Can you explain pressurized airplane cabins?

Answer 1

The article How Tire Pressure Gauges Work explains air pressure. The atmosphere is about 50 miles "deep," and at sea level it exerts 14.7 pounds per square inch (psi). Our bodies think 14.7 psi is completely normal.

When you blow up a tire on a car or a bike, you use a pump to increase the pressure inside a closed space. A car tire typically runs at 30 psi, and a bike tire might run at 60 psi. There is no magic here -- the pump simply stuffs more air into a constant volume so the pressure rises.

A plane flies at about 30,000 feet. The air pressure at 30,000 feet is significantly lower than at sea level (4.3 psi versus 14.7 psi). High-pressure air is used to "pump up" the cabin in much the same way that a tire is pumped up. The high-pressure air on most planes comes from the compression stage of the jet engines.

Answer 2

Pressurized air is pumped into the cabin from a supercharger on the engine, or from a turbochager or compressor of a turbine engine.

It is then cooled by the airconditioning system and enters the cabin through the air ducts.

One or more valves let the air out of the cabin at a cetain rate and this determines how much pressure is in the cabin. The valve closes for more pressure and opens for less pressure.

Cabin pressure is expressed in differential pressure (comparing the inside pressure to the outside pressure) and cabin altitude (a cabin altitude of 5000 feet is equivalent to standing on a 5000 foot mountain.

An aircraft cabin is only good to a certain differential pressure, if it climbs higher, the cabin altitude must climb.

Airliners typically have a cabin altitude of 8000 feet and a maximum differential pressure of 8.5 PSI.

Answer 3

Yes. When you go up and down hills, in a car, your ears pop, right? Well, the road may only changed a few hundred feet and your ears still pop. What if you went from sea level to 40,000 ft of altitude. You ears drums would probably burst unless you could somehow get the pressure to equalize on both sides of the ear drum. Normally, this is done by swallowing a lot and/or chewing gum will help. But at 40,000 ft you wouldn't have to worry about your ears very long because you would probably die in a short time. First, hypoxia (a feeling of "everything is great and funny) and then death.

In an unpressurized airplane a person can only function (according to FAA) up to 10,000 ft in the daytime and at night the limit drops to 5,000 ft unless you have a pressurized aircraft.

Therefore, the pressure inside of an aircraft is usually kept around the 10,000 ft level. This can be done by directing engine bleed air into the cabin OR in smaller aircraft perhaps an air pump is used.

When the aircraft begins to loose altitude for a landing, the air pressure on the inside and outside of your eardrums has to equalize quickly. If it does equalize, a person ends up with an "ear block" which is VERY painful and could rupture the eardrum also. I hope this helps. Pops

Answer 4

Pressurization is required for physiological reasons. Some of the answers in this forum say that the plane's fuselage will crush. This isn't true because the same pressures would exist on both the inside and outside of the airplane just like it is on the ground with the aircraft doors open. Furthermore, following the same logic, if this were true then a rapid decompression would mean the aircraft fuselage would collapse and that also isn't true. Anyway, the atmosphere has constant percentages of nitrogen, oxygen, and other trace gases, regardless of altitude. However, the distance between molecules increases the higher up you go in altitude. This is what causes atmospheric pressure to decrease with an increase in altitude. As such, with each constant volumetric breath, you would have fewer oxygen molecules in your lungs. You reach a point where you aren't getting enough oxygen to survive. Interestingly, at altitudes above 25,000 you can be oxygen deprived even if you are breathing 100% oxygen. People that climb Mount Everist are frequently hypoxic even if they are breathing 100% oxyen.

Cabin pressure systems are designed so that they maintain sea level pressure (or the pressure altitude of the field that the airplane departed from) up until a maximum differential pressure between the outside and inside exists. At that point, as the airplane climbs, the pressurization system will simply maintain pressure at the differential pressure ratio. As such, the cabin altitude (the altitude that you and your ears observe) will also climb. The reverse happens during descent.

To give you further explanation as to how pressurization systems work, basically, air is taken from the engines, its temperature is cooled to make it comfortable, and it is pumped into the cabin under pressure. The aircaft has one or more cabin altitude control valves that allow air from the cabin to escape into the air. To control the pressure of the air inside the cabin, the valve can allow differing volumetric flow rates of air. In other words, it can open and close to adjust cabin pressure.

As for further information, as far as outside temperature is concerned, a standard atmospheric day starts off at 15 degrees celcius at 0 feet MSL. The temperature cools at the rate of 1.5 degrees celcius with each one thousand feet of altitude all the way up until reaching the tropopause.

Now, as far as why airplanes fly as high as they do, the efficiency of an internal combustion engine is directly proportional to the temperature difference of the "hot sink" and "cold sink". In this case, the hot sink is the combustion chamber and the cold sink is the outside air. This is one of the reasons why it is more efficient to fly higher. Furthermore, at any constant fan or compressor RPM, a specific ratio of fuel to air must be introduced into the combustion chamber. The higher the airplane is, the less air there is and as such, the less the amount of fuel that must be sequenced into the combustion chamber. This is the second reason why it is more efficient to fly higher.

Answer 5

Airplane cabins are pressurized because at the high altitudes that airplanes reach the density of air is greatly reduced. This results in a given amount of air (say a lung full) at sea level, now occupies a much larger volume. The effect of breathing normally at high altitudes will be diminished blood oxygen content, loss of consciousness and possibly even death.

Answer 6

When you go up in the atmosphere, the air pressure drops. At sea level, the air pressure is about 14.7 psi. At 18,000 feet, it is about half of that. Planes fly from 25,000 feet to 50,000 feet where the air is much thinner and easier to travel through. This saves a trememdous amount of fuel. Since the air pressure is too low, the amount of oxygen is too small for humans. There have been planes that lost pressure and all the passengers pass out and the plane just flies until it runs out of gas. That is why they have those oxygen masks. Usually, if you are going to fly over 10,000 feet, you need to take oxygen.

Answer 7

airplanes pressurise their cabins at high altitudes because the higher up you go, the less oxygen you have, the oxygen level i believe is maintained until around 12,000 feet and then starts to decrease from there, airplanes therefore pressurise their cabins at that level so the passengers do not suffocate from the lack of oxygen at higher levels

Answer 8

they do it so the passengers, you, can breathe :)