What I meant was to simplify everything, after the rapid descent, considering a/c after its descent at 10.000ft. While Pcab remains constant, to simplify... I would call it the static scenario.
P at 20000 = 6.75 PSI
P at 18000 ft = 7.34 PSI
P at 10000 ft = 10.11 PSI
thus Pcab (18k) - Pstatic around a/c 10k = 7.34-10.11= -2.7 PSI
I completely agree that in real situation, if Pcab=7.34 PSI, negative pressure valve will open at 7.34+1= 8.34PSI (14.900ft).
So in dynamic situation and if Boeing design is working properly (as I assume here): during descent with the same scenario from 18000ft to 10000ft dP will go from 0 PSI (Pcab=Pstatic) to -1 PSI when aircraft reaches 14.900ft,
the. The negative valve starts opening while aircraft continue towards 10000ft, and cabin altitude will descend as air flows inside the vessel. Maintaining around -1.0 PSI as the aircraft reaches 10.000ft, Pstatic=10.11, Pcabin~9.11PSI and the initial scenario is invalid because:
-Aircraft at 10.000ft
-Pstatic =10.11PSI
-dP= -1.0PSI
-Pcab= 9.11
-Cabin altitude =12.700ft.
In the initial scenario, colleagues called “masks off” as a/c was at 10.000ft, instructor called for UAS because cabin alt (in the sim) stayed above 14.000ft so dP > -1.45 PSI during 8min after masks where called off.
Instructor point is : Boeing QRH states : MASKS OFF when cabin altitude at 10.000ft or lower, which is for a pilot operating as per Boeing is correct, and was not respected in this case.
But my interrogation is and remains, what about it in pure technical considerations, considering the negative pressure valve and natural leaks,
dP > -1.45 PSI for 8min seems like a very long time...
This why I tended to simplify with a static scenario, in my last reply, where I get a fictional -2.7 PSI dP as a starting point (disregarding the descent time and equalization during descent).
And 8 min for dP to go from -2.7 to -1.45 seems as well unrealistic as:
Alt cabin with -2.7 psi = 18000ft
Alt cabin with -1.45 psi = 14000ft
8 min time gives a 500ft/min cabin rate of descent, and seems way to low IMO for a safety device, that’s why I asked advice here...