Aircraft Pressurization Beginner’s Guide

Aircraft Pressurization Beginner's Guide - AeroSavvy

How and why are airplanes pressurized?

It’s easy to take flying for granted. We hop on-board a comfy airliner and fly high in the stratosphere without giving breathing a second thought. The aircraft’s pressurization system makes it possible. Here’s how the magic works…

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Balloons on scale - Aircraft Pressurization - AerosavvyHypothetical experiment: If you place a scale in a vacuum chamber and compare the weight of a filled balloon with an empty one, you’ll see that air has mass.

Earth’s atmosphere is about 300 miles thick. At sea level, our bodies are subjected to about 14.7 pounds of pressure from this tall column of air. I’ll bet you don’t even notice! For animals roaming the earth’s surface, a 14.7 psi atmosphere provides the perfect amount of oxygen.

As we climb in altitude, the amount of air pressure acting on us decreases rapidly. You notice the decrease when your ears pop while driving up a mountain or riding a fast elevator. Although the atmosphere is 300 miles thick, most of the air molecules are squashed down to within a few thousand feet of the earth’s surface.

Denver is fine. Going higher spells trouble.

As we climb higher, air molecules are spread farther apart. When we breathe, our lungs take in less air, and less oxygen. Folks living in Denver, Colorado (5600 ft) are quite happy breathing the lower, 12 psi atmosphere. Climbing to a higher altitude, though, and the pressure drops really fast.

At 18,000 feet, the atmospheric pressure is down to 7.3 psi, about half the sea-level pressure. There just isn’t enough oxygen in a breath of air to adequately supply the brain. At this pressure, a healthy adult has only 20-30 minutes of useful consciousness.

NDG_quote-smallAirliners fly between 30,000 and 43,000 feet. At those altitudes the atmosphere provides less than 4 psi of pressure. If you tried breathing at that altitude, your useful consciousness would be less than a minute (followed soon after by death).

To survive high altitudes, occupants of an aircraft need help breathing. The solution is to pump air into the airplane so the interior pressure is high enough to keep the humans happy.

Why bother with pressurization? Why not fly down low?

Airplanes can certainly fly below 10,000 feet where the atmospheric pressure is a comfy 10 psi or higher, but it has some drawbacks:

  • It’s tough to cross a 14,000 foot mountain range at 10,000 ft.
  • Most bad weather is at lower altitudes.
  • Turbofan engines are very inefficient down low.
  • Aircraft ground speeds are slower at lower altitudes.

If you want a fast, smooth ride in a fuel efficient airplane that can fly over a mountain range, we need to pressurize!

How does a pressurization system work?

The airplane body (fuselage) is a long tube capable of withstanding a fair amount of differential air pressure; think of it like a big plastic soda bottle. In theory, we could seal the bottle so, as the airplane climbs, the interior air pressure would stay the same. We can’t do that because it’s hard to perfectly seal a huge airplane fuselage. Even if we could, the passengers would quickly use up the available oxygen. And just imagine the smell inside a perfectly sealed tube on a long flight! Clearly, a big sealed soda bottle won’t work for us without some modification.

Aircraft Pressurization - Soda Bottle - Aerosavvy
A fuselage is a bit like a soda bottle with a hole in the back.

To solve the problems, pressurization systems constantly pump fresh, outside air into the fuselage. To control the interior pressure, and allow old, stinky air to exit, there is a motorized door called an outflow valve located near the tail of the aircraft. It’s about the size of a briefcase and located on the side or bottom of the fuselage. Larger aircraft often have two outflow valves. The valves are automatically controlled by the aircraft’s pressurization system. If higher pressure is needed inside the cabin, the door closes. To reduce cabin pressure, the door slowly opens, allowing more air to escape. It’s one of the simplest systems on an aircraft.

Aircraft Pressurization - B767-Outflow Valve - Aerosavvy
The outflow valve on a Boeing 767-300F

One of the benefits of a pressurization system is the constant flow of clean, fresh air moving through the aircraft. The air inside the airplane is completely changed every two or three minutes making it far cleaner than the air in your home or office.

Pressurization systems are designed to keep the interior cabin pressure between 12 and 11 psi at cruise altitude. On a typical flight, as the aircraft climbs to 36,000 feet, the interior of the plane “climbs” to between 6000-8000 feet.

Altitude graph - Aircraft Pressurization - Aerosavvy
Exterior and interior altitude profile on a typical flight.

Why not keep the cabin at 14.7 psi to simulate sea-level pressure and maximize comfort? The aircraft must be designed to withstand differential pressure, that’s the difference between the air pressure inside and outside the aircraft. Exceeding the differential pressure limit is what makes a balloon pop when it’s over inflated. The greater the differential pressure, the stronger (and heavier) the airplane must be built. It’s possible to build an aircraft that can withstand sea-level pressure during cruise, but it would require a significant increase in strength and weight. A 12 psi cabin is a good trade-off.

outflow-nicotine
That’s just nasty!

Outflow Valve Trivia:

If you look at pictures of airliners taken prior to 1990, you might see brown stains around the outflow valve. The stains are from tobacco smoke. Airlines were thrilled when the industry banned smoking. Tar and nicotine gummed up valves, instruments, and sensors causing thousands of dollars a year in damage. Tobacco is really nasty stuff.

Protecting the Fuselage from Pressurization Problems

Two types of mechanical devices are installed on the fuselage to protect the pressurized section of the aircraft against excessive pressure differential.

Positive Pressure Relief Valves

Every pressurized aircraft has a maximum pressure differential limit. Exceeding this limit (pumping too much air pressure into the fuselage) can cause damage – even blow out doors and windows. To protect the aircraft from over pressurizing, positive pressure relief valves are installed. The devices (sometimes called butterfly valves) are spring-loaded to vent excess air pressure when cabin pressure exceeds the maximum limit.

Positive Pressure Relief Valve - B757 - AeroSavvy
Boeing 757 positive pressure relief valve. Excessive air pressure in the fuselage forces the spring-loaded doors to open, venting excess pressure outside.

