Pressurized cabins?

[Dog Boots]

Just a quick question that's been bugging me every now and then for a few years now...

If the cabin of a commercial airplane is "pressurized" - why the heck does the air pressure in the cabin still change when ascending and descending? :confused:

[Terry]

because they are pressurised to an equivalent height of about 8000' not to sea-level.

[Zep]

Plus the variations in the air-conditioning, etc.

Aircraft pressurisation is like trying to keep a balloon inflated that has a few holes in it. You need to keep blowing into it. It is also why planes will not "explode!" if they get small or even medium holes in the skin from stuff like gunshots (siddown, Claus ;)). It's because they already have small to medium holes in the skin where the air gets out anyway!

[Doubt]

because they are pressurised to an equivalent height of about 8000' not to sea-level.

That is true when the altitude is around 30,000 feet.

My watch as a barometric based altimiter in it. I have checked it while on planes and observed the change with altitude.

It would be interesting if they did pressurise to sea level since a great many airports are well above sea level.

[joobz]

I actually happen to know this. There was a talk a few years back in the institute i worked at (a lung biology group)

Anyway, the reason why they pressurized the cabin in the first place was to maintain a comfortable cabin temperature (the whole PVT relationship thing). Anyway, they could have maintained a constant pressure in the cabin, but then that would mean as you increased altitude, the pressure differential would increase. Now the problem they were worried about wasn't the stress on the cabin hull (which may be an issue), but what would happen if there was a critical failure (like a window breaking).

So they ran a bunch of experiments with a typical cabin and a crashtest dummy by a cabin window. They would establish a pressure differential between inside and outside the cabin and crack open the window. The sudden rush of air would exert a force on the dummy. At pressures differences (I think it was delta >20psi) the dummy would get instantly sucked out the window. Fairly cool old B&W footage of this.

Anyway, they found that a delta 15 psi didn't result in this critical failure and felt it was a good compromise between comfort and safety.

[Rocky]

Anyway, they found that a delta 15 psi didn't result in this critical failure and felt it was a good compromise between comfort and safety.


Since absolute air pressure at sea level is about 14.7 PSI, a delta of 15 PSI would be unlikely.

The times I've had an altimeter inside a commercial airliner it has always read about 8000' while cruising.


-R

[joobz]

Since absolute air pressure at sea level is about 14.7 PSI, a delta of 15 PSI would be unlikely.

The times I've had an altimeter inside a commercial airliner it has always read about 8000' while cruising.


-R
Sorry, A max delta p of 15psi...wait maybe it was 10 now that I think about it.

[blutoski]

I actually happen to know this. There was a talk a few years back in the institute i worked at (a lung biology group)

Anyway, the reason why they pressurized the cabin in the first place was to maintain a comfortable cabin temperature (the whole PVT relationship thing). Anyway, they could have maintained a constant pressure in the cabin, but then that would mean as you increased altitude, the pressure differential would increase. Now the problem they were worried about wasn't the stress on the cabin hull (which may be an issue), but what would happen if there was a critical failure (like a window breaking).

So they ran a bunch of experiments with a typical cabin and a crashtest dummy by a cabin window. They would establish a pressure differential between inside and outside the cabin and crack open the window. The sudden rush of air would exert a force on the dummy. At pressures differences (I think it was delta >20psi) the dummy would get instantly sucked out the window. Fairly cool old B&W footage of this.

Anyway, they found that a delta 15 psi didn't result in this critical failure and felt it was a good compromise between comfort and safety.

Mythbusters reproduced this in one of their episodes. The key factor seemed to be the size of the hole. Bulletholes were harmless - use your finger to stop the rush of air - but a smashed window was surprisingly destructive.

[SteveGrenard]

Anyway, the reason why they pressurized the cabin in the first place was to maintain a comfortable cabin temperature (the whole PVT relationship thing). Anyway, they could have maintained a constant pressure in the cabin, but then that would mean as you increased altitude, the pressure differential would increase. Now the problem they were worried about wasn't the stress on the cabin hull (which may be an issue), but what would happen if there was a critical failure (like a window breaking).

