Originally Posted by
Old Ag
The auto pressurization system doesn't function that way. If both controllers sensed an excessive cabin rate of climb (over 2000 ft/min) it would throw an auto fail without attempting to transfer to the alternate (no ALTN annunciation but a Manual annunciation), requiring manual control of the outflow valve. There is no indication of that happening and the NTSB is still looking into any pressure anomaly even if it doesn't meet the threshold to cause an auto pressurization error.
The primary pressure controller and the alternate pressure controller are independent and cannot know what each other is doing. A transfer from primary to alternate will be attempted whenever the primary detects a problem, if the alternate controller detects the same problem, then AUTO FAIL illuminates and the alternate controller will not take control, the manual annunciation will illuminate only when selected by the crew and both pressure controllers are overridden.
I think it is likely that both controllers in this case may have sensed a temporary change in the cabin rate of climb caused by the door plug shifting position momentarily during previous flights and that this may have generated the reported AUTO FAIL in the previous incidences. It would also confirm why no fault with the pressurisation system was found and why the aircraft was released back to the line.