When the airstair door was closed, at that point the fuselage pressure vessel was sealed. If the pilot/co-pilot had not opened the airstair door, the fuselage would have pressurized itself to the maximum (roughly 6 to 8 psi) until the safety valve opened. It would have remained so until a way was found to shut everything down. Then some way would also have been needed to depressurize but with the OF valve locked closed, and the safety valve normally closed, that could take a long time... There's nothing you can do to the TROV from the outside without damaging it.

At the moment the pilot got the airstair door open, there could have been a fraction of a psi on the door, but do the math... hundreds of pounds pushing it open already. Even if the airstair door has a little equalization vent on it, and even if the pilot thought to use it, it's probably sized only to relieve a slight imbalance after the aircraft is shut down. It would have to be quite large to stay ahead of the inflow when the engines or APU are running. Even if it actually is just big enough to equalize against the APU inflow, given the head start, and the flight attendant could already feel the pressure coming on, then it would have taken a long time to bleed down through the airstair vent. My reading of the report is that the pilot got into a bit of a panic and didn't wait. Then again, maybe there is a vent built into the door buy you have to start opening the handle to activate it, and the difference between venting and locked versus venting and unlatched is 1/16" of handle movement... Would have to go see a G150 to know.

Also not specific, but my reading is that the internal cockpit door wasn't closed. It was swinging loose and struck her when it moved in the outward rush of air. She wouldn't necessarily have seen it coming.

LOTS of Gulfstream jet info (but just the 450): http://code7700.com/g450.htm

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

yeah, I know the pilot fudged the system to no end, that's not what I'm querying. I was thinking of the "means for prevention of (unsafe) pressurisation". I thought all doors (particularly pax main entry door) would've have this.

another day in paradise, or is paradise one day closer ?

From the perspective of the door's mechanism, wouldn't this appear normal?
No interconnection exists between pressurization system settings and the vent valve on a door.
Or are you asking if there's an override on the door latch, to prevent opening when a certain threshold pressure differential is exceeded? That may be a case of some doors have it, some don't... have to look at a G150.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

no, the vent door is supposed to be the first action in opening the door. It's intended to vent cabin pressure, to prevent this very event.

another day in paradise, or is paradise one day closer ?

Somewhat similar occurrence, +25-years, with a C-5A.

Aircraft landed and taxied quickly to a parking ramp. As the engines were winding-down... the crew chief initiated opening of the forward PAX door while standing in the doorway... before full depressurization. The 'whoosh' of slightly pressurized fuselage air... estimated at ~0.1-to-0.2-PSI delta-P... combined with the massive air volume of the fuselage... shoved/blew the crew chief out the door opening: he fell +8-ft onto the concrete ramp, suffering fatal injuries. This was a wide lesson-learned that was integrated into all USAF 'heavy' aircraft operations. I think that actuated doors all incorporate lock-outs and vents in-case-of residual pressurization. NOTE. 737's now have outward opening escape doors... I presume that pressurization would 'help to open these doors' in a crash/damage scenario.

About 20-years ago a KC-135 undergoing maintenance at Tinker AFB inadvertently experienced gross-over-pressurization during a ground engine-run test [pressure-test instrumentation issue]... the aft pressure bulkhead failed [delta-P +18-PSI] and the aft fuselage and empennage came to rest on the ramp with internal debris scattered everywhere. https://theaviationgeekclub.com/time-kc-135-strato...

FYI for grins...
SAE ARP1270 Aircraft Cabin Pressurization Criteria contains the following sections...

3.3 Safety
3.3.1 Overpressure Relief
3.3.2 Negative Pressure Relief
3.3.3 Built in Safety Limits
3.3.4 Ditching
3.3.5 Cabin Altitude Limiting
3.3.6 Cabin Pressure Dump
3.3.7 Cabin Pressure Dump on Landing
3.3.8 Prevention of Initiation of Pressurization If External Doors Are Not Closed, Latched and Locked
3.3.9 Isolation Valve

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]