Negative Pressure Differential Relief Doors

Negative pressure differential means the pressure outside the cabin is greater than the pressure inside the cabin. This situation could occur during a rapid descent. Negative pressure is bad because it pushes inward on doors and windows. These components are not designed for this type of force.

Again, spring-loaded devices are used to protect the fuselage from damage. Air pressure of less than 1.0 psi against the outside of the doors causes them to open inward against the spring load, venting air into the fuselage to equalize the pressure.

Pressurization - Negative pressure differential relief doors - AeroSavvy
Negative pressure differential relief doors on a Boeing 757. Excess pressure outside the fuselage forces the doors to open inward venting air inside the fuselage.

 

Where does pressurized air come from?

Stratocruiser,_BOAC
Boeing Stratocruiser by SDASM

Electric Compressors
Old piston powered airliners, like the Boeing Stratocruiser, used electric air compressors to pump fresh, outside air into the cabin. This system worked well, but the compressors added a lot of weight to the aircraft.


PanAm707_265
Boeing 707 by ClipperArctic CC BY-SA 2.0

Turbocompressors
Early jetliners, like the Douglas DC-8 and Boeing 707 used bleed air from the engines to spin turbocompressors. The turbocompressors then pumped fresh outside air into the cabin.


md-88
MD-88 by Lvco99 CC BY-NC-SA 2.0

Engine Bleed Air
Most modern airliners use bleed air from the compressor section of the engines to pressurize the cabin. This very hot air must be cooled to a comfortable temperature before it’s directed into the cabin.


787-BoeingLivery
Boeing 787 by Tim Wang CC BY-SA 2.0

Electric Compressors (Again!)
The new Boeing 787 Dreamliner brings back the electric compressor. The 787’s electrical system powers compressors, just like on the old Stratocruiser. Advances in technology make this system far more efficient than it’s predecessor from the 1950’s.

What is bleed air?

A jet engine has three main sections: compressor, combustion, and turbine/exhaust. The compressor is at the front of the engine. A series of spinning blades draws in fresh, outside air. As the air is compressed, it becomes very hot. Remember high school physics? As a gas is compressed, its temperature rises. The hot, compressed air then enters the combustion chamber where it is mixed with fuel and burned. The expanded gasses continue through turbine blades which power the compressor blades before exiting the engine producing thrust.

Turbofan_operation
Turbofan Operation by K. Aainsqatsi – CC BY-SA 3.0

Bleed air is fresh, clean, hot air taken from the compressor section of the engine before it is mixed with fuel or exhaust gasses. Common uses for hot bleed air are wing and engine ice protection, cabin pressurization, engine starter motors, and air driven hydraulic pumps.

 

How do pilots control the pressurization?

Boeing 757 and 767 pressurization panel - AeroSavvy
Pressurization controls on a 757 & 767

It’s really, really easy. The cabin altitude control panel on the 757 and 767 is super simple. During preflight checks, pilots turn the “LDG ALT” knob to display the altitude of the landing airport. That’s it! We don’t touch it for the remainder of the flight. The automatic mode takes care of the outflow valve for us.

The remaining indicators and knobs are for redundancy in case of a malfunction. There are two separate automatic modes. A manual mode allows us to adjust the position of the outflow valve should both auto systems fail. Pressurization systems work great and rarely cause any trouble.

Effects Of Flying In A Pressurized Cabin

Muga-Glass-of-red-wineThe air inside an aircraft cabin is very low in humidity. On a long flight it’s important to drink plenty of water to stay hydrated. When the flight attendant offers you a bottle of water, drink it. You may not notice that you’re dehydrated.

Alcohol consumption: Dehydration increases the effects of alcohol on your body. To make matters worse, alcohol increases dehydration; it’s a double-whammy. If you choose to drink alcohol on a flight, be sure to drink plenty of water and have something to eat while enjoying your cocktail. Don’t be that guy. Drink extra-responsibly when flying.

Does this food taste bland? Yes! There’s a good chance your in-flight meal really does taste bland. The aircraft cabin’s low humidity and lower air pressure reduce your sense of taste and smell by as much as 30% according to a Lufthansa commissioned study. Airline food kitchens often add extra spices and flavoring to meals to compensate for your crippled taste buds!
Special thanks to my Twitter friend (and fellow blogger@Jen_Niffer for tipping me off to the Lufthansa study!

Further Reading About Pressurization:

What happens if there is a problem with the pressurization system?
Your Oxygen Mask vs My Oxygen Mask

134 Comments

  1. Ken , as always very good article and clear for all of us. Thank you again taking time to share those things with rest of us. Probably on a “second” life I would have been a pilot..

    • It’s amazing that we used to fly in airplanes coated in that nasty stuff. “Smoking Section?” The whole aircraft was a smoking section! Sorry if I grossed your out. Thanks for reading!

      • Yes, I remember those days…I’ve had asthma all my life. Even though I was seated as far as possible from the “smoking section”, we were ALL breathing it. That’s probably responsible for at least some of the irreparable damage to my lungs and COPD which now hinder my travel. Thank goodness those days are over! Thanks for your clear and concise explanations!

        • Hi Melinda,
          It’s disappointing that it took us all these years to finally recognize and respond to the public safety hazards of smoking.

          I couldn’t agree more… Thank goodness those days are over!

          Thanks for reading,
          Ken

  2. Great post Ken! Informative but also quite entertaining. Loved the graphics — and I had no idea the 787 used electric motors to compress air for the pressurization system. I also didn’t know abut the tar stains on the outflow valve… but I should have. That feces-color staining is unmistakable. Truly disgusting.

  3. Hi Ken, I really enjoy your site. Could you please explain the “LDG ALT” knob? What would an altitude of 0070 be? I’m very curious about aviation, but obviously not a pilot! Thanks

    • Hi Joe, great question!

      Before we take off, we adjust the LDG ALT (Landing Altitude) knob so the display shows the elevation above sea level of the landing (destination) airport. When I took the photo in the article, we were in Shenzhen, China (near Hong Kong) getting ready to depart to Kuala Lumpur, the capital city of Malaysia. Kuala Lumpur’s airport elevation is 70 feet above sea level, so that’s what we set in the window.

      As we begin our descent for landing, the pressurization system uses the LDG ALT information to make sure the air pressure inside the plane is the same as the outside pressure when the airplane touches down on the runway.

