If the system limits a pressure above the local atmosphere, which is what a Safety Relief valve normally does, then the plane would remain pressurized. Being as pressure is relative, if it was keeping 5 psig (delta pressure) at altitude it could maintain that on the ground. Apparently cruise is typically 7-8.5 psig with the lower value used to increase allowable cycles and the upper limit to be more comfortable for passengers.
The question then is, will the air back-flow to the engines from the fuselage the same way it came in. There appears to be a valve that will close on low-pressure sensing to prevent back-flow, but I doubt it would automatically close on engine shutdown. If it doesn't the remaining pressure should only make it impossible to open the doors until it bleeds through the typical leak paths passenger jets seem to have. (Like the over wing door seal that I saw a 1/8th gap until they ran the engines up and forced the door out. I noticed it from the amount of engine noise next to my head.)
To add - if one were to seal the plane like a bottle at altitude and not maintain that pressure difference continuously and then return it to the ground there would be more pressure outside than inside - there are relief valves for that. One system my company made for B-52s had that valve fail and the electronics package inside the cylindrical enclosure got shrink-wrapped when the plane returned to the ground.
Example: at 12,000 feet the outside air pressure is roughly 9.3 psia. At 30,000 ft it is roughly 4.3 psia with the cabin at 12,000 ft it would require a difference of 5 psi.
If one were to land with a 9.3 psia in the plane it would experience nearly -5psig and likely beer-can large portions of the fuselage; there are under pressure valves to prevent that. OTOH if one maintained 5 psig to the ground it would be about 19.7 psia inside, but still only 5 psig, just the same as when flying at 30,000 ft.
Last edited by MechEngr; 20th Dec 2022 at 08:13.