jumbojim747
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Presurising On The Ground Why?

Sun Jun 19, 2005 6:29 pm

Why do the pilots pressurize the aircraft on the ground.?
Wouldn't it be too early to do so as they would have a fair amount of time to do it while airborne.
Im thinking maybe it would be a bad idea for the aircraft as it sometimes needs to go back to the gate for different reasons and then it has to be un presureized again.
Why not just do it while say at climb.
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Jetlagged
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RE: Presurising On The Ground Why?

Sun Jun 19, 2005 9:08 pm

It's done automatically as soon as throttles are advanced to takeoff power on most modern airliners, not a pilot action.

All this does is pre-pressurise the cabin to about 200 feet below aircraft altitude so dumping this for an immediate landing is not a problem.

In the case of an aborted takeoff, the cabin would automatically depressurise on closing the throttles.

Pre-pressurising puts the outflow valves in a controlling mid position, rather than fully open. This helps smooth out cabin pressure bumps which might occur on rotation. It also means the crew can't forget to start pressurising the aircraft during the climb  Smile

On older, three crew aircraft such as the 747-200 it was usual to take off unpressurised, with the packs being switched on one at a time during initial climb. A two man crew would not necessarily have the time to carry out this procedure properly.
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HAWK21M
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RE: Presurising On The Ground Why?

Sun Jun 19, 2005 9:13 pm

In ref to Pressurisation in case of Chapter 21.On the B737 pre pressurisation is carried out to prevent a pressure bump inside the cabin when the Aircraft gets Airborne.Its a matter of a flick of a switch & is not a lengthy process.

If "Presurisation" refers to the Chapter 29,Hydraulic system,then its to check for leaks & any warning lts.

regds
MEL
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SlamClick
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RE: Presurising On The Ground Why?

Sun Jun 19, 2005 10:22 pm

It is not pressurized to the full allowable differential, but just to maybe 200 feet below field elevation - a fraction of a pound per square inch.

In addition to the benefits above, relating to the outflow valve, consider this: A small amount of pressure will seat the plug-type doors and windows fully. Also, a slightly pressurized fuselage tube is stronger than an unpressurized one. Open an aluminum beer can some time. As soon as you break the pressure seal the sides become really flexible. Metal is the same thickness as before, the difference is a very small amount of internal pressure, or simply, resistance to compression.
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thegreatchecko
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RE: Presurising On The Ground Why?

Sun Jun 19, 2005 10:55 pm

When I fly the King Air, we pressurize it for the above reasons, but also because it provides for a slightly quieter cabin due to the better gap seal. It provides for a slightly more comfortable cabin for the passengers and thats what its all about!

GreatChecko
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Jetlagged
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RE: Presurising On The Ground Why?

Mon Jun 20, 2005 2:04 am

Slamclick, you jest surely?

Aluminium beer cans don't have ribs and stringers.  Smile

I doubt 0.1 psi differential would strengthen the structure noticeably.
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jumbojim747
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RE: Presurising On The Ground Why?

Mon Jun 20, 2005 6:56 am

Thank you all for your help.
All the points make sense here i appreciated very much
Cheers
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SlamClick
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RE: Presurising On The Ground Why?

Mon Jun 20, 2005 7:44 am

Quoting Jetlagged (Reply 5):
Slamclick, you jest surely?

Absolutly not.

Of course a pressurized tube is stronger than a non-pressurized tube. (So long as the amount of pressure applied is not, itself, a strain on the structure.)
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XFSUgimpLB41X
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RE: Presurising On The Ground Why?

Mon Jun 20, 2005 10:36 am

the CRJ does the same as the 737 as SlamClick described...It's only when power is advanced for the takeoff roll, not during taxi.
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Jetlagged
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RE: Presurising On The Ground Why?

Tue Jun 21, 2005 8:26 pm

Quoting SlamClick (Reply 7):
Of course a pressurized tube is stronger than a non-pressurized tube. (So long as the amount of pressure applied is not, itself, a strain on the structure.)

Stiffer yes, stronger no. It will fail in tension at the same load. So it won't flex as much but will only support the same bending moment from the tail for example.

If an aircraft fuselage required pre-pressurisation to be strong enough it shouldn't be flying.
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lightsaber
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RE: Presurising On The Ground Why?

Wed Jun 22, 2005 1:25 am

Quoting SlamClick (Reply 7):
Of course a pressurized tube is stronger than a non-pressurized tube. (So long as the amount of pressure applied is not, itself, a strain on the structure.)



Quoting Jetlagged (Reply 9):
Stiffer yes, stronger no. It will fail in tension at the same load. So it won't flex as much but will only support the same bending moment from the tail for example.

Stronger from a buckling standpoint, an important criteria with aluminum aircraft.


Quoting Jetlagged (Reply 9):
If an aircraft fuselage required pre-pressurisation to be strong enough it shouldn't be flying.

True. But it might minimize fatigue and thus reduce life cycle costs. (I'm speculating... )

Lightsaber
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SlamClick
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RE: Presurising On The Ground Why?

Wed Jun 22, 2005 3:03 am

Quoting Jetlagged (Reply 9):
It will fail in tension at the same load.

I'll keep that in mind next time someone tries to pull my airliner in half. Smile
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FDXmech
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RE: Presurising On The Ground Why?

Wed Jun 22, 2005 12:32 pm

On a different note, the DC10 pressurization schedule maintains field elevation until 5000 feet. This mode in case the aircraft needs to return to the airport.
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XFSUgimpLB41X
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RE: Presurising On The Ground Why?

Wed Jun 22, 2005 1:53 pm

On the CRJ, if we begin a descent before we reach 6000 feet the airplane automatically thinks that we are returning to the field and sets cabin elevation for where we just departed from.
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2H4
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RE: Presurising On The Ground Why?

Wed Jun 22, 2005 9:13 pm




Quoting XFSUgimpLB41X (Reply 13):
On the CRJ, if we begin a descent before we reach 6000 feet the airplane automatically thinks that we are returning to the field and sets cabin elevation for where we just departed from.

What constitutes a descent to activate this? If, for example, you have to level off at 5000 feet for traffic, and during your level-off you sink 30 or 40 feet, does that set cabin elevation to the departure airfield?



2H4


Intentionally Left Blank
 
XFSUgimpLB41X
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RE: Presurising On The Ground Why?

Thu Jun 23, 2005 3:27 am

Don't have my manuals with me..but i believe its a 500FPM descent.
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Jetlagged
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RE: Presurising On The Ground Why?

Fri Jun 24, 2005 6:27 am

Quoting Lightsaber (Reply 10):
Stronger from a buckling standpoint, an important criteria with aluminum aircraft.

So perhaps the wings and tail should be pressurised too? Big grin

Quoting Lightsaber (Reply 10):
True. But it might minimize fatigue and thus reduce life cycle costs. (I'm speculating... )

Pressurisation cycles decrease fatigue life.
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