      Thanks for the questions and thanks for reading!
      Ken

  4. Hi Ken, I always enjoy your writing. A quick question though, what if both engines are dead? Does that mean the cabin is not pressurized?

    Cheers
    Pete J

    • Hi Pete,

      This is one of those worst case scenarios! Systems on every aircraft model are different, so my answer will be based on a “generic,” bleed air pressurized aircraft; we’ll call it the AeroSavvy AS-100.

      In the event of all engines failing, we’ll lose our primary sources of pressurization. With no air being pumped into the cabin, the outflow valve will close in an attempt to maintain cabin pressure. Even though the valve is closed, the air still leaks out through various cracks and crevices causing the cabin pressure to slowly drop. It won’t be a rapid depressurization, but your ears will start popping.

      IF the aircraft’s APU (auxiliary power unit) is running, it may provide enough bleed air to pressurize the cabin and keep the masks from dropping. The crew will likely turn on the APU as soon as they see the engines failing.

      In the meantime, the crew will have the aircraft in a controlled, gradual descent as they run through the engine restart checklist. Once the engines are running again, normal pressurization will be restored.

      The good news is that this has never happened on an AeroSavvy AS-100. 🙂
      Sorry for the long answer.
      Thanks for reading!

  5. Ken…follow you on Twitter and enjoy your Tweets and these articles so much…I live my aviation fantasies vicariously through you…thanks so much…a Canadian fan

  6. Thanks Ken
    This article explains aircraft pressurisation so clearly. I have forwarded the link to my book group. Why? One of the members wondered why her smuggled boxes of eggs burst in her hold suitcase.

    • Hi Judith,

      Thanks for the kind words.

      Were they raw eggs? Eggs have very little air inside of them which should make them fairly immune to the pressure changes in an airliner. If she checked her bag, it was likely the baggage crews that caused the eggs to break.

  7. I recently flew on a 787 for the first time. I was hoping that the extra cabin pressure on a 787 would help my symptoms from sinus issues be less apparent. By midway of the long flight I had all the various pain issues that I associate with sinus problems while flying. This made me wonder if the 787 actually is flown at the advertised pressure you see in the literature or airlines can set the amount as they see fit? (I also realize that my discomfort might be from something else; dehydration or just sinus problems not related to the pressure.) – Hans

    • Hello Hans,

      I’m sure the airline was operating the 787 pressurization exactly as it was designed to be used. It’s an automated system that works beautifully. Even though the system is an improvement over older systems, it still does not maintain a ground-level cabin pressure. While a typical airliner might maintain a 7000-8000 foot cabin altitude, the Dreamliner has a 6000-7000 foot cabin. While this is a big improvement, someone with sinus problems will likely still have problems in a 787.

      I’m sorry about your discomfort. You might check with your doctor before flying again. He or she may be able to recommend something like a Benzedrex inhaler that will help relieve your discomfort.

      Thanks for reading!
      Ken

  8. I see an aircraft pressurization system advertised as 5.5 psi. Is there a chart or formula which would indicate the cabin pressure versus sea level pressure at 5.5? As I am reading various responses, I am guessing that it may be close to 13,000 ft MSL. Yet I have been unable to find a chart.

    • Hi Alvin,

      Something doesn’t sound right. If you were in a 5.5 psi atmosphere, that would be equivalent to 25,000 feet. You’d be unconscious within 3-6 minutes. Here’s a chart I found with Google: http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

      If the 5.5 psi reference you saw referred to differential pressure, it wouldn’t be nearly high enough to pressurize an airline cabin. Airliners typically run about 7-8 psi differential pressure at cruise (differential pressure is the pressure inside the cabin minus the pressure outside).

      If you can provide a link to where you saw the information, perhaps I can decipher it for you.

      Thanks for reading!
      Ken

      • The various versions of the Piper Malibu it has a maximum cabin pressure differential between 5.5 and 5.6; that is probably the plane he saw that data on. Because they fly at a much lower altitude the lower differential is not as much of an issue as in an airliner.

    • Military tactical aircraft are typically pressurized to a 5.5 psi differential above 23,000 feet to save structural weight. With only one or two crew members on-board, it’s lighter to have them on stored or processed oxygen the entire flight, rather than keep the cockpit at a lower pressure altitude.

    • Great question, Ben!
      The very first aircraft with pressurization was the Airco DH.9A. The British WWI bomber was modified in Dayton, Ohio to have a pressurized compartment. In 1921, this redesignated aircraft, the US D-9A, flew the first high altitude pressurized flight. I don’t know if any one person “invented” pressurization. The US D-9A work was done by the Aviation Section, U.S. Signal Corps and its successor the United States Army Air Service.

      From Wikipedia: https://en.wikipedia.org/wiki/Airco_DH.9A

      Thanks for reading!
      Ken

  9. How are air crew and passengers in commercial airlines, without filtration or detection systems warned, protected and informed in the event of a fume incident from contaminated bleed air?

    • Hi Owen,

      On the aircraft I fly, there are no detection or warning systems to let us know of contaminated bleed air. There are only a few things that can contaminate bleed air.

      Occasionally, deicing fluid can get into the system; we have procedures that we follow to minimize this risk. The very small amounts that get in the system (usually through the APU inlet) are not hazardous. The large amount of fresh air flowing through the system quickly dissipates the residual vapor.

      Sometimes vapor from engine lubricants can find themselves in the bleed air system as the engines are started. Again, the small amounts of vapor are quickly eliminated from the system by the huge quantity of fresh air being moved through the aircraft.

      It’s possible that a temperature control valve can malfunction causing excessively hot air in the system. We’ll get an indication in the cockpit that this is happening and we will likely smell it. The crew has a checklist for this and the system responsible for the problem will quickly be shutdown. This is pretty rare and redundant systems assure that it’s not a big deal.

      That’s about it for bleed air. Because of the way the system is designed, it is very difficult for the air supply to become compromised.

  10. Great article, Ken. On a recent flight to TPA on an older 737-300, ears were popping as usual on the way up and down. Return trip on a 700, the pressurization was so smooth the thing I noticed was the very little bit of ear popping I had. So, was it the aircraft, or my own “plumbing” that made the difference?

