737 pressurization and Off Sched Descent
All very correct except that the Max Diff for T/O and Landing is .125 psi which according to my manual is an AFM limitation from Boeing. If this number was used because of JAR requirements etc. I do not know.
As for the OFF SCHED DESCENT light. It is connected to the actual A/C Pressure Alt and is only functional when using the Auto Mode. As the A/C climbs to within 1000’ (so the good book says) of the altitude set in the flight alt window of the Auto controller a rely closes or trips whichever you like to say that signals the controller to begin maintaining a constant cabin pressure differential of 7.45 or 7.8 psi. (there may be others but these are the max normal diff’s the controllers in our A/C are designed to maintain) If this rely does not get activated prior to the A/C starting a descent then the aforementioned light will come on which is telling you that the controller has started to schedule a descent back to the T/O field alt as memorized by the controller prior to T/O (it’s actually the cabin alt at T/O it remembers) In short it thinks you are in an emergency return to departure airport and thus relieving the crew of having to deal with changing the pressurization controller during times of high work load.
You can imagine what would happen to the diff pressure if you departed from an airport at sea level and were going to land at one at say 3000’ ASL or visa a versa and never achieved you selected flight alt. Ie: You would not have .125 psi diff. Thus the Warning / Information light.
It should be noted also that if at anytime during the flight the flight alt selector knob is depressed as if to change the flight alt in the window this function is lost and could result in over pressurization etc. at landing.
Now that I have rambled on about things you may already know. The answer to your question is no you will not get this light during a slow decompression. You would if the rely had not closed / tripped prior to you starting the descent no mater what the current cabin pressure.
Cheers all and hope this helps.