737NG Cabin Pressurization

[B737NG_Pilot]

The 737NG pressurization schedule is designed to meet FAR requirements as well as maximize cabin structure service life. The pressurization system uses a variable cabin pressure differential schedule based on airplane cruise altitude to meet these design requirements.

At cruise altitudes at or below FL 280, the max differential is 7.45 PSI. which will result in a cabin altitude of 8000’ at FL280. At cruise altitudes above FL280 but below FL370, the max differential is 7.80 PSI. which will result in a cabin altitude of 8000’ at FL370. At cruise altitudes above FL 370, the max differential is 8.35 PSI. which will result in a cabin altitude of 8000’ at FL410.

This functionality is different from other Boeing models which generally use a fixed max differential schedule thus can maintain lower cabin altitudes at cruise altitudes below the maximum certified altitude.

The above 3 paragraphs are from The Boeing 737 Technical Guide.

I have set my Flight Altitude to L350. I climb to L330 and maintain L330, without resetting my cruise altitude. How does this help the life cycle of an airframe?

After a while I rest my my Flight Altitude to L330. Now the cabin starts to descent why?


How does variable differential schedule differ from the fixed one?

[flyboyike]

You actually fly the 737?

[B737NG_Pilot]

Yes I do. 700/800900ER

[flyboyike]

You sure? Absolutely sure? 'Cause I gotta tell ya, I have my doubts.

[B737NG_Pilot]

Well you can keep your doubts to your self. I ain't a push button pilot. If u can then answer my questions, or let the experienced pilots/technicians answer them.

[Dynamite1]

Flyboyike! Take it easy Guv.....

[Dynamite1]

My guess 737ng is .....that at FL 330 the ac could already be in cruise phase- because of probably being within 0.25 psid of the FLT ALT selection; FL 350 in your example. Now resetting FLT ALT to 330 may thus initiate a slight cabin descent.

As per the MRG ....refer pressurisation schedule.....

"This table explains why the cabin rate of climb indicator shows a descent when the airplane makes a step climb from a level flight below FL370 to a (new) cruising level above FL 370."

[B737NG_Pilot]

Thanks Dynamite.... I can't find it in the MRG. If could tell me where exactly.... Thanks

[Dynamite1]

Air Systems in the MRG under Limitations...rest my undertanding from cockpit companion.....
Do we share the same paymaster?? ......THE JOY OF FLYING

[flyboyike]

Originally Posted by Dynamite1

Flyboyike! Take it easy Guv.....

Take what easy? He's asking questions I've never seen or heard a line pilot ask. And look at his reply "I ain't a push button pilot". What does that even mean? Or is that something lost in translation from the Hindi?

[Dynamite1]

Tech log-The very best in practical technical discussion on the web!!!!!.......just to remind you flyboyike...
And for those who may be technically challenged...its never too late to learn....if they want to!!
Nyways who are you to decide what questions a line pilot should ask?
if you have something to contribute by all means sqwak....if not kindly maintain radio silence....
And what was that racial comment on translation from Hindi all about?
Do I see a Bleach Boy here........?

[flyboyike]

Easy on the nerves, Dynamite, it's all good. I'm sure Bleach Boy is not a racist statement at all.

[framer]

Flyboyike... I've just finished a conversation on another thread about this exact sort of thing. Someone asks a question ( presumably trying to better themselves ) and they are met with aggression right off the bat.
Pprune is becoming a very negative place to spend time. I think that we need to speak up if people wade in like you did at the start of this thread, otherwise all the mature knowledgable pilots won't bother coming back.
Nothing good was achieved by questioning the OP's honesty and employment, it diverted a tech thread and injected a fraction more confrontation into the forum.
This is what I said in another thread but I think it applies pretty much anywhere, Internet or real life:

The inclination to dive straight in with sniping snarling attacks when someone tries to better themselves lowers the forums tone. It smacks of either oversized ego or undersized confidence.

A group of pilots wouldn't put up with your aggressive questioning of whether the OP is in fact a pilot if we were standing around in a hangar, we'd tell you to wind your head in. Also, I may be wrong but I doubt in real life you'd ask him that question.

[Ozlander1]

The question was "Does this help the life cycle?"

Since none of you have a answer, I going to say, NO it doesn't. A cycle is a cycle whether it's to FL 240 or FL 400.

[JeroenC]

The NG is certified to higher alts (FL410) than older models. In order not to re-certify the whole pressurisation controller the regime below FL370 was kept and a higher differential regime was "added". So the goal was not to preserve cycles but to ease certification.

