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Presurising On The Ground Why?  
User currently offlineJumboJim747 From Australia, joined Oct 2004, 2464 posts, RR: 44
Posted (9 years 4 months 5 days 20 hours ago) and read 3901 times:

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.


On a wing and a prayer
16 replies: All unread, jump to last
 
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 1, posted (9 years 4 months 5 days 17 hours ago) and read 3844 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 2, posted (9 years 4 months 5 days 17 hours ago) and read 3847 times:

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



Think of the brighter side!
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 3, posted (9 years 4 months 5 days 16 hours ago) and read 3823 times:

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.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineTheGreatChecko From United States of America, joined Mar 2004, 1130 posts, RR: 2
Reply 4, posted (9 years 4 months 5 days 15 hours ago) and read 3802 times:

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



"A pilot's plane she is. She will love you if you deserve it, and try to kill you if you don't...She is the Mighty Q400"
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 5, posted (9 years 4 months 5 days 12 hours ago) and read 3729 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineJumboJim747 From Australia, joined Oct 2004, 2464 posts, RR: 44
Reply 6, posted (9 years 4 months 5 days 7 hours ago) and read 3600 times:

Thank you all for your help.
All the points make sense here i appreciated very much
Cheers



On a wing and a prayer
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 7, posted (9 years 4 months 5 days 6 hours ago) and read 3592 times:

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.)



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4211 posts, RR: 37
Reply 8, posted (9 years 4 months 5 days 3 hours ago) and read 3528 times:

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.


Chicks dig winglets.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 9, posted (9 years 4 months 3 days 18 hours ago) and read 3329 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineLightsaber From United States of America, joined Jan 2005, 13251 posts, RR: 100
Reply 10, posted (9 years 4 months 3 days 13 hours ago) and read 3284 times:
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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



Societies that achieve a critical mass of ideas achieve self sustaining growth; others stagnate.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 11, posted (9 years 4 months 3 days 11 hours ago) and read 3262 times:

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



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineFDXMECH From United States of America, joined Mar 2000, 3251 posts, RR: 34
Reply 12, posted (9 years 4 months 3 days 1 hour ago) and read 3183 times:

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.


You're only as good as your last departure.
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4211 posts, RR: 37
Reply 13, posted (9 years 4 months 3 days ago) and read 3169 times:

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.


Chicks dig winglets.
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 14, posted (9 years 4 months 2 days 17 hours ago) and read 3121 times:
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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
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4211 posts, RR: 37
Reply 15, posted (9 years 4 months 2 days 11 hours ago) and read 3080 times:

Don't have my manuals with me..but i believe its a 500FPM descent.


Chicks dig winglets.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 16, posted (9 years 4 months 1 day 8 hours ago) and read 2980 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
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