How does a pressurized airplane cabin work?

How does a pressurized airplane cabin work -- more importantly, what happens when the pressure fails? Join Marshall Brain as he breaks down the science behind cabin pressurization.

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Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstoff from houstof works dot com, where smart
happens him Marshall brain with today's question, how does a
pressurized airplane cabin work? And what happens when that pressurization fails.

(00:21):
This month, the world was treated to an unexpected surprise.
It was the site of an airplane cabin with a
big hole in it, open to the sky. We can
imagine that this spectacle was even more unexpected to people
who were on the flight. If you saw photos or
video of the whole you may have also been struck
by how little there is between the inside and the

(00:43):
outside of an airplane. There was a piece of plastic
headliner on the inside of the plane, some insulation, and
then a thin aluminum skin on the exterior of the plane.
That's it. It brings up an interesting question, what is
going on inside an airplane cabin when it's cruising at
thirty three thousand feet. It turns out the passengers are

(01:04):
flying in something that vaguely resembles a space capsule. Let's
take a look at how the space capsule works. The
first thing to understand is that people dressed in normal
clothing definitely cannot survive at thirty three thousand feet. This
altitude is roughly the equivalent to standing at the summit
of Mount Everest. If there were some way you could

(01:24):
stick your arm out the window at thirty three thousand feet,
the first thing you would notice is that it's incredibly
cold minus forty degrees fahrenheit or colder. The second problem
is incredibly low air pressure. The pressure is so low
that people would pass out very quickly from lack of oxygen.
The air at that altitude and temperature is also extremely dry.

(01:47):
So how are we able to sit in an airplane's
comfy chairs at thirty three thousand feet feeling like we're
sitting in someone's living room. The first thing that has
to happen is pressurization. The air it's sea level is
about fourteen point seven p s i, or pounds per
square inch. The pressure at thirty three thou feet, roughly

(02:07):
six miles up, is approximately four p s i. Something
has to be done to increase that pressure or people
would quickly pass out from lack of oxygen at four
p s I. Fortunately, the jet engines on the aircraft
act like big air compressors. If you take a part
of jet engine and look at it. It has four
main sections. At the front where the air is coming in,

(02:29):
there is a compressor stage blades second air, and compress it.
The fuel is injected into the compressed air stream and
ignited in the combustion stage. That air expands greatly from
the heat of combustion and flows through another set of blades,
turning them as it passes through. The energy from those
turning blades is going to be transmitted forward by a

(02:52):
shaft to turn the compressor blades, and then the exhaust
gases flow out of the engine at high speed to
create thrust to keep the airplane in the air. By
creating and opening in the engine between the compression stage
and the combustion stage, high pressure air can bleed out
of the engine and feed into the cabin to pressurize it.

(03:12):
Because this air has just been compressed, it's hot. Therefore,
the ventilation system on the plane will first cool it
down using the extremely cold outside air that's already available
to a comfortable temperature. The air pressure inside the plane
is not sea level pressure. It's more like Denver pressure,
where Denver is the mile high city. You can think

(03:34):
of the airplane's cabin like a big pressurized tube that's
been pressurized to about the air pressure that you'd experienced
in Denver. Now we have a cabin that is pressurized
and warm, but because the outside air is so incredibly dry,
some consideration has to be given to humidity. Fortunately, the
plane is full of humidifiers. People give off moisture every

(03:57):
time they exhale, and also through perspiration, So the dry
air from outside is mixed with air already in the
cabin and recirculated. The ratio of new air to existing
air is typically fifty fifty. The recirculated air passes through
filters that remove any airboard particulates. The air in the
cabin is still dry even after this recirculation process, but

(04:20):
not nearly as dry as it could be. What happens
if cabin pressurization fails. This can occur if the airplane
skin ruptures or a window breaks. I've been on a
flight where the copilot's windows simply cracked, and that was
enough to depressurize the cabin. When that happens, the masks
overhead will deploy and the pilot will immediately start descending

(04:44):
down to a safe altitude like eight thousand feet. The
masks get their oxygen, not from pressurized tanks of oxygen
they would be too heavy, but instead from a chemical
reaction involving something like potassium chlorate. When heated, potassium chlorine
gives off lots of oxygen, and a chemical oxygen canister

(05:04):
like this is very light relatively speaking. So the next
time you board an airplane take a moment to marvel
at what's happening. You'll be sitting in a chair at
thirty three thousand feet, just like you might sit in
your living room. An amazing amount of technology makes that possible.
Be sure to check out our new video podcast, Stuff

(05:26):
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