker 1: the ventilation system on the plane will first cool it

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Speaker 1: down using the extremely cold outside air that's already available

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Speaker 1: to a comfortable temperature. The air pressure inside the plane

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Speaker 1: is not sea level pressure. It's more like Denver pressure,

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Speaker 1: where Denver is the mile high city. You can think

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Speaker 1: of the airplane's cabin like a big pressurized tube that's

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Speaker 1: been pressurized to about the air pressure that you'd experienced

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Speaker 1: in Denver. Now we have a cabin that is pressurized

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Speaker 1: and warm, but because the outside air is so incredibly dry,

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Speaker 1: some consideration has to be given to humidity. Fortunately, the

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Speaker 1: plane is full of humidifiers. People give off moisture every

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Speaker 1: time they exhale, and also through perspiration, So the dry

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Speaker 1: air from outside is mixed with air already in the

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Speaker 1: cabin and recirculated. The ratio of new air to existing

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Speaker 1: air is typically fifty fifty. The recirculated air passes through

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Speaker 1: filters that remove any airboard particulates. The air in the

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Speaker 1: cabin is still dry even after this recirculation process, but

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Speaker 1: not nearly as dry as it could be. What happens

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Speaker 1: if cabin pressurization fails. This can occur if the airplane

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Speaker 1: skin ruptures or a window breaks. I've been on a

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Speaker 1: flight where the copilot's windows simply cracked, and that was

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Speaker 1: enough to depressurize the cabin. When that happens, the masks

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Speaker 1: overhead will deploy and the pilot will immediately start descending

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Speaker 1: down to a safe altitude like eight thousand feet. The

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Speaker 1: masks get their oxygen, not from pressurized tanks of oxygen

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Speaker 1: they would be too heavy, but instead from a chemical

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Speaker 1: reaction involving something like potassium chlorate. When heated, potassium chlorine

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Speaker 1: gives off lots of oxygen, and a chemical oxygen canister

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Speaker 1: like this is very light relatively speaking. So the next

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Speaker 1: time you board an airplane take a moment to marvel

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Speaker 1: at what's happening. You'll be sitting in a chair at

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Speaker 1: thirty three thousand feet, just like you might sit in

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Speaker 1: your living room. An amazing amount of technology makes that possible.

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