So I was reading about the 737's cabin-pressurization system, and then I came across these two bits of information:


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?


1 Answer 1


That second quote which states that "outflow valves on NG and MAX operate at 8.95 PSI" is incorrect. According to the Boeing 737 FCOM (page 751):

Two pressure relief valves provide safety pressure relief by limiting the differential pressure to a maximum of 9.1 psi.

Accordingly, that "limitation" table appears to be for the pressure at which the outflow valves operate and not the operational limit. Regarding the operational limit, according to the link you shared:

At cruise altitudes above FL370, the max differential is 8.35 PSI. which will result in a cabin altitude of 8000’ at FL410.


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