    • Hi Derek,

      That’s an interesting question! Both the new and old 737s are designed so that the cabin altitude during cruise flight doesn’t exceed about 8000 feet.

      However, the pressurization system on the newer 737 is an improved digital system that includes features designed to increase comfort. The pressure changes, while still there, should be a little smoother and less noticeable, making the flight a little easier on your ears. As you noticed, passengers like it.

      Thanks for reading!
      Ken

  11. Very informative, thanks. I was just diagnosed w a tiny brain aneurysm and am nervous about flying next week although 2 neurologists said it was fine and a nurse told me that its a myth that you can’t fly. Someone else said your brain is protected ftom pressure bc it’s in a fluid sac and i read that it’s ok bc the cabin is pressurized which led me to this article. Do you have any insight about how flying affects the brain and blood vessels and if there are any dangers?

    • Hi Sydney,

      I’m sorry to hear about your medical issue. I really can’t comment because I’m not a doctor. If you aren’t sure, please don’t hesitate to get a second opinion from a neurologist.

      Thanks for reading!
      Ken

  12. Hi Ken,
    Many thanks for your explanation because I recently traveled on an Australian domestic flight on which I experienced pain in one ear, and I was wondering how and at what level the pressure was maintained.
    I also noticed that perfumes seemed to suddenly be detected in the cabin air. Can and do pilots inject perfumes or other chemicals into the cabin air stream?
    Thanks again

    • Hi Rod,

      Aircraft pressurization systems generally maintain the cabin pressure altitude at about 7000-8000 feet or about 11.3 psi. If you have a sinus blockage, it doesn’t take much change in pressure to experience inner ear pain.

      I’ve never heard of anyone injecting perfumes or chemicals into the cabin air stream. No aircraft that I am familiar with has that capability. Most likely, another passenger was putting on, or spraying perfume. You might also have been smelling a deodorizer in a lavatory (which is far better than smelling the lavatory!).

      Thanks for reading!
      Ken

  13. Hey anyone please tell me, if the pressurization system fails to do its intended function while the aircraft is in cruise, how does it regain/maintain the cabin pressure?

    • Hello, Renjith,

      I think you are asking: “What happens when the pressurization system fails?”
      If the system and its redundant, back up systems stop working, the outflow valve is designed to close which will slow (but not stop) the aircraft from depressurizing. In this situation, the flight crew will begin an emergency descent. At lower altitudes there will be sufficient atmospheric pressure for normal breathing.

      Thanks for reading!
      Ken

      • Thanks Ken for your reply.. and here comes my real doubt… at the time of depressurisation, when the aircraft starts to descent to a lower altitude, how the cabin pressure will get equal to the outside pressure.. I mean fuselage is air tight right? Then how its done…?

        • Hi Renjith,

          It surprises many people that an aircraft fuselage is not airtight. Even with the outflow valve fully closed, air still leaks out of it. Window and door seals also leak a little bit of air. So, if the pressurized air source is interrupted, the fuselage will slowly lose pressure. This is why flight crews will immediately begin descending the aircraft if there is a serious pressurization problem.

          Thanks for reading!
          Ken

  14. hi Ken! great article! i loves read..its fits my style..I’m from malaysia..currently stdying dip in aircraft maintenance..could you explain well about how outflow valve operate? thanks

    • Hi Ezzat,

      Pressurization systems on every aircraft are a little different. The valves are usually operated by an electric motor that receives a signal from the controller. As you go through your training, you’ll learn the small details!

      Thanks for reading!
      Ken

  15. Thanks for an informative article and a great blog!

    Unfortunately, the experiment of “fill a balloon and weigh it” won’t work the way that you describe. Of course, air *does* have mass… but filling the balloon also displaces atmospheric air… so the balloon “floats” in the atmosphere by exactly the same amount of additional weight imparted by the mass of the air inside it. So a balloon inflated with air will weigh the same on a scale as an unfilled balloon.

    An easy way to think about this is to consider a helium-filled balloon on the surface of the earth. Helium has some mass, but it’s less dense than air. Helium balloons float because they displace more air-mass than the mass of the helium inside of them. So filling a helium balloon will make it apparently weigh less, even though you’ve increased the mass.

    Best wishes!

  16. does a small amount of decompression take place on all flights ( to a certain extent) or is it considered decreased air pressure that occurs on all fights ?

    • Hi Rupert,

      Decompression and “decreased air pressure” are the same thing: the reduction of air pressure in the cabin of an aircraft. After an aircraft takes off, the pressure inside the cabin decreases at a slower rate than the pressure outside the aircraft as it climbs.

      Thanks for reading!
      Ken

  17. Hi Ken:

    Your posts are great and interesting for me and I always read it… I am Iranian and I translate your post to Persian for my channel in telegram…I learn so much information from your posts and thank you for writing these posts.

    • Hello Reza,

      I’m glad you enjoy reading AeroSavvy! I occasionally fly over southern Iran when flying from Dubai to Germany. You have a beautiful country.

      Thanks again for reading!
      Ken

  18. Hello Ken,
    I have a billion questions coming more as a result of curiosity than anything else. Recently we had a little baby and wife and I were trying to fly but the airline told us baby needed to be at least a month old to fly but Google said otherwise, what is your proffessionl opinion on flying a week old baby?

    • Hi there,

      If it were my kid, I wouldn’t trust Google or a pilot. Please ask your pediatrician. Your child’s doctor is the best resource you have to determine whether he or she is ready to fly.

      Thanks for reading!
      Ken

  19. Hello Ken,
    Great stuff on pressurizing the cabin. Quick question. How does the system maintain a cabin oxygen concentration (21% at sea level) of cabin air at cruise of 35K feet When there is very little oxygen in the outside air. My guess this is where the compression of outside air to be vented through the cabin plays a role???

    • Hi Byron,

      Don’t try to overthink the system, it’s very simple. The air in our atmosphere is 21% oxygen. This percentage is roughly the same at all altitudes that an aircraft operates. At sea level, air molecules (including oxygen molecules) are close together. At 35,000 feet, the air molecules are spread out (but the oxygen content is still 21%). The pressurization system simply takes the “thin” outside air and pumps it into the cabin with enough to force to push the air molecules close together so they’re breathable by us humans.

