How Things Work: Cabin Pressure
Why you remain conscious at 30,000 feet.
- By George C. Larson
- Air & Space magazine, January 2002
WE HUMANS NEED AIR TO LIVE, so we do best around sea level. Airplanes are at their best up high, where the air is thin and smooth. And therein lies the rub: We invented a machine that thrives where we don’t. This became obvious as soon as engine power increased to a point at which aviators could reach altitudes where they lost consciousness.
At first, fliers coped by filling tanks with pressurized oxygen and inhaling the gas through rubber tubes; later, form-fitting face masks made oxygen delivery more reliable. In many high-flying light airplanes and military aircraft, oxygen systems and face masks are still used to keep the pilot alive and conscious.
In 1937, the U.S. Army Air Corps began research flights in a modified Lockheed Electra; the XC-35 was the first airplane built with a pressurized cabin. The fuselage was designed with a circular cross-section to eliminate stress points when the fuselage expanded under pressure. Openings were sealed to prevent air from escaping. Windows were reduced in size and strengthened, and the cabin inside became a pressure capsule—like a big aluminum can—that held five people. In 1937, the XC-35 earned the Air Corps the Collier Trophy for most significant development of the year.
Two years later, Boeing submitted a design to the Air Corps for a long-range bomber, the B-29 Superfortress, which would have pressurized compartments for the crew. And in 1940, Boeing’s 307 Stratoliner began flying passengers in pressurized comfort at 20,000 feet. Today all airliners are pressurized, and although the details vary among them, the basic elements of cabin pressurization systems are almost universal.
Air is pressurized by the engines. Turbofan engines compress intake air with a series of vaned rotors right behind the fan. At each stage of compression, the air gets hotter, and at the point where the heat and pressure are highest, some air is diverted. Some of the hot, high-pressure air, called bleed air, is sent to de-ice wings and other surfaces, some goes to systems operated by air pressure, and some starts its journey to the cabin.
The cabin-bound air has to be cooled first in an intercooler, a device like a car radiator that sheds the heat to the ambient air scooped aboard for that purpose. From there the air travels into the airplane’s belly, where air packs cool it further using air cycle refrigeration. An air cycle cooler is perhaps the simplest air conditioner ever invented, because it doesn’t need a refrigerant as an intermediate fluid to dump heat. The air packs compress the incoming air to heat it before sending it to another intercooler to dump the heat to the outside. The air then expands through an expansion turbine, which cools it the way blowing with your lips pursed results in a cool flow of air. (Test the principle by blowing with your mouth wide open to see how warm the air would be if it weren’t compressed and then allowed to expand.)
Now the air is ready to mix with air from the cabin in a mixer, or manifold, that adds the new air to the recirculating cabin air, which is moved by fans. To maintain a comfortable temperature for the passengers, automatic systems regulate the mixture of heat from the engines and cold from the air packs. To maintain the pressure in the cabin equal to that at low altitude, even while the airplane is at 30,000 feet, the incoming air is held within the cabin by opening and closing an outflow valve, which releases the incoming air at a rate regulated by pressure sensors. Think of a pressurized cabin as a balloon that has a leak but is being inflated continuously.
On the ground, the airplane is unpressurized and the outflow valve is wide open. During preflight, the pilot sets the cruise altitude on a cabin pressure controller. As soon as the weight is off the main wheels at takeoff, the outflow valve begins to close and the cabin starts to pressurize. The airplane may be climbing at thousands of feet per minute, but inside the cabin, the rate of “climb” is approximately what you might experience driving up a hill. It might take an average airliner about 20 minutes to reach a cruise altitude of, say, 35,000 feet, at which point the pressurization system might maintain the cabin at the pressure you’d experience at 7,000 feet: about 11 pounds per square inch. Your ears may pop, but the effect is mild because the climb rate is only 350 feet per minute. When the airplane descends, the pilot sets the system controller to the altitude of the destination airport, and the process works in reverse.
