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Pressurization In Aircraft

Mon Feb 12, 2007 12:53 pm

I am hoping that someone can explain how the pressurization system in a commercial aircraft works to keep it at a constant 8,000 feet.. Does the pressurization begin at a given altitude on ascent and does depressurization occur on descent? Thanks in advance.
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RE: Pressurization In Aircraft

Mon Feb 12, 2007 2:18 pm

It's only 8K at the aircrafts service ceiling. If the aircraft was cruising at 35K and it has a crevice ceiling of 41K then the cabin altitude will only be around 6K. The pressurization works by measuring the outside pressure. Most aircraft maintain a cabin psi of about 7-8 and will keep a sea level cabin altitude untill the outside pressure meets that pressure, then it will start to lower the cabin pressure according to the cruising level. Some aircraft like learjets have very high cabin psi number and can fly at 0 cabin altitude over 20K feet.
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RE: Pressurization In Aircraft

Mon Feb 12, 2007 2:19 pm

It depends on the particular aircraft, but presurization begins at takeoff (sometimes before sometimes right after), and the cabin climbs at a much slower rate than the actaul aircraft. While the aircraft might be climbing at 1,500 fpm, the cabin could would most likely be climbing at only 3-500 fpm. The systems logic used to regulate this vary greatly by aircraft, but it is [almost?] always an outflow vavlve that opens and closes to maintain the preasure that the copmuter wants.
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RE: Pressurization In Aircraft

Mon Feb 12, 2007 4:36 pm

Yes the air conditioning packs pump air into the aircraft all the time, and the outflow valve (or valves) let the air out. The pressurisation control computor moves the outflow valve open and closed to keep the get the cabin pressure right. After engine start (B737) or after take off the outflow valve will move from full open towards closed to control a steady rate of ascent in the cabin. The rate of change of cabin pressure is kept constant at about 600fpm climbing and 300fpm descending. All the pilot has to do is to tell the computor the altitude of the airport where you will be landing. In newer aircraft this is done automatically by an input from the FMS. The pressurisation computor works out for itself what to do when the climb or descent is stepped, and will calculate that you have started the descent. The rate of climb and descent can be adjusted by a knob but is usually left on the pip.
There are safety valves fitted. One or two inwards relief valves that open if the cabin pressure goes below outside air pressure, and usually two safety valves that open if the cabin air pressure goes too high. The inwards relief valve is just a sprung loaded flap with a 0.5psi spring. The safety valve is more complicated and is set to around 8.5psi differential. These valves very rarely operate as the system is very reliable.
I had a problem two months ago. The B757 was descending normally when at 6000ft there was a bang under the floor. The crew looked up and saw that cabin pressure was also at 6000ft. It took a while to work out that what had happened was that someone had played with the rate of change knob and it was set to minimum which is about 100fpm. During the descent the computor had descended the cabin from 8000ft at 100fpm. The aircraft had descended from 31000ft at 3000fpm, and at 6000ft had caught up with the cabin. The aircraft was at 6000ft descending and the cabin was also at 6000ft descending very slowly. So the inwards relief valves opened and the bang as they opened was heard on board.
This is quite rare in our operation and took some time to work out what had happened
The other defect we get is when the oytflow valve freezes up when you start descent. In the days of smoking on aircraft this was quite common as the outflow valve was always covered in nicotine, but nowadays it is a rare occurance.
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RE: Pressurization In Aircraft

Mon Feb 12, 2007 6:54 pm

Here you see a Pressure diagram of a A320:

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RE: Pressurization In Aircraft

Mon Feb 12, 2007 10:19 pm

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RE: Pressurization In Aircraft

Tue Feb 13, 2007 1:48 am

In most aircraft, the pressurization controller is automatically set, the cabin altitude rate of climb and descent can be varied as set. Usually when the aircraft begins its descent from cruise altitude the pressurization controller will begin descending the cabin altitude to the landing airport elevation (as adjusted). Manual mode is usually available for specific control enabling the crew to do things like maintain a higher differential as necessary. The diff cannot exceed the maximum design limitations for each specific aircraft. While automatic, crews must monitor the settings as it will only do what it is command it to do.
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