      I hope that helps clarify it.

      Thanks for reading!
      Ken

  20. Hi Ken, Thanks for a really informative article that answered all the questions that went through my mind on a flight from Singapore to Perth yesterday. It’s interesting that you busted the myth planes recycle the air through the cabin which makes it easy to catch a cold on a plane.
    My question is does using bleed off air increase fuel consumption? I was on a Royal Jordanian Tri-star some years ago and several passengers fainted (including me) and we were told that the pilots would turn down the air-con to save fuel. Does that make sense?

    • Hi Phil,

      Thanks for the comments and questions! I’ll try to clarify the system a little bit…

      Does bleed air increase fuel consumption?
      YES! We are essentially “stealing” compressed air from the engines and that comes at the cost of burning a little extra fuel. The less bleed air we steal, the lower our fuel burn is. The amount of fuel is relatively small on any flight, but it all adds up at the end of the year.

      “Recycled” air: Actually, most modern aircraft do, sort of, recycle air. A typical component of the air conditioning system is the recirculation (or recirc) fan. The fan takes air, usually from a lower compartment, and pumps it back into the air conditioning ducts. The fans increase overall cabin air circulation while lowering the airflow required from the pressurization system. This saves a little bit of fuel ($$). Don’t worry, even with the recirc fan, there is always fresh air being pumped into the cabin and stale air exiting the outflow valve. Even the most efficient aircraft pressurization system has more air turn-over than your home or office.

      Your last comment has me stumped. Pilots really can’t “turn down” the pressurization. Some systems allow us to increase/decrease the flow, which affects the noise level and fuel burn slightly, but the aircraft will still maintain proper pressurization. If the cabin pressure is reduced to a dangerous level, the oxygen masks will drop down. I have no idea what could have been going on in your Tri-Star (one of my favorite classic jets!).

      Thanks again for writing!
      Ken

  21. Hi Ken. I came across your article while trying to research whether or not I can ship an exterior steel patio door by way of airfreight on a major passenger airline without any Argon gas escaping. I was told by the airline, after they checked with their Dangerous Goods (DG) Manager, that it was okay because it was not compressed. I am still concerned that the gas may escape under pressure in the cargo area and the door would be rendered useless once it gets to its destination, because once the gas escapes, fog, or steam, builds up between the glass and appears cloudy. My concern arose when the shipping worker told this to me when he asked how I was shipping it. I’d appreciate any input on this concern. Thank you.

    • Hi Shari,

      That’s a very interesting question. You need to ask the door manufacturer. They will know if their doors can be safely shipped by air.

      Thanks for reading!
      Ken

  22. Hi Ken,

    Very Informative Article.

    Came across this Article, while searching for Cabin Pressure Related Psychological / Physiological effects. During our Last flight with Family (My Wife’s Maiden Night Flight), she started feeling suffocated, when the cabin lights were dimmed, which could be attributed to Psychological effect, but soon (even before and during Take OFF) started reporting symptoms of Barotrauma (Pain in various parts of “gastrointestinal tract”), which could not be attributed to Psychological effort.

    My Question here is:
    1. Will the Cabin Pressure be altered even before Take OFF (During Engines are Revved up in full Throttle, in preparation of Take OFF in Few Seconds).

    2. I came across another article (Link Given Below), which Claims that Human capacity to adapt to low pressures differ between day & Night. Is It TRUE!!!?

    Link: https://www.quora.com/Why-does-cabin-pressure-change-during-flight-Why-cant-it-be-kept-constant-and-just-adjusted-once-i-e-after-landing

    (Check First Answer, which claims of Day & Night Difference).

    Thanking you in Advance.

    • Hi Vishnu,

      There are a lot of really bad (incorrect) responses in the Quora link. Hypoxia effects are the same on the body, day or night. At night, we may notice a difference in vision due to the way rods and cones in the retina work. Without oxygen, you’ll die in the same amount of time, day or night. 🙂

      Most airliners pressurize the cabin slightly just before takeoff. This small amount of pressurization should theoretically cause gasses in the body to be reduced in size (take up less space in the digestive system and sinuses). If this small amount of pressurization has any effect on the body at all, it would reduce gas pains (the gas takes up less space). Once airborne, the cabin pressure will slowly begin to drop (cabin altitude climbs). The slow drop in cabin pressure causes bodily gasses to expand. That’s what usually causes gas pains in the digestive system and clogged sinus pain.

      It’s difficult to say exactly what was going on with your wife. I hope her next flight goes better!

      Thanks for reading!
      Ken

  23. Ken, maybe you can help with this question.

    Was flying a King Air 200, very high pressure system across the region and PA/DA was extremely low.
    Field Elev. 1000 (Cabin Controller setting)
    DA -3,200 ft
    PA -2,700 ft
    Alt.Setting 30.66
    Temperature -16C

    During the descent the aircraft started to depressurize at 4,000ft MSL. Is this normal due to the environmental conditions? the Cabin Controller would only adjust to -1500ft.

    Kevin

    • Hi Kevin,

      That doesn’t sound like normal behavior for a pressurization system. However, I’m not familiar with the King Air. You should consult the manufacturer’s operating manual or your company’s training documentation for more information.

      Thanks for reading!
      Ken

  24. Hi Ken, a first class article. I train commercial divers in the UK and often use the analogy of pressurised aircraft to help understand that the partial pressure of oxygen is what sustains life rather than the percentage of oxygen in the mix. A “deep sea” diver living in saturation will survive breathing oxygen percentages of less than 5% at depth and this can be difficult to understand for many. Your article helps make it very clear for many in the way you present it. Forgive me if I borrow some of your knowledge to educate the commercial diving community. A fantastic article, and very well presented, many thanks.

  25. Hi Ken, this is really great, thank you! Really helped me to understand how pressurization more.

    I did have one question, however. For narrow body (737, a321) shorter duration flights <45 mins that fly at a lower altitude of 15-20k feet, would cabin pressure be higher than one flying at 35,000 feet for 2 hours? So, instead of a psi between 11-12, it would maybe be even higher at 12-13 and thus cabin altitude would be lower than 5-6k feet?

