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mats
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Questions About Cabin Pressurization

Sun Feb 05, 2006 10:39 am

Some questions about pressurization:

1. When does it take place? During pushback? Taxi? Before takeoff? During takeoff?

2. Is it a conscious action by the pilot? In other words, does he or she say, "Now I'll pressurize the cabin. I'll press this button." Or does it just happen automatically?

I always thought that it was a gradual process, but I've heard stories about how taxi-ing aircraft are pressurized.
 
spacecadet
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Questions About Cabin Pressurization

Sun Feb 05, 2006 11:22 am

Quoting Mats (Thread starter):
I always thought that it was a gradual process, but I've heard stories about how taxi-ing aircraft are pressurized.

Pressurized to what? When taxiing, the outside pressure is already greater than the interior pressurization's ever going to be. There's no reason to pressurize a plane to greater than sea level. It'd be pretty uncomfortable for the passengers, like being underwater - not to mention uneconomical.

Someone else probably knows the technical term for this, but there is probably a switch thrown during taxiing that closes the air bleeds. This doesn't pressurize the cabin, though; it just seals it.

As the airplane climbs, pressurization is automatic. There's a pre-programmed pressure altitude that the airplane maintains, usually around 8,000 feet. It's not an all-or-nothing thing, though - it's not as if there's no pressurization below 8,000 feet and then it's suddenly switched on after crossing that threshold. At 8,000 feet the plane may be pressurized to, say, 4,000 feet - at 10,000 feet it might be pressurized to 6,000 feet, etc. up to a maximum pressure altitude of 8,000. There's a balance struck between economy and comfort, and it is a gradual process. This is something that can be selected by the pilot or engineer, depending on the airplane, but it's usually set once and then left alone for every flight based on manufacturer recommendations and airline policy.
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FlyDeltaJets
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Questions About Cabin Pressurization

Sun Feb 05, 2006 12:00 pm

When I pushback SU & OK they always asked to pressurize at the gate and you would hear the noises of the action.
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Dalmd88
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Questions About Cabin Pressurization

Sun Feb 05, 2006 5:21 pm

On climb out. To pressurise on the ground you have to bypass the safety valve. If the plane is in auto mode as soon as the mains come off the ground the aircraft will begin to pressurize.
 
Matt72033
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RE: Questions About Cabin Pressurization

Sun Feb 05, 2006 6:04 pm

Quoting FlyDeltaJets (Reply 2):
When I pushback SU & OK they always asked to pressurize at the gate and you would hear the noises of the action

thats hydraulics isnt it?
 
2enginesonly
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RE: Questions About Cabin Pressurization

Sun Feb 05, 2006 7:44 pm

Mats,

Yes, aircraft are pressurized on the ground but when depends on the a/c type.
Fokkers for instance are pressurized as soon the cabindoors are closed to 70' below sealevel ( if there's apu-bleed avail ofcourse....if not, it happens during engine start ). As soon as the powerlevers are advanced to take-off the cabinpress goes to 200' below sealevel. This is done to avoid pressurebumps during the take-off cycle thus to aid in passengercomfort.
For the B767 the cabinpress goes to 70' when the powerlevers are around 10-11 degrees past ground idle.
All this is done automatically by computers without any pilot actions.
These computers also take care that when an aircraft lands, the cabin is unpressurized and any remaining pressure will be dumped at touchdown. The computer will receive a signal from a ground/flight switch which is on the main landing gear.

For maintenance we sometimes have to pressurize the cabin on the ground and in some cases this will be to full max differential press but usually 4-5 psi is sufficient to check for leaks. This is normally done by 3 persons: 2 in the cockpit ( just in case somebody gets into problems ) and one outside to check if nobody gets near the a/c or tries to open a door ( which is hard to impossible due to the press difference ). The fun is to release the pressure in the cabin quick enough so you can get snow inside ( water in the air will turn to snow Embarrassment) but personally I don't like that because of pain in the ears Embarrassment)

I hope this explains it a bit. Ofcourse there's much more to it but this should do it.

Arjan
 
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HAWK21M
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RE: Questions About Cabin Pressurization

Sun Feb 05, 2006 8:42 pm

Quoting Mats (Thread starter):
When does it take place? During pushback? Taxi? Before takeoff? During takeoff

On a regular flight after the Cabin Exits are closed.A prepressurisation schedule begins after the Pressurisation mode is selected & the Flt-Grd switch is moved to flt posn to commence the same.Thereafter as the Flt Alt varies,the Cabin will climb accordingly depending on the mode selected.

Quoting Mats (Thread starter):
Is it a conscious action by the pilot? In other words, does he or she say, "Now I'll pressurize the cabin. I'll press this button." Or does it just happen automatically

Yes.

regds
MEL
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CosmicCruiser
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RE: Questions About Cabin Pressurization

Sun Feb 05, 2006 10:16 pm

Some of you nice folks really ought to specify what a/c you're talking about when you make a post like this. You can't say you "flip a switch" and that's true for every jet. 2enginesonly made the most accurate post and did specify certain types.
I've never heard of pressurizing at the gate and since I fly the md-11 I can say it pressurzes to .5psid on eng. start, packs can be on or off for t/o depending on perf. requirements.
 
vc10
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RE: Questions About Cabin Pressurization

Sun Feb 05, 2006 10:33 pm

I have been retired now for a while so rules might have changed, but here in the UK it used to be mandatory for an aircraft to be unpressurized whilst it is on the ground, otherwise if an acident happened whist on the ground the doors would be hard if not impossible to open.
The engine air bleeds can be selected open for airconditioning but the outflow valves will probably be wide open until the aircraft gets airbourne, which was the case on the last aircraft that I was involved in.

little vc10
 
fr8mech
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RE: Questions About Cabin Pressurization

Sun Feb 05, 2006 11:07 pm

SOME aircraft are pressirized to a very low PSID in order to begin the process as the throttles are advanced. OTHERS don't begin the process until take-off. SOME are fully automatic in their function, OTHERS require crew intervention. Do we see a pattern here. Different aircraft have different procedures and schedules.
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A/c train
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RE: Questions About Cabin Pressurization

Mon Feb 06, 2006 3:16 am

dont forget you can manually control cabin altitude on the ground, if you switch from auto to manual mode. How else do you do a pressurisation run.

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