How Are Airplane Cabins Pressurized?

Cabin pressure systems haven't changed much over the decades, but the controls for them have gotten safer and more sensitive. Learn more in this episode of BrainStuff, based on this article: https://science.howstuffworks.com/transport/flight/modern/airplane-cabin-pressure.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff, a production of iHeart Radio, Hey
brain Stuff Lauren vogelbone here. Back in the nineteen thirties,
aviation manufacturer Boeing came up with a new airliner, the
Model three oh seven Stratoliner, which featured a game changing innovation.
It was equipped with a pressurized cabin, which enabled the

(00:23):
plane to fly more swiftly and safely at altitudes above
the weather without causing passengers and crew to have difficulty
getting enough oxygen from breathing the thinner air at twenty
feet or about six thousand meters. Since then, cabin pressurization
has become one of those technologies that most of us
who fly probably take for granted. Cabin pressurization works so

(00:46):
well that passengers barely even notice it, in part because
it gradually adjusts the air pressure inside the plane as
it climbs in altitude, and then adjusts it again on
the way down. Before the article this episode is based
on how stuff works, spoke with Chuck Horning, who's been
an associate professor in the Aviation Maintenance Science Department at
em Very Riddle Aeronautical University since two thousand five. Before that,

(01:10):
he was a mechanic and maintenance instructor at Delta Airlines
for eighteen years. He explained that the basic technology of
airplane pressurization has pretty much stayed the same for decades,
though the advent of electronic computerized controls has made it
more precise. Essentially, the aircraft uses some of the excess
air that's pulled in by the compressors and its jet engines.

(01:32):
He said, it's not a terribly complex system. The engines
don't need all that air for combustion, so some of
it is tapped off and used for both air conditioning
and pressurization. The excess air from the compressors is cooled
and then pumped into the cabin. It's regulated by a
device called the air cabin pressure controller, which Corning describes

(01:53):
as the brains of the pressurization system. He said that
controller automatically regulates the pressure is sation. It knows from
information that the flight crew enters in what the cruising
altitude is, its schedules the pressurizing so that as the
airplane climbs and the external pressure goes down, it goes
to work. Pressurizing. An aircraft too much could put its

(02:15):
main body. Also called the fuselage under too much stress
from differential pressure inside and outside the fuselage as the
plane climbs. To avoid that, airliners don't try to duplicate
the air pressure at sea level. Instead, at a cruising
altitude of thirty six thousand feet that's eleven thousand meters
on most commercial jets, simulate the air pressure at an

(02:36):
elevation of eight thousand feet, or about the same as
Aspen Colorado. However, the Boeing seven eight seven Dreamliner, which
has super strong carbon fiber in its air frame, is
able to get down to the equivalent of air pressure
at six thousand feet or eighteen Horning explained that's better

(02:57):
because as the cabin altitude goes up, you have less
oxygen in your blood. That's why when you get off
a plane you may feel tired. We'd add that other
factors of air travel, like dehydration and stress can add
to that. But back to pressurization. How much air needs
to be added in order to pressurize the cabin depends,
of course, on the volume of the cabin, but it's

(03:20):
more complicated than simple volume because the aircraft's pressurization system
works in combination with the air conditioning system. It's also
continuously cycling that air through the cabin, recirculating some of
it and venting the rest as it draws in fresh
air from the engine compressor. Most airplanes will completely exchange

(03:40):
the air inside the cabin in three to five minutes.
Airliners have to be careful to pressurize gradually as they ascend,
and depressurize just as gradually when they descend towards the destination,
because humans are pretty sensitive to change. Is an air
pressure something anyone who's ever suffered from airplane air already knows.
That's one reason why the air pressurization system has automated controls.

(04:04):
As Horning explains, if the controller were to malfunction, the
aircraft's pilot could manually depressurize the aircraft during the descent,
but it might be an uncomfortable experience for passengers and
crew since it's tough to do it deftly by hand.
The air pressurization system also contains safety mechanisms designed to
ward off mishaps, namely two pressure release valves. The positive

(04:28):
pressure release valve will pop open if inside pressure gets
too high because too much air is being pumped in
the cabin. This release valve relieves that pressure. There's also
the negative pressure valve, which protects the aircraft from the
effects of a shift in which the outside pressure becomes
greater than inside the cabin, it vents air in This

(04:48):
might occur during a sudden descent, but it also occurs
during normal operation. Horning said, airplanes are not designed to
be submarines. They're designed to have a higher inside pressure
than the outside. That's why the negative pressure relief valve
is much more sensitive. As a result, when you're on
a plane that's descending, once in a while, you actually

(05:09):
hear a loud rush of air. That's the negative pressure
valve kicking in. In the rare event that deep pressurization
fails during a flight, there are other safeguards. There's a
sensor that detects when the pressure declines to the equivalent
of twelve thousand feet or thirty seven in elevation. That
switch automatically drops oxygen masks into the cabin so the

(05:32):
passengers can continue to breathe without difficulty. In some aircraft,
the oxygen comes from cylinders, while others get it from
generators that release oxygen through a chemical reaction. Today's episode
is based on the article how are airplane cabins pressurized
on how stuff works dot Com written by Patrick J. Tiger.

(05:54):
Brain Stuff is production of by Heart Radio and partnership
of how stuff Works dot Com, and it is produced
by Tyler Clane. Four more pod casts from my heart Radio,
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