mm43
Thanks for answering the question. Reference to the Cabin Pressure Controller schematic shows that in Automatic Mode the barometric reference comes from the ADIRUs, but my take is that should that reference fail, then it selects Manual and the Safety and Relief valves will attempt to keep Cabin Pressure equal to the outside when descending below 8000/7350 feet. Here's the schematic for good measure -
Cabin pressure is normally controlled automatically but can be controlled by the aircrew in a manual mode if certain failures prevent the normal auto operation.
But, more importantly, if all else fails, the overpressure relief valves work on simple springs that operate autonomously should the pressure in the cabin go above normal differential limits. Also a negative pressure relief valve also works autonomously on simple springs to relieve pressure differential between inside the cabin and outside, if the internal cabin pressure becomes too negative.
I love your dedication, research and posts...many thanks and well done!