That’s how I imagined the design, as we can also hear the cockpit window seals leaking when dP < 2 PSI
Originally Posted by
NSEU
The relief valve is simply a flapper door with a spring on it (low tech). Like all springs, they will have an operating range. They are not light switches with instantaneous on/off actions. They "meter" the flow (according to the magnitude of differential pressure.
Then again. With a spring metered to open at -1.0PSI, by the time -1.45 PSI is attained, that valve must have opened some amount and let outside air in the cabin quite fast.
Back to that earlier scenario.
Starting data :
Packs OFF
Outflow valve Closed.
Reaching altitude 10.000ft after a rapid descent from fl200
Cabin altitude at starting point let say 18.000ft
At this starting situation, dP= -2.7 PSI
Negative pressure relief valve is opened by outside pressure flowing inside.
As it is springloaded cabin altitude vertical speed is at least proportionate if not squared with the dP.
The negative relief pressure valve is around 15cm x 25cm, I would imagine the cabin rate of descent at that starting point way above 1000ft/min. It could be determined by thermodynamic calculus but I am to rusty to take a blank paper!
From the sim experience of operating the Outflow valve manually, a very light squeeze of the OPEN/CLOSE switch (1/40 of the travelling) gives you already 500ft/min difference.
My guess of the relief valve opening at -2.7 PSI giving 1000ft/min shall be very conservative.
With this assomption, I cannot believe it would take more than 5min for cabin alt to decrease from 18.000ft to 14.000ft which relate in dP as -2.7 to -1.45 in 5min, as at -1.45 PSI negative relief valve is still opened by outside air so cabin rate of descent should still be of some fair amount...
What is your opinion on this ?