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Air Pressure In Cabin

Sat Apr 02, 2005 11:39 pm

What is the air pressure in the cabin during flight in, for example, a 767 or A330 at flight level 300? I know for sure that the cabin is pressurized, but it can't be the same pressure as it is at sea level. I've learned that from opening a juice or yoghurt container with the foil raising like a dome  Wink

/Micke
 
SQ325
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RE: Air Pressure In Cabin

Sat Apr 02, 2005 11:44 pm

In the RJ85 we have 6100ft cabin altitude in FL300 thats something about 6.7 psi
most likely the cabin of a A330 is better and you have a lower pressure than in the RJ!
 
ACDC8
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RE: Air Pressure In Cabin

Sat Apr 02, 2005 11:44 pm

Cabin air pressure is usually set at 8000 feet. The new B787 is supposed to have cabin air pressure set at 6000 feet. Hope that helps!

cheers,
Patrick
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troubleshooter
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RE: Air Pressure In Cabin

Sun Apr 03, 2005 6:13 am

Quoting ACDC8 (Reply 2):
Cabin air pressure is usually set at 8000 feet. The new B787 is supposed to have cabin air pressure set at 6000 feet

This is valid only when operating at max differential pressure. Below that, the cabin altitude is regulated according to the schedule of the cabin pressure controller. At FL300 it will be a bit lower that the mentioned 8.000 ft.

The pressurization system of the ERJ for example maintains a cabin altitude of 8000 ft at FL370. Then you have max. differential pressure of 7.8 PSI.
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Tod
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RE: Air Pressure In Cabin

Mon Apr 04, 2005 11:25 pm

The lowest relative altitude I've seen on a regular basis is 5600ft on 744.

The highest was 8400ft once on a 734 last year.

Per the FAR25.841 the maximum is 8000ft .
 
loggat
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RE: Air Pressure In Cabin

Tue Apr 05, 2005 1:31 am

Quoting Troubleshooter (Reply 3):
The pressurization system of the ERJ for example maintains a cabin altitude of 8000 ft at FL370. Then you have max. differential pressure of 7.8 PSI.

The ERJ will hold a differential pressure near it's maximum value, long before it will hold maximum cabin altitude as Troubleshooter has rightly noted. I typically see 7.4 PSI with cabin altitudes around 4-5 thousand.
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troubleshooter
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RE: Air Pressure In Cabin

Tue Apr 05, 2005 3:02 am

Quoting Loggat (Reply 5):
I typically see 7.4 PSI with cabin altitudes around 4-5 thousand

This improves pax comfort. The ERJ pressurization system (automatic mode) starts to blow up the cabin when the theoretical max. differential pressure is reached or after a time delay of 15 minutes after take off. Whichever comes first. Theres a formula to calculate this in the AMM, but I´m to lazy to search for it at the moment.   

[Edited 2005-04-04 20:09:36]
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Arrow
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RE: Air Pressure In Cabin

Tue Apr 05, 2005 3:17 am

I wear an altimeter/watch (which I use for route finding on backpacking/mountaineering trips), and every time I fly I watch the cabin altitude. Just did a YVR-JFK trip on Air Canada A319, and the cabin altitude west to east was 7800 feet while the plane was at 37,000 ft. Similar numbers coming back. I've seen the cabin altitudes vary from 6000-8000 feet depending on aircraft and cruise altitude.

Legally, I don't think cabin altitude can ever get near 10,000 feet because at that level flight crew would need to be on oxygen, and the casual wine and beer drinkers in the cabin would all be seriously drunk (not to mention short of breath). Would be easier on the airframe, though.
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troubleshooter
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RE: Air Pressure In Cabin

Tue Apr 05, 2005 7:25 pm

Arrow,

most aircraft I know give a "cabin high altitude" warning on the EICAS at about 10.000 ft cabin altitude. You are right, the pressurization system will normally prevent this.
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PhilSquares
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RE: Air Pressure In Cabin

Tue Apr 05, 2005 8:19 pm

Just to give some idea of cabin pressure. On a recent, yesterday, flight from SIN-SEL we had a cruise altitude of FL370. Our cabin pressure was 4300' the cabin differential was 8.6psi.
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Tod
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RE: Air Pressure In Cabin

Tue Apr 05, 2005 11:49 pm

Quoting PhilSquares (Reply 9):
Our cabin pressure was 4300'

On a 744?

