Pressurization, Boeing 737-800

[KristianNorway]

Hello colleagues

Could you guys enlighten me when it comes to the pressurization system on the Boeing 737-800?

We are told that if you fly above your selected FLT ALT you may experience an overpressure (<9.1 psi) situation. Why is this?

[ajd1]

FCOM Vol 2, 2.40, pages 7-9 describe it, although not in any great detail. I'll check the maint books.

[KristianNorway]

Yeah, I've read the FCOM. It didn't help. :}

[Kuchan]

Normal differential pressure modulated by Discharge Valve is below 8.7 psi approx.

Either flying higher than than the allowable altitude or increasing decreasing cabin altitude would cause the positive relief valve to open at approx 9 psi. That is the diff press between the cabin and the ambient.

Attempting to go higher than the allowable altitude would result in increasing the cabin altitude that eventually deploying all oxygen masks.

[KristianNorway]

Thanks, but you're not telling me why. The primary pressure modulator is the outflow valve. Above FL290 but below- and including FL370 it regulates to a differential pressure of 7.8 psi, and when climbing above FL370 it regulates to a differential pressure of 8.35. All is, as you mentioned, below 8.7 psi.

Other than that you seem to contradict yourself.

Let's say you set 370 on the FLT ALT-selector and climb to FL400. Why may you get an overpressure situation?

[Kuchan]

It is the Delta D=differential pressure not over-pressurization when Discharge Valve and over-press valve are functioned on.

Flying higher would result less efficient in engines to produce sufficient pressure to maintain cabin altitude thereby inadvertently increasing the diff press. regardless of your selected cabin altitude.

Below 8.7 Diff, Discharge vlv is operating, Above 8.7 Diff, both Discharge vlv and Over Press vlv are operating. Above 9 diif., cabin press is decreasing regardless what setting you selected.

[KristianNorway]

Sorry, but with all due respect that's wrong in almost every sense.

Does anyone else have a clue?

[sheppey]

Word of caution. Notice the After take off Checklist in the 737 makes no mention of checking the pressurisation instruments. Most pilots new to the game only check what the checklist tells them to check. For example bleed switches and pack position. People have died because the pressurisation during the early part of the climb after take off was not checked to be working. An occasional glance at the two pressurisation instruments is vital in the climb to cruise altitude. Helios comes to mind.

[Al Murdoch]

It says to check Aircon and Press in my ATO checklist...

[KBPsen]

Let's say you set 370 on the FLT ALT-selector and climb to FL400. Why may you get an overpressure situation? Because the CPC calculates a target cabin pressure based on what is set on the cabin pressure control panel. This will then result in the expected differential pressure if you are at the same flight level as selected on the control panel. Go above the selected FL and the differential pressure will increase. Basic stuff really.

[ImbracableCrunk]

Posted by KBPsen:

Because the CPC calculates a target cabin pressure based on what is set on the cabin pressure control panel. This will then result in the expected differential pressure if you are at the same flight level as selected on the control panel. Go above the selected FL and the differential pressure will increase. Basic stuff really.
Agreed. The last paragraph in the FCOM where it talks about the cruise phase of flight reads:
Deviations from flight altitude can cause the pressure differential to vary as the
controller modulates the Outflow Valve to maintain a constant cabin altitude.

So, I think the computer looks at your FLT ALT and then says, "Ok this is the target PSID I will give them when they are there." (eg. 8.35PSID) Once within the 0.25PSID of FLT ALT, it goes into a "alt hold" type of mode where it now varies the PSID to keep a constant cabin alt.

If you go above FLT ALT, then the computer can't keep the cabin alt constant without increasing the PSID.

[KristianNorway]

Thanks guys. Those are the best explanations yet.
The reason I was asking is that it's easy to interpret the FCOM in a way that leads you to think the CPC monitors the cabin differential pressure even when on the selected flight altitude. But as ImbracableCrunk noted, it seems to try to maintain a constant cabin pressure once it reaches the selected FLT ALT (within 0.25 psi).
Pilots tend to answer "why"-questions with a statement of the obvious. Ofte n you get a procedure or the FCOM-text. It's refreshing to get a good "because"-answer.

So thanks ImbraceableCrunk :D

[Centaurus]

Mine too , reverse L scan.

As I understood his point, Sheppey was referring to the commonsense airmanship check of glancing at the pressurization instruments as part of the after take off checks. My 737 QRH does not mention this check - only (as Sheppey said) pack and bleed switch positions.

[KBPsen]

So, I think the computer looks at your FLT ALT and then says, "Ok this is the target PSID I will give them when they are there." (eg. 8.35PSID) Once within the 0.25PSID of FLT ALT, it goes into a "alt hold" type of mode where it now varies the PSID to keep a constant cabin alt. No. The CPC calculates a target cabin pressure, or cabin altitude if you will. If will then decrease the cabin pressure until the target pressure is reached and then maintains it whenever that is. The 0.25 PSI has to do with cancelling the off schedule descent function.

The CPC does not control differential pressure, it is a mere by-product. The CPC does however monitor differential pressure as if it is too high it may indicate a controller failure.

The FCOM is poorly worded as it gives the impression that differential pressure is an active control parameter, when it actually is a theoretical parameter used in determining the target cabin pressure.

[ImbracableCrunk]

I think that's basically what I said, minus the bit about about off-sched descent.