There is another important reason aircraft cabins are pressurized to 8K feet when flying at 30K feet, and that is the PO2 of the inspired air. This is why a sudden loss of cabin pressure causes the oxygen mask to drop, triggering supplemental airflow of 100% O2 until the plane reaches an altitude where this is no longer a problem. For a good discussion of this see:

http://www.ispub.com/ostia/index.php?xmlPrinter=true&xmlFilePath=journals/ijamt/vol1n1/altox.xml

[Soapy Sam]

I thought it was so the coffee stayed warm.
I asked a stewardess once what the cabin pressure was, as a kid had a bag of crisps (potato chips) which was about to pop it's welds.
The answer from the flight deck was "Twelve", which caused as much confusion as it cleared.

[Zep]

8,000 feet was chosen for these reasons:

1) Above about 10,000 feet, most humans will need additional oxygen - the air is too "thin" to allow long-term life (without acclimatisation, anyway). And as hinted at above, trying to maintain "sea-level" at altitudes up to 30,000 feet for 200+ passengers would likely require prohibitive technology and power. A good compromise was about 8,000 feet - enough oxygen can be provided with weight- and fuel-efficient technology.

2) The oxygen levels at 8,000 feet are still measureably lower than towards sea-level, and this causes a measure of drowsiness. Therefore the passengers are far more likely to stay seated and passive (also why they add games and movies on the flights now), and not turn it into a flying bar and dance-party. Makes managing them all MUCH easier for the cabin staff, who can then provide better service (because there are fewer incidents to deal with).

[As explained to me by QANTAS tech and cabin crew folks! ;)]

[Dog Boots]

Thank you all, my question has been answered!

Just for clarification, is the following an accurate summary?

The ideal pressurization would be to make a hull that's completely airtight and no pressure difference would be felt during flight. This is too dangerous, though, because a broken window or other breach of the hull would be catastrophic. Therefore a compromise is made.

[Zep]

No, that's not right. Remember: They need to refresh the air in the cabin continuously in order to keep the passengers alive and not suffocate them. In a 747, for instance, about 4 tonnes of cabin air is replaced with fresh every minute (i.e. farts do not accumulate down the back). Therefore there is a clear airway to outside the aircraft - its is not sealed at all. The trick is to keep the pressure differential between inside and outside sufficient to maintain an allowable liveable oxygenated atmosphere inside the cabin.

Technical note: Pressurisation is used as the most effective way to keep many passengers oxygenated and warm. Only above about 60,000 feet does the lack of atmospheric pressure become lethal (liquids - blood - boils at body temperature), as does the extreme cold. Below that, if you have sufficient oxygen and stay warm, you can survive at altitude reasonably well. Hence high-altitude climbers and WW2 aircrew needing oxygen (and warm clothes) but not pressurisation.

[Ladewig]

The trick is to keep the pressure differential between inside and outside sufficient to maintain an allowable liveable oxygenated atmosphere inside the cabin.


I think there are really two tricks going on. One is, as you say, to keep the pressure differential comfortable. The second is to keep the temperature comfortable. The outside air is in the neighborhood of -50 degrees Farhenheit. Modern commercial airliners have to run the air close enough to the engine to make it exceptionally hot and then expose it to the cold to bring it down to a comfortably warm temperature while balancing the pressure.

[Foster Zygote]

Now the problem they were worried about wasn't the stress on the cabin hull (which may be an issue)

It's a big issue. Every time an airframe is pressurized and de-pressurized it exerts a level of stress on the structure. Reducing the pressure differential to a minimum greatly reduces the level of metal fatigue and greatly increases the operational life of the airframe.

[CurtC]

Modern commercial airliners have to run the air close enough to the engine to make it exceptionally hot and then expose it to the cold to bring it down to a comfortably warm temperature while balancing the pressure.Not sure what you mean about running the air close to the engine - the act of pressurizing the outside air heats it up so much that they then have to cool it before sending it into the cabin. As far as I know, proximity to the engine is not a factor.

[Ladewig]

Not sure what you mean about running the air close to the engine - the act of pressurizing the outside air heats it up so much that they then have to cool it before sending it into the cabin. As far as I know, proximity to the engine is not a factor.