So I was told, standing by for correction.

[Dynamite1]

Very well said Framer....
In all the smoke missed the basic question...spot on abt the life cycle Ozlander..
Saves costs JeroenC not to re-certify like you said.
And Flyboyike..its all indeed good mate..
Happy ppruning!

[rottenray]

Wow, let's beat up someone who asks a question...

Originally Posted by flyboyike

Take what easy? He's asking questions I've never seen or heard a line pilot ask. And look at his reply "I ain't a push button pilot". What does that even mean? Or is that something lost in translation from the Hindi?

First. that's pretty racsist.

Next, you could do something different than you usually do - you could simply answer the question, share the knowledge you have with others so everyone can learn.

Frankly, you and a few others have badly polluted these fora with snippy, crass remarks.

I'm not a pilot, merely an enthusiast who at one time greatly enjoyed Pprune, before the likes of you with your "private club" attitude, and a few others who actually tell folks posting questions they should be using google instead, have taken over the discussions here.

I really, really hope I never have the pleasure of trusting my life to someone with your attitude.

If you behave this way here - in an innocent discussion setting with zero pressure - I would hate to trust your CRM in an emergency.

[B737NG_Pilot]

Thanks Dynamite, Framer, Ozlander1, JeroenC. My quest to learn about differential pressure control, as led to me learn about various other types of pressure controllers used on other aircrafts.

After asking the same posted question on pressurisation to various pilots / techs , I have always got a different answer.

The Boeing FCOMs, are good but written on a need to know basis.

Dynamite when you say it saves cost? You mean for the airframe or the controller it self?

By the way when I used the term "Push Button Pilot", its a term used to describe pilots, who fly modern flight decks, and actually push buttons to fly the a/c. You will need to be in the industry long enough to know about it. Most people don't want to know why?

Thanks a lot people.

[de facto]

At cruise altitudes at or below FL 280, the max differential is 7.45 PSI. which will result in a cabin altitude of 8000’ at FL280. At cruise altitudes above FL280 but below FL370, the max differential is 7.80 PSI. which will result in a cabin altitude of 8000’ at FL370. At cruise altitudes above FL 370, the max differential is 8.35 PSI. which will result in a cabin altitude of 8000’ at FL410

No.
The MAX cabin altitude the cpc will allow is 8000 ft.
At intermediate levels the cabin altitude will be dependent on the max psi as you mentionned above but also on the cabin altitude you have on top of your head when seated on the right seat.
At FL280 for example,the cabin would be about 5000ft.

If you want a lower cabin altitude,you can set in the FL a level which will make the CPC schedule a higher PsI therefore a lower cabin.

Example:if you set 375 and fly at 330 , the psi will be limited to 8.35 instead of 7.8,therefore a lower cabin,7000 ft instead of 8000ft.

The difference is not worth messing about,however on the planning ground stage,setting the highest level you can reach within your SOP,personal preference(wx/margin),will be benefitial in two ways.

First if you dont get your optimim level,you may keep a lower cabin altitude until you reset your CP Cruise level,again some sop are requesting it to be done when the final level is reached,ATC final FL 330,you level off and set the CP FL.
Or you can wait to reach before you descend(before descent checklist).

So in that case you may have a cabin altitude rise from lets say 6000 to 7000 ft.

Secondly if you forget to reset the CP fL during descent,you will simply get an off schedule descent.
Much better than setting 270 on the ground but climbing to 410 and not resetting the CP fl which will possibly get the pressure releief valves to operate.

I have set my Flight Altitude to L350. I climb to L330 and maintain L330, without resetting my cruise altitude. How does this help the life cycle of an airframe?

After a while I rest my my Flight Altitude to L330. Now the cabin starts to descent why?

The CPC may have scheduled at fl 350 the psi to be 7.8 and cabin of 7000f,at fl 330 psi of 7.8 and cabin of 6500.
When you reset the CP fl, the cabin will descend to the new calcualted cabin altitude.
The CPC knows your actual altitude via the alternate static and will readjust the cabin descent/climb rate depending on your actual rate of descent.

Now you have obviously never looked at your actual cabin altitude at various FL,which leads me to believe you are either not rated or doing your rating and havent been in the sim yet.
If you are not rated nor going to be on the NG,maybe changing screen name would help getting sensible answers to your type related questions.

[B737NG_Pilot]

Thanks for the brilliant explanation de facto. Appreciate your effort and time. Well I am flying the 737NG series for a year now, but this was doubt which no was able to explain the way u did it..



Thanks and Happy Landings.