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readytotaxi
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Trying To Understand Cabin Pressure?

Mon Jan 10, 2011 7:48 pm

I would like to know the procedure in the cockpit from take off to landing for setting the cabin pressure.
As a passenger I enjoy a comfortable cabin from start to finish, what is going on in the cockpit during my flight?
From the ground to our cruising height and then the decent are the crew adjusting the cabin pressure as we go or is this computer generated? How does it work pls.  
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roseflyer
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RE: Trying To Understand Cabin Pressure?

Mon Jan 10, 2011 8:21 pm

After engine start the bleed system and cabin pressure is pretty much set to auto. The only times when the pilots are working actively with the pneumatic systems are during engine start. Everything is done automatically. Not all planes us computers however. Automatic cabin pressurization has been around since the stratocruiser in the 40s!

The amount of air for ventilation is actually more than needed for pressurization. The air comes off the Air Conditioning Packs under the cabin forward of the wing and goes through the main air distribution system. Passenger air distribution is controlled by quite a few valves and sensors. Air also goes to electrical equipment for cooling. It's far too difficult to easily explain.

Here's a good website for explanations:
http://www.b737.co.uk/

[Edited 2011-01-10 12:36:00]
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Mir
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RE: Trying To Understand Cabin Pressure?

Tue Jan 11, 2011 12:16 am

Quoting readytotaxi (Thread starter):
what is going on in the cockpit during my flight?

Normally nothing. Most modern airliners have automatic pressurization systems that handle all the dirty work themselves, taking information from the FMS for destination altitude, cruise altitude, current altitude, and all sorts of other parameters, and then developing and running the pressurization schedule itself. All the pilots have to do is make sure it's doing what it's supposed to do. There are, of course, manual backups, and then the pilots will have to run the pressurization themselves by metering how much air exits the cabin through the outflow valves. You've got charts that tell you what pressure the cabin should be at for a certain cruise altitude, and you try and make the necessary changes as smoothly as you can. But it isn't easy, and you're liable to get some ear-popping, especially if you're changing altitude quickly.

-Mir
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ShyFlyer
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RE: Trying To Understand Cabin Pressure?

Tue Jan 11, 2011 12:20 am

In modern aircraft, the process is automated. The computers aim to maintain pressure equivalent to between 5,400 to 7,000 feet above sea level. The exact pressure depends upon the aircraft design, the altitude the aircraft is cruising at, and the elevation of the destination.

The system will either get the needed information itself from the flight management system or require that the crew input the destination airport's elevation into the pressurization control panel.

On older aircraft, these duties were handled by the Flight Engineer. Although, as RoseFlyer mentioned, some of those systems were automated in some fashion as well.
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tdscanuck
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RE: Trying To Understand Cabin Pressure?

Tue Jan 11, 2011 4:33 am

Quoting readytotaxi (Thread starter):
From the ground to our cruising height and then the decent are the crew adjusting the cabin pressure as we go or is this computer generated? How does it work pls.

A modern automated system takes three inputs...takeoff field altitude, landing field altitude, and cruise altitude. These are used to design the pressurization schedule. The nice systems pull all this information from the flight management computer, so there's no extra inputs for the flight crew (which means "nothing" is happen in the cockpit vis a vis pressurization during the flight).

The system will start out at the ambient altitude of the takeoff airport (outflow valves full open). Some time during the takeoff roll the system will pressurize slightly to reduce pressure transients as the ECS system fires up during takeoff. From there, it will increase cabin altitude at some fraction of the climb rate so that you end up at the target cabin altitude when you hit cruising altitude. Depending on the nuts and bolts of the pressurization controller, this may stay constant for cruise or may change slightly with step climbs. The process is reversed on descent, ramping from the cruise cabin altitude to the landing field altitude.

In the infrequent case that the landing altitude is actually higher than the normal cabin altitude (i.e. airports above about 8000') the cabin actually *climbs* during the descent to match up to the landing altitude. Extra steps are necessary if the landing airport is above the altitude that triggers cabin warnings or oxygen mask deployment.

Tom.
 
ThirtyEcho
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RE: Trying To Understand Cabin Pressure?

Tue Jan 11, 2011 7:38 am

Laughably, this reminded me of the first time I operated a pressurization system as a pilot on a training flight. My ears haven't forgiven me for that one and neither has the instructor.
 
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DocLightning
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RE: Trying To Understand Cabin Pressure?

Tue Jan 11, 2011 8:55 am

Quoting tdscanuck (Reply 4):
Extra steps are necessary if the landing airport is above the altitude that triggers cabin warnings or oxygen mask deployment.

Like what? What's the altitude at which the emergency triggers...um...trigger?
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tdscanuck
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RE: Trying To Understand Cabin Pressure?

Tue Jan 11, 2011 12:45 pm

Quoting DocLightning (Reply 6):
Like what? What's the altitude at which the emergency triggers...um...trigger?

The exact figures are type dependent but, generally, above 10,000' the cabin altitude warning horn goes off and above ~13,000' the cabin oxygen masks drop.

If you're in the band between 10,000' and 13,000' you just have to deal with the warning horn...above the oxygen mask trigger altitude you need to do something to prevent the masks from dropping until the plane pressurizes. On the newest stuff I think this is automatic, but there are a lot of airplanes running around with systems that were designed before airports that high were factored in.

Tom.
 
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readytotaxi
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RE: Trying To Understand Cabin Pressure?

Tue Jan 11, 2011 5:53 pm

Wow people, many thanks for the feed back.
The detail was great and easy to understand.
Cheers.   
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