    Thank you again!

    • Hi Stephen,

      That’s exactly right. If the aircraft is cruising at a lower altitude, the pressurization system will provide a lower cabin altitude. A few nights ago we had a short hop on the 767. Our cruise altitude was 24,000. I think our cabin was about 2,000 feet.

      Thanks for reading!
      Ken

  26. First time reader and thoroughly enjoyed the article on pressurization. I especially appreciate the way you wrote if for fliers instead of engineers! TW

    • Hi Dan,

      We have the same problems that passengers have. Same tricks apply to everyone. If your ears need a little help to equalize, holding your nose and gently blowing through the nose usually works for me. Chewing gum can also help.

      Thanks for reading!
      Ken

  27. Very Interesting.
    I was looking for explanation of airtight strength requirements of 6000Pa for High Speed Trains and landed up here.
    Great Explanation and Very Informative.

  28. Great article and conversation. I have been using and flying with an oxygen concentrator (I have idiopathic pulmonary fibrosis) but I can only get a decent blood oxygen level with it full on (5 litres a minute) on short haul propeller flights. Given that the battery life is limited at 5 litres I needed to know if I should have the POC working while taxiing and in ascent. It appears from the some of the submissions I will only need it at a low level – say, 2 litres a minute – or perhaps not at all. Am I right Ken?

    • Hi Chris,

      I’m sorry to hear about your problem. I am not familiar with your medical equipment and I’m not a doctor so I won’t comment on your question. I would feel terrible if I gave you wrong information. Contact your physician or specialist and talk to them about flying. They should be familiar enough with aircraft pressurization and your concentrator to help you make an informed decision.

      Thanks for reading and good luck!
      Ken

  29. Interesting!

    But if I could understand you better, you said aircraft is not airtight as we used to think. My question is, why is it that an aircraft loses pressure and eventually get crashed when for instance something penetrate the fuselage?

    • Aircraft don’t crash because of a hole in the fuselage or rapid depressurization. Aircraft can still be controlled and safely landed when depressurized.

  30. Hi Ken,

    I’m a airline pilot and can’t figure this question out. Can you help please?

    If I’m at FL300 and increase cabin pressure from 4000-6000′, does the Cabin differential pressure increase or decrease?

    • Hi Tauseef,

      If you maintain FL300 and change the cabin interior from 4000 feet to 6000 feet, you are increasing cabin altitude (decreasing cabin pressure). This moves the pressure inside the cabin closer to the atmospheric pressure outside, thus decreasing your differential pressure (less difference between inside and outside).

      Thanks for reading!
      Ken

  31. The last time I flew was over 20 years ago and during landing the pressure got really bad. I could feel pins and needles all over and my head being squeezed. I felt like I was about to have a nosebleed but didn’t. My empty water bottle was sucked in/crushed in the middle from it. My husband didn’t feel the effects as badly as I did. Is this normal or was there a problem? Are there people with extreme sensitivity to the pressure and if so, is there anything I can do to make myself more comfortable?

    • Hi Stacy,

      The water bottle thing is perfectly normal and will happen on every airline flight. You drank the water when the aircraft was in cruise. The cabin altitude was probably 6,000 feet or so. Then you put the lid on empty the bottle. As the aircraft descended, the pressure increased inside the cabin. As the cabin pressure increased, it crushed the bottle.

      As for your personal symptoms, I have never experienced them. I would recommend asking your physician. They will be familiar with the effects of altitude changes.

      Thanks for reading!
      Ken

  32. Sure Captain I just want to ask you! Can a APU pressurise the aircraft on ground without your engines running

  33. Nice article,
    I have several questions, first, will typical airliner have pressurized cabin up to 2400 meters equivalent before actually ascending over that height ( I mean will I ever feel air pressure higher than 2400 meter/8000 feet equivalent at any stage of flight)? second, do airliners typically pressurize cabin before takeoff, for example may i feel that pressure of 8000 feet equivalent even before the plane is at airport sea level height of 10 feet while taking off? and thirdly, which airliners provide of 7000 feet or lower air pressure for more comfort? because I am sensitive to pressure change and feel a bit breathless with pins and needles even above 1900 meters see level on mountains, equivalent to 6500 feet, thanks ahead!

    • To clarify a common misunderstanding, when air pressure in an aircraft cabin DECREASES, the cabin altitude equivalent INCREASES and vice-versa.

      7000-8000 feet is about the highest altitude (lowest pressure) the cabin of a typical airliner will reach.

      The cabin pressure will never decrease before takeoff. There are negative pressure relief valves on the fuselage to keep this from happening (cabin pressure psi can never be lower than ambient outside pressure).

      On some aircraft, the pressurization system will increase cabin pressure (lower cabin altitude) slightly before takeoff to seal all the doors and increase comfort as the aircraft begins to climb. You can feel this small increase in your ears just before takeoff.

      As the aircraft climbs to cruise altitude, the cabin altitude climbs (pressure decreases) at a slower rate so when the aircraft reaches cruise altitude (ex: 37,000 feet) the cabin reaches its target altitude of about 7000 feet.

      Opposite happens when descending. As aircraft altitude decreases, cabin altitude also decreases so the cabin reaches airport elevation at about the same time as the aircraft.

      I think the Boeing 787 and some newer Airbus models have lower cabin altitudes at cruise. 6000-7000 feet range.

  34. Hey Ken,

    Great article. It is rare that puts Internet in a really good use. Please keep it up!
    All the best!

    EM

  35. Hello Ken

    I am hoping that you can help me find a solution to the following. I have to fly a lot. However, I have been suffering from barodontalgia [tooth squeeze] on an increasing basis during ascents – not descents over the last 18 months-2 years. Unfortunately, medical/dental people cannot find a cause. Thus I have now been grounded for several months.

    By studying altitude graphs, I have noticed that the pain seems to start when the aircraft is climbing and attains 25,000 feet – at about 15 minutes after take-off and continues for about 30 minutes thereafter whilst the cruising height of about 35,000 feet is achieved. The discomfort is about 5/10 on an Airbus A320-214 and about 7/10 on an Airbus A321-231 but is about 15/10 on the most recent Boeing 737-8F2.

