Positive Pressure and Negative Pressure relief valves MEL B777

[extricate]

Hi there,

I'm looking at the MEL item for Positive and Negative Pressure relief valves.

Positive Pressure Relief Valve: No. installed: 2, No. required: 1

Negative Pressure Relief Valve: No. installed: 4, No. required: 0 but flight has to be conducted unpressurized.

Seems like Negative Pressure Relief Valve is more important than Positive Pressure Relief Valve, why is that so?

Any examples will be good. Thanks for your time

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[Highway1]

Might be an airline difference but in my MEL there is the option to have both Positive Pressure Relief Valve's inop and operate unpressurized in the same way as having all Negative Press Relief Vents inop.

Therefore is you have at least 1 valve and one vent operational then normal operations are allowed.

[mustangsally]

Go back to basic definition:
Positive Relief valve prevent an over pressure within the fuselage. Most are set to open some where around 8.5 psi differential and fully open at something north of 9.0 psi.
Negative Relief valve are to prevent a lower pressure within the fuselage than atmospheric pressure around the fuselage.

When a door or escape hatch is opened, to much positive pressure with in the fuselage would/could prevent this door/hatch from being opened. With a negative pressure within the fuselage the door/hatch would blow in causing possible personal injure.

[Chris Scott]

Originally Posted by extricate

Hi there,
I'm looking at the MEL item for Positive and Negative Pressure relief valves.
Positive Pressure Relief Valve: No. installed: 2, No. required: 1
Negative Pressure Relief Valve: No. installed: 4, No. required: 0 but flight has to be conducted unpressurized.
Seems like Negative Pressure Relief Valve is more important than Positive Pressure Relief Valve, why is that so?
Any examples will be good. Thanks for your time

The fact there's 4 doesn't necessarily mean in itself that negative relief is more important, but I think it probably is. The fuselage is simply not designed for negative differential pressure. Also, negative diff-pressure is typically caused by the crew performing a long-term high rate of descent - usually because circumstances have delayed the TOD. (We used to call it "catching up with the cabin.") It happens quite frequently on older types, where cabin-pressure control is manual and the crew may even forget to start the cabin descent at TOD.**

If you fly the a/c unpressurised, the outflow valve(s) will be locked open. That should keep the cabin pressure close to ambient. But that doesn't explain, admittedly, why the option of flying unpressurised is not included in your quoted list for the case where both positive pressure-relief valves are u/s.

** As an aside, one of the few clues available to the approximate time of the demise of AF447 was the "low differential-pressure" warning transmitted to Paris by ACARS, which we were able to infer would have likely happened as the a/c descended through about 6000 ft.