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How Are Airplane Cabins Pressurized?
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
BY PATRICK
J. KIGER
Cabin
pressure on a plane is something passengers don't really think about until
their ears start popping or an emergency occurs.
Back
in the 1930s, aviation manufacturer Boeing came up with a new airliner, the
Model 307 Stratoliner, which featured a game-changing innovation.
It
was equipped with a pressurized cabin, which enabled the 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 20,000 feet (6,096 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 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, explains Chuck Horning.
He's
been an associate professor in the aviation maintenance science department at
Embry-Riddle Aeronautical University in Daytona Beach, Florida, since 2005 and
before that, a mechanic and maintenance instructor at Delta Airlines for 18
years.
"It's
not a terribly complex system,"
says Horning, who explains that the basic technology 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 in its jet engines. "The engines
don't need all that air for combustion, so some of it is tapped off and used
both for 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 Horning
describes as "the brains of the pressurization system."
.
"That controller automatically regulates the pressurization," Horning explains.
.
"That controller automatically regulates the pressurization," Horning explains.
"It knows from information that the flight crew
enters in what the cruising altitude is. It 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 fuselage under too much stress from
differential pressure as the plane climbs, Horning says.
To
avoid that, airliners don't try to duplicate the air pressure at sea level.
Instead,
at a cruising altitude of 36,000 feet (10,973 meters), most commercial jets
simulate the air pressure at an elevation of 8,000 feet (2,438 meters), about
the same as Aspen, Colorado.
The
Boeing 787 Dreamliner, which has super-strong carbon fiber in its airframe, is
able to get that down to the equivalent of air pressure at 6,000 feet (1,829
meters).
"That's better, because as the cabin altitude
goes up, you have less oxygen in your blood," Horning explains. "That's why when you get
off a plane, you may feel tired."
How
much air needs to be added to pressurize depends on the volume of the cabin,
Horning says.
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 the air inside the cabin in three to five
minutes, according to Horning.
Gradual
Pressurization Is Key
Airliners
have to be careful to pressurize gradually as they ascend and depressurize just
as gradually when they descend toward the destination airport, because humans
are pretty sensitive to changes in air pressure — something anyone who's ever
suffered from airplane ear already knows.
That's
one reason why the air pressurization system has automated controls. 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 as
deftly by hand.
.
The air pressurization system also contains safety mechanisms designed to ward off mishaps.
.
The air pressurization system also contains safety mechanisms designed to ward off mishaps.
The
positive pressure release valve will pop open if inside pressure gets too high
because too much air is being pumped in the cabin. It will relieve that
pressure.
There's
also the negative pressure valve, which protects the aircraft from the effects
of a shift in which the outside pressure would become greater than inside the
cabin. (This might occur during a sudden descent, as Aerosavvy details.)
"Airplanes are not designed to be
submarines," Horning says. "They're
designed to have a higher inside pressure than the outside. That's why that
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
hear a loud rush of air. That's the negative pressure valve kicking in.
In
the rare event that depressurization fails during a flight, there are other
safeguards, Horning notes.
There's
a sensor that detects when the pressure declines to the equivalent of 12,000
feet (3,658 meters) in elevation.
That
switch automatically drops oxygen masks into the cabin, so that the 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.
NOW
THAT'S INTERESTING
Sudden
depressurization is depicted in the climactic moment in the classic James Bond
movie "Goldfinger," in which the pressurized cabin is punctured and
the eponymous villain gets sucked out a window to his demise.
"If there is a rapid depressurization of cabin,
you've got that huge volume of air that will try rushing out of whatever hole
is letting air out. That's going to create a pretty good disruption inside the
cabin. You're going to be disoriented. The scene in the movie may have overdone
it a bit, though," says Horning.
Patrick
J. Kiger has written for HowStuffWorks
since 2008 covering a wide array of topics, from history and politics to pop
culture and technology. He worked as a newspaper reporter for the Pittsburgh
Press, and the Orange County Register in California, where he covered one of
the biggest serial murder cases in U.S. history, and also as a staff writer at
Baltimore Magazine. As a freelancer, Patrick has written for print publications
such as GQ, Mother Jones and the Los Angeles Times, and on the web for National
Geographic Channel, Discovery News, Science Channel and Fast Company, among
others. In recent years, he's become increasingly interested in how
technological advances are altering urban life and the design of cities, and
has written extensively on that subject for Urban Land magazine. In his spare
time, Patrick is a longtime martial arts student and a fan of crime fiction,
punk rock and classic Hollywood films.
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