    Oddly enough, I have discovered that I have not experienced the same problem on inter-continental longhaul flights – for example, Boeing 777-3FX (ER)]. However, I note that it flew at a slightly lower altitude, the lower 30s for much of the flight.

    I am considering experimenting with a series of flights on a Bombardier Dash 8 Q400 because it only flies to 25,000 feet. I am guessing that the effects of cabin pressure will not be as severe.

    I would be grateful for any ideas/comments/suggestions you might have as regards this course of action.

    Thanking you in anticipation of anything constructive that you can say.

    Ted

    • Hi Ted,

      I’m sorry to hear about the discomfort you experience during flight. I did a little bit of research and found that the Q400 cabin altitude at a cruise altitude of 25,000 is about 8,000. This is similar to most large airliners at cruise. You are correct that when an airliner cruises at a lower altitude, the cabin altitude will be a little lower. Most aircraft pressurization systems are designed to keep the cabin at around 6000-8000 feet.

      During an increase in altitude, gasses in our bodies expand which might explain why you have pain only during climb. When descending, those same gasses are compressed as the atmospheric pressure increases.

      I wish I had some advice to give you. Because you may have a medical problem, the best advice will come from a medical professional.

      Good luck. I hope you can find out what is causing the discomfort!

      Ken

  36. Hi Ken

    Thanks for responding.

    I have attended various medical/dental specialists over the last few months. Unfortunately, they cannot locate the source of the problem, which may or may not be neurological in origin.

    My personal belief, based upon the fact of the same event happening on two consecutive days, at the same point on different flights, is that there are indeed gasses in my body expanding to cause me the pain – I believe that the condition is called barodontalgia. I have researched various academic articles online in the context of how it affects pilots and aircrews.

    My reason for putting up my query on this website is to try and identify the precise trigger. In other words, might it be the rate at which the cabin pressure is changed or the amount by which it is changed, that is causing me the problems?

    If I may put it this way. (1) Pick up a pen and press it between your thumb and forefinger. You can feel the pen. You know how much pressure you are applying to hold it. It is not painful. (2) Imagine you are walking along somewhere. You trip and fall forwards. You put out your arms and hands to stop your fall. The surface on to which you fall is rough. The palms of your hands are grazed and, perhaps, slightly cut. You swear. Slightly painful but bearable. You know that the pain in your hands was caused by the pressure of the fall of your hands on to the rough surface and you promise yourself to try and be more careful in future. (3) You are captured by a James Bond type villain. You are bound to a chair with the palm of your hand facing upwards. He/she has a drill and starts to drill into your hand to obtain information. You scream because of the force of the drill going into the tissues of your hand. You would not wish this experience on anyone.

    As I stated previously – by examining flight altitude and speed graphs, I have identified two specific points in time during the ascent (namely, at about 25,000 feet during an ascent to 35,000 feet) of two different types of aircraft, namely, the Airbus A321-231 and the Boeing 737-8F2, on two consecutive days). Some invisible force then attacks the nerves in my teeth and it is like example (3) above and it lasts for 30 minutes in the whole of my upper right teeth. I do not believe that the events are purely coincidental.

    Thus I believe that some aircraft system is engaged either by the pilot or an online computer at that point in the ascent. The process is to change the cabin pressure from “X” to “Y”. If I understand the position correctly, then the flaps at the tail end of the aircraft are opened/closed to a particular angle to set the appropriate level of cabin pressure.

    On the assumption that something to do with cabin pressure is having an adverse effect on my dental/neurological system, I want to try and ascertain just what specific variation is causing that effect so that I might then try and avoid that particular situation.

    If, however, you are saying that the mechanical aspect of the rate and amount of pressurization in a Q400 rising to a maximum altitude of 25,000 feet is no different in degree/operation/amount/rate/whatever etc. than the ascent of the A321-231 and the 737-8F2 from 15,000 up to 35,000 feet (with the pain setting in as the plane reaches 25,000 feet), then I may be wasting my (and your) time on this line of enquiry.

    The only advice the medical/dental people can give me is – try a flight in the Q400 to the lower cruising altitude and see what happens!

    If there is no more that you can ascertain and/or state about the technical/mechanical side of the cabin pressure changes during the ascent of the aforesaid three aircraft, then I would be grateful if you could consider whether or not you could identify any body/airline/department/agency/person or whatever/whoever anywhere in the world, to whom I could address my enquiry as I am desperate to find an answer.

    Thanking you again for your time in any event.

    Ted

    • Hi Ted,

      I understand your frustration. The crew doesn’t do anything to the pressurization system at 25,000′. The pressurization system is automated on most every modern airliner. We set the destination airport elevation before we take off and the system takes care of the rest. The system maintains a “comfortable” rate of climb and descent inside the cabin during the flight. It’s possible that a Q400 cabin has a slower rate of climb since it’s not climbing as high as a jet.

      I’m not a doctor so I won’t suggest anything for you to try. If your medical professionals suggest trying a Q400 flight, then it’s certainly worth a shot. Let me know how it goes.

      Good luck!

    • Hi Ted,
      When you find the appropriate person to ask, here is what I would want to know if I was you: Is the plane fully pressurized by the time it reach’s 15,000 feet? If not, is the rate of pressurization constant or does it happen more rapidly near the end of the process? Since the cabin is not sealed, does the pressure fluctuate (even minor fluctuations would not be a natural state for the human body), and if so is this more pronounced at altitude?
      -=Bob

      • Bob,

        Aircraft cabins are under pressure, but they don’t really “pressurize.” Cabin pressure actually decreases (the cabin altitude climbs) at a rate that is much slower than the pressure decrease outside the aircraft as it climbs.

        The rate of pressure decrease (cabin altitude increase) inside the cabin is fairly constant throughout the climb – about 300-500 feet per minute. If the aircraft makes an intermediate level off, cabin pressure will stabilize. When the aircraft resumes climb to cruise altitude, the cabin altitude will again start to climb at 300-500 feet per minute.

