Pressurization, Boeing 737-800
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Pressurization, Boeing 737-800
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?
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?
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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.
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.
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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?
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?
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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.
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.
Last edited by Kuchan; 24th Mar 2012 at 12:31.
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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.
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Originally Posted by KristianNorway
Let's say you set 370 on the FLT ALT-selector and climb to FL400. Why may you get an overpressure situation?
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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.
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.
Deviations from flight altitude can cause the pressure differential to vary as the
controller modulates the Outflow Valve to maintain a constant cabin altitude.
controller modulates the Outflow Valve to maintain a constant cabin altitude.
If you go above FLT ALT, then the computer can't keep the cabin alt constant without increasing the PSID.
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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
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
Mine too , reverse L scan.
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Originally Posted by ImbracableCrunk
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.
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.
