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Name: Megan
Status: Student
Age: 9-12
Location: LA 
Country: United States
Date: March 2006


Question:
Why do airplanes have pressurized cabins? How is the pressurized cabin maintained?



Replies:
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).

Pretty cumbersome!

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 shirtsleeve environment.

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 smoothly.

Hope this helps!

David Brandt


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.

Ryan Belscamper


Dear Megan,

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



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