Pressurized Cabins

[ed84c]

In most modern aircraft the cabin is pressurized. Despite this you still get the caracteristic (and often painful on descent) ear popping. Why does the pressure change when it is meant to be a controlled environment?

[Sayonara]

Because there is a time lag while the pressurisation system catches up with the change in pressure outside.

[Ophiolite]

The cabin is pressurised, but not to sea level pressures. Instead the pressure is equivalent, if my memory serves me correctly (and it probably doesn't), to that at an altitude of around 7,000'. Since nobody plans to run a marathon in the aircraft the lower oxygen level is not a problem.

 

I believe the primary reason for this is to reduce the structrual load on the hull. So there is still quite a bit of pressure decrease/increase to deal with at the start and end of the flight, hence the discomfort in the ears. (And sinuses, if you have problems with them.)

[Ophiolite]

Sorry Sayonara, you're definitely wrong on this one. Just tracked down a site from which I quote the following:

 

The pressurisation system ensures that the cabin altitude does not climb above approx 8,000ft in normal operation. However in late 2004 the BBJ will be certified to keep cabin altitude to below 6,500ft thereby increasing passenger comfort.

 

Source:

http://www.b737.org.uk/pressurisation.htm

[Firedragon52]

Not all planes are pressurized, are they?

[Sayonara]

Sorry Sayonara, you're definitely wrong on this one. Just tracked down a site from which I quote the following.

What?

 

Essentially, you added specific details to what I said. Unless you're proposing that pressure changes aren't the same as pressure changes.

 

[Firedragon52]

Not all planes are pressurized, are they?

Ha...ok...

 

My post was edited for errors (I think), but nobody answered my question.

 

I only asked because it is was going to lead me to my next question.

 

What would be the notable differences, in comfort, if the plane was pressurized. Or is pressurization needed to fly at those heights?

[Ophiolite]

What?

 

Essentially' date=' you added specific details to what I said. Unless you're proposing that pressure changes aren't the same as pressure changes.

 

[/quote']Your original post said:

Because there is a time lag while the pressurisation system catches up with the change in pressure outside.

 

That bears little relationship, as I read it, to reality. There is not a time lag. The pressure system does not have to catch up. When ascending the pressure is reduced in a measured and controlled fashion, the reverse occuring when descending. If this is what you meant the words did not convey it to me. Indeed the pressurisation system never catches up when depressurising during ascent. Its not meant to. That the whole point of it. So your statement is wrong, or very badly worded.

[Ophiolite]

What would be the notable differences' date=' in comfort, if the plane was [b']pressurized[/b]. Or is pressurization needed to fly at those heights?

Hopefully one of the posters is pilot and will confirm this. I think if you are flying much above 10,000' - 12,000' you need to be pressurised because the oxygen content is too low for comfortable breathing. (Unless of course you have been living at altitude and are acclimatised.) Since most commercial jets are flying at 25,000 - 40,000 feet, it is obviously essential for them.

If the plane were to depressurise at that altitude you would be unconscious within one or two minutes - so grab that oxygen mask if it ever appears above you.

Military fighter pilots use oxygen, and I believe their planes are unpressurised, but thats something of a reasoned guess rather than knowledge.

[Sayonara]

So your statement is wrong, or very badly worded.

Or i need to read the question more carefully

[Ophiolite]

Or i need to read the question more carefully

A common failing. I think I am junking about three draft posts a day because when I re-read them and the question I realise I had missed the point entirely.

 

Which reminds me of a classic remark I heard from an accountant at a business meeting on one occasion. (This is completly off-topic, but I've started so I'll finish.) He had been asked an akward question which several of us knew he would be unwilling to answer. His response - "Ah. Well, that's not actually the question you've asked." And he proceeded to answer a completely different one. It worked.

[ed84c]

by the way what exactly is that in your avitar ophilote?

Thanks everybody for the great and speedy reponce youve given to this query.

[Ophiolite]

by the way what exactly is that in your avitar ophilote?

.

It's a partial view of a used 12 1/4" PDC (Polycrystalline Diamond Compact) fixed cutter drill bit. This particular one drilled approximately 3' date='500' of Mezozoic sediments in the southern North Sea, on a gas field, suffering severe vibration during the run as evidenced by the impact damage to some of the cutters. Price as new, approximately $80,000. (Use once and discard.) I imagine that's more than you wanted to know.[img']http://www.scienceforums.net/forums/images/icons/icon7.gif[/img]