AN EMB145 EXPERIENCED LOSS OF PRESSURIZATION IN AUTO MODE AT FL330. CREW DECLARED EMER AND REGAINED CONTROL DURING DSCNT IN MANUAL.

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ON INITIAL CLB OUT; UPON LEVELING AT FL330; CABIN BEGAN TO LOSE PRESSURIZATION. RAN APPROPRIATE CHKLISTS AND REQUESTED LOWER ALT. ATC BEGAN TO 'STEP US' DOWN A COUPLE THOUSAND FT AT A TIME. WHEN WE REALIZED THIS WOULD NOT SUFFICE; WE DECLARED AN EMER IN ORDER TO GET US DOWN TO 10000 FT. AFTER RUNNING APPROPRIATE CHKLISTS; THE SIT BECAME UNDER CONTROL AND A CONTINUOUS CONTROLLED DSCNT WAS INITIATED DOWN TO 10000 FT. DISPATCH MADE THE DECISION TO RETURN THE ACFT BACK TO ZZZ WHERE A MAINT INSPECTION WAS TO BE COMPLETED. FAULTY OUTFLOW VALVE IS BELIEVED TO HAVE CAUSED THE PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF PRESSURIZATION AT LEVEL OFF STARTED SLOWLY THEN RAPIDLY INCREASED AND IMMEDIATELY ATC WAS ADVISED AND STARTED STEPPING THEM DOWN. THE RPTR SAID THE MANUAL MODE WAS BEING OPERATED AND PRIOR TO 10000 FT; THE PRESSURIZATION WAS BEING CONTROLLED. THE RPTR STATED THAT MAINT REPLACED THE OUTFLOW VALVE AND THAT WAS SUSPECTED BY THE FLT CREW.