I read it in an in-flight magazine. I cannot attest to the veracity of it. My bad.

[Zep]

In some aircraft, they bleed the AC inlet air directly off the jet engine compressor section (http://en.wikipedia.org/wiki/Bleed_air), before it goes into the burner and turbine stages. It is, indeed, quite hot, having been considerably compressed. So it has other obvious uses as well for high-flying aircraft.

[Foster Zygote]

In some aircraft, they bleed the AC inlet air directly off the jet engine compressor section (http://en.wikipedia.org/wiki/Bleed_air), before it goes into the burner and turbine stages. It is, indeed, quite hot, having been considerably compressed. So it has other obvious uses as well for high-flying aircraft.

Some aircraft can rout the hot air through the leading edges for de-icing.

[Ziggurat]

8,000 feet was chosen for these reasons:

1) Above about 10,000 feet, most humans will need additional oxygen - the air is too "thin" to allow long-term life (without acclimatisation, anyway).

Uh, no. 11,000 feet, for example, is hardly life threatening. It will cause noticeable shortness of breath if you exhert yourself (doesn't happen much on a plane), and it gives some people headaches and nausea, but it is NOT life threatening in any way.

2) The oxygen levels at 8,000 feet are still measureably lower than towards sea-level, and this causes a measure of drowsiness. Therefore the passengers are far more likely to stay seated and passive (also why they add games and movies on the flights now), and not turn it into a flying bar and dance-party.

Urban myth. If you make the passengers drowsy, you make the pilots drowsy. A plane crash is far too costly an event to increase the risk of, just to keep passengers seated.

And you don't have to, anyways. There's nothing to do while standing, and you get in the way of those service carts. And movies and games are provided to keep passengers HAPPY (so they become return customers), not to keep them docile.

[As explained to me by QANTAS tech and cabin crew folks! ;)]

They were feeding you a line.

[Major Billy]

The new B787 Dreamliner (http://www.popularmechanics.com/technology/transportation/3493516.html).Unlike aluminum aircraft, which maintain a cabin pressure of 8000 ft., the 787’s more resiliant composite body can be pressurized to 6000 ft. The 787 reduces complexity by pressurizing the cabin using electric compressors, rather than bleed air from the engines.

[Skeptical Greg]

It's a big issue. Every time an airframe is pressurized and de-pressurized it exerts a level of stress on the structure. Reducing the pressure differential to a minimum greatly reduces the level of metal fatigue and greatly increases the operational life of the airframe.

Wasn't the cause of that 737 coming apart in Hawaii a few years ago determined
to be metal fatigue from frequent re-de-pressurization ?

[Dog Boots]

No, that's not right. Remember: They need to refresh the air in the cabin continuously in order to keep the passengers alive and not suffocate them. In a 747, for instance, about 4 tonnes of cabin air is replaced with fresh every minute (i.e. farts do not accumulate down the back). Therefore there is a clear airway to outside the aircraft - its is not sealed at all. The trick is to keep the pressure differential between inside and outside sufficient to maintain an allowable liveable oxygenated atmosphere inside the cabin.


Ok, I wasn't quite clear enough in the way I put my question. The thing I had a problem understanding, was why the pressure in the cabin would not be kept at "normal" Earth surface pressure (which obviously varies, but still). But is it accurate to say that this is the compromise between safety (in case of hull breach) and passenger comfort?

[Dog Boots]

No, that's not right. Remember: They need to refresh the air in the cabin continuously in order to keep the passengers alive and not suffocate them. In a 747, for instance, about 4 tonnes of cabin air is replaced with fresh every minute (i.e. farts do not accumulate down the back). Therefore there is a clear airway to outside the aircraft - its is not sealed at all. The trick is to keep the pressure differential between inside and outside sufficient to maintain an allowable liveable oxygenated atmosphere inside the cabin.


Ok, I wasn't quite clear enough in the way I put my question. The thing I had a problem understanding, was why the pressure in the cabin would not be kept at "normal" Earth surface pressure (which obviously varies, but still). But is it accurate to say that this is the compromise between safety (in case of hull breach) and passenger comfort?