Related topics: Aerospace Manufacturing Aerospace Science Aircraft Golden Age of Flight
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Comments (22)
when the cabin door closes on 14.7 # pressure [assuming sea level takeoff]and the objective is to maintain pressure at say the 8,000 ft. level [10.9psi] is pressure first extracted
until that altitude is attained and then pressure maintained
through the bleed system? thanks for clearing this up, george
Posted by George Sites on May 30,2008 | 02:35 PM
To answer George's question, the climb through 8000 feet only takes a few minutes, and the cabin pressure controller allows the cabin pressure to follow the decrease in outside pressure until the target pressure is reached, then maintains that pressure throughout the remainder of the flight until the descent through 8000, then allows cabin pressure to again follow ambient outside pressure to touchdown. At least that's how I understood it when banging around in back of C-141B's as a USAF flight nurse. I remember my ears popping on the way up to a particular point, and again on the way down after a particular point.
Posted by Ken Hodges on July 11,2008 | 09:54 PM
My wife are missing 2 bones in her head (parietal)and I want to Know if presure in the cabin will affect her.
I will apreciate this information.
Editors reply: We aren't qualified to answer. We suggest you ask a neurosurgeon.
Posted by Thomas on September 22,2008 | 12:40 AM
George,
Aircraft are not designed to ever have a negative pressure differential (more pressure on the outside than on the inside). If the cabin altitude is set at 8,000ft before takeoff, the air inside the fuselage will gradually escape through the aircraft's outflow valves as the aircraft climbs. For example, if the aircraft is climbing at 4,000 feet per minute, the cabin altitude might only be increasing at 500 feet per minute. Once the cabin altitude reaches its set value (8000 ft in this example) the outflow valves will modulate to maintain the cabin altitude. If an 8000 ft cabin altitude can be maintained with a 5 psi differential (as would be the case if the aircraft were flying at ~25,000ft) and the aircraft climbs to a higher altitude (35,000 as an example), the aircraft will have to increase the differential pressure (~8 psi).
Posted by Titan on November 9,2008 | 05:07 PM
I would like to know the pressure inside the luggage part of a plane. Is it the same as in the passenger area?
And what about pressure inside the freight area?
This is specific information I need for transport of sensitive medical products.
Posted by Hans Middelbeek on February 24,2009 | 10:41 AM
I am searching for a chart or graph plotting aircraft altitude versus cabin altitude at various pressure differentials.
For example, what would be the aircraft cabin altitude flying at 15,000 feet with a differential of 2.5 psi?
Posted by Frank E. Adams on June 8,2009 | 09:06 AM
The text says that the air pressure change is kept to a climb rate of 350 ft per minute whilst the aircraft climbs at thousands of ft per minute.
If this is true then the airframe will be stressed beyond its design limits.
The reason for reducing the cabin pressure is to reduce the weight of the airframe. (It would need to be stronger and therefore heavier to withstand the extra pressure)
The pressure chosen is a trade off between weight and passenger comfort, and safety for medical reasons.
The rate of change of cabin pressure is determined by the climb rate and will match it until the minimum pressure is reached. This is automatic and ensures that the airframe is not overstressed.
When descending the cabin pressure is maintained until the outside pressure equals the interior pressure and they remain equal for the rest of the descent.
Richard. (ex airframe fitter)
Posted by Richard on December 8,2009 | 05:20 PM
I have just flown and experienced a first ever problem, in many years of flying, prior to take off with the cabin pressure. I would be grateful if someone could answer what the worst case scenario would be if a commercial plane took off with the cabin presurisation computer not working? Between taxi off the stand to the far away start of the runway, pax on board started having ear pain discomfort, children crying, me personally started to feel sick and light headed. Having to keep swallowing every 10 seconds or so to clear the ears, the experienced crew member immediately knew their was a problem and rang the captain. he aborted the flight and went back to base where the engineers very swiftly replaced the controller/computer for the cabin pressure. thank goodness these tests are done prior to take off, a very worrying time.
Posted by Shelley Beard on February 21,2010 | 06:51 AM
Is the air pressure in the cabin maintained at that of sea level or is it lower than that of sea level? I have weak lungs and I suffer from altitude sickness and need oxygen in form of liquid cans or a concentrator if I have to travel somewhere which is more than 800 ft. high. Does this mean if I go on a flight I would need extra oxygen if the air pressure is not at the equivalent of sea level?
Posted by vinny on March 2,2010 | 07:49 PM
Vinny,
The cabin pressure is reduced to a value equivalent to about 8000 feet.
You would not be allowed to fly with your condition without a doctor's approval.
Posted by Richard on April 24,2010 | 01:20 PM
Answer to Frank E. Adams: Only today I see your request. I have a table that may be usefull for you. It is a table made from compiled data of several aircrafts AOM's.
