Country: United States
Date: March 2006
Why do airplanes have pressurized cabins?
How is the pressurized cabin maintained?
Hi, Megan. As you go higher in the atmosphere, there is less air
pressure. That makes it harder to breathe and leads to a host of other
problems. Before pressurized cabins, you couldn't fly much over 10,000
feet high without using oxygen to keep your senses about you. Too high
and you lose consciousness.
But flying higher in thinner air results in less drag, so it would be
good to fly there if you can, especially for long trips like airliners
use. It saves them fuel and increases the speed they can go. Flying
higher also allows them to use high altitude wind currents like the jet
stream to their advantage.
Wiley Post was one of the first to fly higher than 10,000 feet to set
aviation records of the time. You can find a picture of him in the
pressure suit he designed on the net at
http://en.wikipedia.org/wiki/Wiley_Post (scroll down for the picture).
Obviously, you cannot have everyone who wants to fly put on one of those
get-ups, so it is better to pressurize the cabin. Since most airliners
today are powered by turbine engines, there is already a high volume
air compressor built in as part of the engine. To pressurize the
cabin, some of this higher pressure air is bled off of the airflow
through the engine before the combustion section. this reduces engine
efficiency a little, but it means that the inside of the airplane has a
To control it, there is a main pressure regulator that measures the
pressure in the cabin and lets air out or in from the compressor to
maintain the pressure. There are also emergency relief valves that let
air out at certain levels, so the airplane does not overpressurize. The
main systems have been mechanical in the past, but some of the newer
systems are computer controlled, and they deliver a much smoother
transition, so your ears do not stop up like they would with some of the
older systems that changed pressure in large steps rather than
Hope this helps!
There are several advantages to a pressurised cabin in an
aircraft. One of the big ones is allowing passengers and pilots to
breath comfortably at altitudes where the air would otherwise be to
thin. Another is taking advantage of the air's desire to move
outwards through any tiny cracks or weather seals around the
aircraft. Since the air is always moving out, It becomes easier to
heat the cabin since there is no cold air flowing in.
Some aircraft even use their cabin pressure to increase thier
structural strength, much like how a balloon becomes more rigid as
it is inflated.
Pressurising a cabin is a simple affair, all you need to do is pump
air in from outside the airplane, and regulate how much air you are pumping.
Airplane cabins are pressurized (and heated) to keep the passengers
comfortable. In fact, since at 33,000 feet of altitude, where
planes tend to fly these days (they can go faster with less air
resistance and so less fuel consumption at higher altitudes where
the air is less dense) the air density is about 1/4 the density at
sea level. So you have to breath four times as often to get the
same amount of oxygen into your lung. That may not do the job,
since it probably takes longer to get the oxygen out of the air than
the time between inhaling and exhaling when you are breathing four
times faster than normal.
There are other problems. Apparently capillaries start leaking even
at lower altitudes such as 12,000 feet. The fluid can gather in
body cavities and cause serious problems.
The pressure is maintained by air pumps which pressurize the
incoming air. To minimize costs, the air pressure is allowed to
decrease to pressures found at altitudes around 3,000 feet, which
most people can handle without trouble.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Click here to return to the Engineering Archives
Update: June 2012