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.