        If the aircraft climbs slowly, the cabin altitude will increase slowly; about 300 fpm. If the pilots need to expedite the climb (due to traffic or weather) the cabin will climb more rapidly (closer to 500 fpm).

        Cabin pressure controllers have been around for years. The devices have been perfected and control cabin pressure accurately and smoothly.

        As for the natural state of the human body… Flying in an aircraft is most definitely not a natural state for our bodies. Even a smooth, 300 fpm climb can be uncomfortable for some.

  37. This is one of the best presented articles I’ve ever read. Thanks for breaking out the crayons on this!

  38. How is it that we don’t get altitude sickness in airplanes. Heck some people even get altitude sickness in Denver. But if the pressure of the cabin is equal to 6k-8k feet, some people should have the same effect? no?

    • Altitude sickness (which is a reaction to and/or symptoms of hypoxia) tends to begin at altitudes above 8,000 feet. Most people won’t notice hypoxia symptoms until above 10,000 feet. Aircraft cabins are maintained at around 6000-8000 feet which is well below the altitude most people will experience negative effects.

      WebMD has a good article about this subject: https://www.webmd.com/a-to-z-guides/altitude-sickness

  39. Exhaust gas doesn’t “exit the engine as thrust” – thrust is a force and exhaust is a gas. Exhaust may *produce* thrust as it exits the engine but they are two different things. : )
    Also worth mentioning that the exhaust gas’s thrust contribution is small compared to the thrust from the fan.

    • “Exhaust may *produce* thrust as it exits the engine”
      A little nit-picky but, fair enough. I updated the sentence to reflect your comment.

      Your second comment only applies to bypass engines. Pure turbojet engine thrust is produced by 100% exhaust gas.

      Thanks for reading!

  40. Dear Ken ,
    Excellent and Very informative article , thank you very much! Please share your thoughts on below comment.
    I suppose there must be a different reason as to why 14 psi is not maintained inside since the design would’nt change much when we are dealing with pressures below atm pressure.( Im guessing maybe more compressor power required to maintain 14 psi inside ( or ) increased weight of aircraft due to higher pressure leading to higher fuel consumption ???)
    The maximum differential that can happen when we are dealing with pressures below atmospheric pressure on outside is 1 bar or 14 psi and the current fuselages must suffice for the same.

    • Hello Ravichandra,

      To maintain sea level pressure inside an aircraft cabin at cruise altitudes, the aircraft structure needs to be significantly stronger (and heavier). If the pressure outside the aircraft at cruise is 4 psi and the pressure inside the aircraft is 12 psi, that there’s a differential pressure of 8 psi.

      This means that 8 pounds of pressure acts on every square inch of the aircraft interior. That’s a huge number. A passenger window that is 9″ x 12″ (108 square inches) has over 800 pounds of pressure pushing against it. Increasing the interior cabin pressure to 14 psi significantly increases the forces on the aircraft.

  41. Hi Ken

    I enjoyed reading your article from beginning till the end.
    Not that I read the article but I read all comments and your responses.
    Article reflects your knowledge and your talent in writing.
    Responses reflect your skill in providing accurate and adequate information.
    Thanks for a great article written by a talented Author/Pilot.

    Joseph Bader

  42. Dear Ken,
    I am Mohammed Khan, from India. Got interested to know abour “cabin pressure” when today i read in our local newspaper, where the cabin crew forgot to turn on the switch which controlls the cabin air pressure, thereby resulting in nose and ear bleeding of passengers.

    While surfing i came across “aeerosavvy” and how grateful i was to know the basics. Thanks a lot for your englightening article. I plan to read more and brush up my general knowledge. Thanks once again.

    By the way i happen to be a Marine Engineer (sailed for almost 45 years, including 15 years as Chief Engineer). So your article was a delightful read.

    Warm Regards,
    Mohammed K. Khan
    emkaysails@yahoo.com

  43. Hi Ken,

    I am guessing that the cabin pressurisation is via the overhead passenger (comfort) vents? If my assumption is correct, what happens if all of the passengers set theirs to the closed position? Thank you for your informative articles.

    • Hi Fred,

      Only a small part of pressurization air passes through the punkah louvres – yes, that’s really what they’re called! So nothing bad will happen if they are all closed. Most of the pressurization air enters the fuselage through vents in the cabin hidden by trim panels.

  44. Hi Ken,

    Why we need to add 500 feet to destination airport elevation when landing with bleeds off?

    Thanks and kind regards,
    Miljenko Radic

    • Not knowing the type of aircraft or your airline’s procedure, I can only guess. My guess is that for a bleeds off landing, you turn off the bleeds off at about 500 feet AGL. So you set the pressurization system to 500 feet above airport elevation so when the bleeds are switched off, the cabin and outside pressure are equal. No “bump” to the passenger’s ears.

  45. Dear Ken,
    First, an amazing blog – as others have said – finally a good use for the internet!
    My question is on the Boeing 767. I’d read it doesn’t have an APU (auxilliary power unit) so to the earlier question asked – what would be done if power on both engines was lost at altitude ( I know rare – but it happened on an Air Canada flight some years ago – they landed safely thanks to a pilot who was also a glider pilot! ). Much appreciated!
    ( oh, PS. I just rode a 787 to Japan – spectacular trip – even the food tasted better ! )

    • Hi Robert, thank you for the kind words.

      The 767 most definitely has an APU. If both engines fail, a ram air turbine deploys to provide hydraulic power to flight controls. The crew will start the APU to provide electrical power to the flight instruments and navigation system.

      During descent, the crew will attempt to restart the engines.

      Glad you enjoyed your 787 flight. It’s a wonderful aircraft!

  46. What is the pressurisation situation, with the fuel? surely the fuel tank is pressurised while in flight right? yes you’ve got release valves for moisture, and over pressurisation. but what is the tank pressure situation, does it drop in a flight? do you feed air in? and if so what do you do to remove said air from tank, when refueling.

    And don’t call me shirley! 🙂

    • The tanks on the 757/767 are not pressurized. There are several pumps that carry fuel to the engines. In the event of a low pressure pump failure, fuel can actually gravity feed to the high pressure pumps in the engine. There are vents in the wing tanks to allow air pressure to equalize during fueling and flight.

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