That's lower than I usually come across as a pax.
Thank you to you and your airline for being generous with the air!

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Jetlagged
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RE: Air Pressure In Cabin

Wed Apr 06, 2005 8:53 am

Quoting PhilSquares (Reply 9):
Just to give some idea of cabin pressure. On a recent, yesterday, flight from SIN-SEL we had a cruise altitude of FL370. Our cabin pressure was 4300' the cabin differential was 8.6psi.

At FL370, ambient static pressure is around 3.2 psi. That makes the cabin pressure 11.8 psi, which is around 6,000 ft. A typo perhaps?

Quoting Tod (Reply 10):
Thank you to you and your airline for being generous with the air!

It's not a question of the airline being generous. The pressure controller would do the same job on any 744. Boeing's design sets the maximum Pdiff and everything else follows. It doesn't cost much in bleed air terms to maintain a lower cabin altitude. The cost is in the weight of the fuselage structure.
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peterpuck
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RE: Air Pressure In Cabin

Wed Apr 06, 2005 1:49 pm

The 727 has a normal operational max. diff. of 8.6 as well, and we would see a cabin altitude of 6300' at FL370. So you are right, just a typo on phil's part.
 
Bellerophon
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RE: Air Pressure In Cabin

Thu Apr 07, 2005 4:11 am

Jetlagged

Normal max diff pressure on a B747-400 is 8.60 psi, which at FL 370 would give a cabin altitude of around 6,000 ft, so, as you suggested, it appears there is a typo in the figures posted by PhilSquares.

Interestingly however, the typo need not be the cabin altitude, as one might at first glance assume, but is probably in the aircraft altitude, and, just possibly, in the quoted max diff figure.

Substituting a 2 for a 7 in the quoted altitude (thus FL320 not FL370) makes the figures correct:

Max differential pressure......8.60 psi
Ambient pressure FL320......3.98 psi
Cabin pressure at FL320....12.58 psi
Cabin altitude at FL320.......4,300 ft


It is also, just, possible that they were operating at the limiting max diff of 9.4 psi on the B747-400 (can’t think why you would, but never mind) which would again make the figures correct:

Max differential pressure......9.40 psi
Ambient pressure FL370......3.15 psi
Cabin pressure at FL370....12.55 psi
Cabin altitude at FL370........4,300 ft


However, my best guess is that our friend simply typed FL370 instead of FL320.

Regards

Bellerophon
 
PhilSquares
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RE: Air Pressure In Cabin

Fri Apr 08, 2005 11:25 pm

Sorry for not responding earlier, however, I was on a trip.

Sorry for the typo, it should have been 6300' at 370.
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skywatch
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RE: Air Pressure In Cabin

Sat Apr 23, 2005 11:32 pm

In an MD11, the ideal DP at cruising is 8.6psid or less. Cabin pressure relief valves open around 8.76psid. If the cabin altitude exceeds 9,500ft, then all three ACC's and an independent aneroid baromemter will issue a Level 3 Alert. Oxygen masks drop from the ceiling at a cabin altitude of 15,000ft. On the Air Synoptic Display screen, the cabin vertical speed indicators are usually white, but turn amber if the cabin climb rate is above 1,500fpm for 1 minute, of if the cabin rate of climb is 3,000fpm for 5 seconds. The alert is issued if the cabin descent rate exceeds 750fpm for one minute, or 1,500fpm for five seconds. The maximum cabin DP for landing is 0.5psi.

Source: Delta MD-11 Pilot's Reference Manual
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Santhosh
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RE: Air Pressure In Cabin

Sun Apr 24, 2005 10:51 pm

Are spring loaded valves used in the cabin to release the pressure if it exceeds the nominal value of cabin pressure?

Regards
George
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HAWK21M
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RE: Air Pressure In Cabin

Sun Apr 24, 2005 11:35 pm

Quoting Santhosh (Reply 16):
Are spring loaded valves used in the cabin to release the pressure if it exceeds the nominal value of cabin pressure?

Also referred to as Safety Relief Valves.Spring loaded Diaphram Valves that dump Cabin Air overboard in case of Excess Diff Pressure.There are Two on the B737s.
Purely Mechanically operated.
regds
MEL
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