A Change In Cabin Pressure

A boring explanation of how it works.

Cabin Pressurization is one of those things that everybody knows about but hardly anybody really knows much about. The thing we generally call “pressurization” is really two systems working together. The Air Conditioning system provides the air that is pumped into the airplane, usually from the engines. The pressurization system controls how much of it is let out.

To breathe we need more than just air. We need air under pressure. At low altitude the air pressure is provided by the weight of the atmosphere pushing down on us. This is the same as when you dive to the bottom of a pool and feel the weight of the water.

Above 5,000 feet most people will notice a difference in the way they feel due to a decrease in blood oxygen level. This affects everyone differently, and not everyone the same way each time.

Jets typically cruise between thirty five and forty five thousand feet high. Due to the nearly complete lack of atmospheric pressure at these altitudes this is practically outer space as far as our bodies are concerned . Like David Bowie said “it’s cold in outer space”. The temperature is about 40 below zero at high altitude.

We could all wear a pressure suit and space helmet but that would make it difficult to eat the peanuts and pretzels. A more practical alternative is to pump the entire airplane up with excess air. Pumping the cabin to about 9 PSI simulates the atmosphere at a lower altitude, allowing us all to breathe without supplemental oxygen or the space suit.

Airliner cabins DO NOT keep the equivalent of sea level pressure though. Since there is a limit to how much pressure can be safely applied to the airplane cabin sea level atmosphere can only be maintained up to about twenty five thousand feet of airplane altitude. At normal cruise altitudes the 9 PSI of cabin pressure will result in a cabin altitude of about 8000 feet.

Most light airplanes aren’t pressurized at all. Since they generally fly well below ten thousand feet this isn’t much of a problem. Doctors will often tell expectant mothers that it is fine to fly during pregnancy so long as the airplane is pressurized. This is ignorant. An unpressurized airplane will almost always cruise below 6,000 feet but a pressurized airliner will almost always have a “cabin altitude” above 8,000 feet. For a long flight expectant moms and anyone with compromised respiratory function might consider bringing along an oxygen concentrator or supplemental oxygen. Airlines have differing rules as to what is allowable.

We’ve all heard the FAA mandated announcement Flight Attendants make while doing Tai Che with a margarine cup in one hand. What really happens in the event of a cabin pressure failure depends on several things.

If the cabin pressure fails after takeoff during the initial climb you may not even know what the problem is before the crew explains. This sort of failure is generally due to a cargo door seal that is leaky or some other unexpected hole in the airplane that the air conditioning supply cannot overcome. A return for landing may cause you to be late but there is no real cause for concern. The masks probably won’t even drop.

Up at high altitude if the cabin pressure fails as a result of an air conditioning supply problem the decrease in cabin pressure will be fairly gentle. The pilots will already be working to restore proper airflow to the cabin as the masks drop from the overhead panels. Cockpit warnings start at 10,000 feet cabin altitude, the masks drop at about 14,000 feet.

Generally, you can expect a fairly rapid descent if you were already at high altitude, so get in a seat and fasten your seat belt. Passenger oxygen is not delivered under pressure but it will help give you something to do to pass the time. The oxygen supply is either provided from a tank or may be generated chemically by a canister in the overhead compartment.

The oxygen generators get pretty hot so the cabin may smell like there is an electrical fire. This is just a smell so don’t freak over it. If the masks have dropped and the airplane is headed down before you expected it, everything is going to be just fine. Paper work will be generated by the crew and your flight will be late, but everything should be just fine.

A rapid depressurization is very, very rare. This is a rapid decrease of cabin pressure that is caused at high altitude by the failure of a compartment door, window or something that results in the equivalent of a large hole in the airplane. When the air conditioning system cannot keep up with the leak the cabin pressure will be lost rapidly. An emergency descent will be made. Most people will pass out. At high altitude a rapid loss of pressure will allow only a few seconds of consciousness. If you get on the oxygen right away you may not pass out. You may see Flight Attendants walking around carrying emergency walk around bottles checking on the passengers. When a safe altitude is reached everybody should be fine. The flight will probably divert to the nearest suitable airport.

During any cabin pressure failure event the pilots will be wearing pressure masks that do a pretty good job of keeping them functioning.  This assumes they were functioning before.


5 Responses to “A Change In Cabin Pressure”

  1. It was so interesting I read it twice!

  2. There was an earlier version that repeated four times!

  3. Glad you fixed it — I love this one!

  4. Fantastic blog. You’re great with words.

    A friend of mine sent me your blog with the subject header – “Your new favorite website”. It just might be…:-) I miss flying immensely.

    I’m a writer and I keep wanting to sit down and write about my flying experienecs (I have a Private/Instrument – but, not current due to an injury) Your blog takes me back to a very wonderful time in my life … I miss the sky.

  5. I love these informative entries. I’ll definitely be sharing with my husband who is kind of a fraidy cat about flying but fortunately doesn’t let that keep him from traveling. This just might make him white-knuckle it a little less next time. Thanks!

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