When will positive pressure relief valves automatically open?

[B18210]

Hi folks

My question is as subject and I still cannot figure why even though the IP has given me two scenerios...
1. outflow valves failed and fully closed
I'd like to know what kinda situations will cause the cabin differential exceeds the limitation to make the valves open when outflow valves are fully closed.
2. CONDITIONAL, wind switch from strong tailwind to headwind
I have no clue on this statement

Thanks for clearing up the hints for me! :confused:

[tom35]

Hi,

I give you one scenario that will lead the valve to open.

Basically, the main target of the Cabin pressure unit is the comfort of passengers. So the best way is to handle the flight with a regular rate of 500 to 700 feet/minutes. That mean, from cabine cruise altitude (around 8000 feet) to ground level almost 15'

If you start descent from high altitude with very good rate, the plane could reach the cabin altitude, causing positive diferential pressure and opening your valve.

Today fuel saving purpose is a top priority for airlines and the good old descent with a 3° angle is not the best one to save fuel. We target a 4° descent, that mean, at 500 Kts a vertical speed of 3500 feet/minutes and less than 15' to get landed.

Rgds

[mono]

Tom35,

the senario you depict will result in NEGATIVE pressure differential (where the cabin pressure is less than the ambient pressure)

B18210,

The positive pressure relief valves will open when cabin pressure excedes a given value. Nominal max diff is around 8.6 psi for most commercial a/c. In the event of a failure of the pressurization system there are relief valves fitted which will open to prevent over-pressure (typically 8.8-8.9 psi diff)

Reasons for them opening would be for example a system failure or an error where the system is left selected to manual and the system outflow valves were closed (in this case as the a/c climbed the cabin altitude would remain at or below take-off elevation and eventually the max diff would be exceded).

Your second condition has nothing to do with pressurization. At a guess you are being given conditions which would lead to an overspeed situation.