So I was reading about the 737's cabin-pressurization system, and then I came across these two bits of information:
Limitations
Max differential pressure:
Series Max Diff 1/200's [sic] 7.5psi 200Adv's [sic] 7.8psi Classics 8.65psi NG & Max 9.1psi [...]
Pressure (Safety) Relief Valves
These two valves, located above and below the main outflow valve, protect the aircraft structure against overpressure if the pressurisation control system fails. they are set at Originals 8.5psi, Classics: 8.65psi , NG & Max: 8.95psi.
According to these figures, the 737 NG and MAX have a maximum allowable pressure differential that exceeds (by 0.15 PSIg) that at which the cabin-pressure relief valve opens and vents the cabin pressure overboard.
Given that the purpose of the cabin-pressure relief valve is to prevent overpressure-induced structural damage, and that its activation threshold is presumably set where it is in order to give the necessary margin of safety between the point at which the valve opens and that at which the aircraft's structure would begin to suffer actual damage, allowing the cabin to be pressurized beyond the relief valve's activation threshold seems like it would inevitably come with an erosion of this safety margin and an increased risk of (potentially-serious) structural damage. (Not to mention how one would pressurize the plane beyond this point in the first place, what with the open relief valve venting the cabin pressure overboard.)
I'm assuming there's something I'm missing here, but what?