As I don't know how to attach documents here, if you send me a mail, I can answer with the attached table (thadeuamello@gmail.com).
Posted by Thadeu A. Mattos Mello on August 12,2010 | 09:31 AM
Does anyone know of any specific airlines or airplanes that pressurize their cabins at say, 7000 or 7500 feet? I tend to feel disoriented above 8000 feet, and am trying to find an airline and/or airplane that is pressurized below 8000 feet. Thank you.
Posted by Steven Borkowski on October 20,2010 | 11:36 PM
Hello, i've a doubt. The outflow valve is a "hole" in the fuselage that controls pressurization, and we know if there's a hole in another area of the fuselage caused by a gunshot for example, it can get bigger and despressurizes the aircraft. As the outflow is a "hole", why doesn't it get bigger and the structures aroud it don't crack? Is there a reinforcement material aroud outflow?
Posted by Christina on February 11,2011 | 09:23 AM
In response to Vinny's question/comments about cabin pressure & altitude sickness. There are a couple of different types of portable concentrators or oxygen generators out there that are FAA approved and run on 120V, 12V and have a 6-8 hour battery life dependent upon litter flow/usage. This option allows car & flight travel and the ability to be portable for several hours. Ask your doctor if this option would be a viable option with your specific diagnosis or if you are already working with an oxygen supply company ask about rental options.
Posted by DA Morford on May 9,2011 | 06:32 PM
This is for Steven Borkowski...
Hi, I am a flight simulation enthusiast and as a virtual pilot I find this subject extremely important to read and understand in order to know how the system functions and how to control it. Although its late I just want to answer Steven Borkowski's question about airlines pressurizing their cabins below 8,000 ft.
I came accross this link http://en.wikipedia.org/wiki/Cabin_pressurization in Wikipedia. It says that the Boeing 767 does it at 6,900 ft.
Cheers,
Posted by Fouad Safi on May 31,2011 | 06:46 AM
Sometimes when i fly on some local flights i get this very painful feeling on my forehead that spreads to the rest of my head as we land but disappears a few minutes after landing. Is it normal and what causes it? EDITORS' REPLY: We're not qualified to answer, except to advise: Call your doctor before your next flight.
Posted by Anne Gatere on June 21,2011 | 12:59 AM
If an aircraft is cruising at, say, 35000 ft. and all packs stop working, will the aircraft be able to maintain its cabin pressure? Or will the air start leaking and the pressure slowly drop?
Posted by badegol on July 14,2011 | 05:18 AM
In answer to Shelly Beard's question from 21 Feb 2010. I would say the Helios 522 Crash would be a worst case and concerns pressurization albeit from a different aspect. The account in the wikipedia link below tells just about all. Pre-flight is absolutely essential. The pressurization switch was set to manual rather than auto and they missed this fact several times during pre-flight.
http://en.wikipedia.org/wiki/Helios_Airways_Flight_522
Posted by Ed Donahue on July 28,2011 | 10:33 AM
I am concerned about flying since I have had two small heart attacks after skiing in the Rockies. Both times they happened the day after I got back from the ski trip. The first time it happened my cardiologist said it would have happened no matter where I was, but the second time he said "I guess you have a problem with the altitude."Now I'm not sure what to expect since I have not flown since both episodes. I live in Iowa so I'm used to the flatlands.I have a wedding to go to in SanDiego over a weekend and aren't sure if I should go or not.
Any ideas would be welcome.
Posted by Marsha Rasmussen on September 15,2011 | 12:16 PM
Very well explained. Thanks
Posted by on September 20,2011 | 01:28 AM
hey everyone i guess this blog is old but ill try to post my question .. im taking this flight attendant course and i got this . how much time it takes the a/c to drop from 35000 ft to 10000 ft incase of rapid decompression .. ty EDITORS' REPLY: It would depend on the aircraft model, its volume, etc.
Posted by amjad on December 1,2011 | 11:59 PM
I have flown twice a week for the past ten years without a physical incident. (1000 plus flights)
9 months ago I had a heart attack 6 hours after getting off a flight. Recent Danish studies show a cumulative effect of the differential pressure on humans. (DVT's etc.)
It is common knowledge that airline manufacturers test the fuselages of aircraft thousands of times to determine when the differential pressure will cause the windows and the cabin itself to rupture.
What data is available on how this pressurization effects humans ?
Posted by Joe Flynn on December 20,2011 | 10:04 PM