Narrative:

A normal preflight; taxi; takeoff; and climb were conducted and all checklists completed through FL180. At about FL280 and around XA05Z; the altitude warning horn sounded. Oxygen masks were donned; communications established; and the cabin altitude was noted at just above 10000 ft and slowly climbing. No warning or abnormal lights were noted on the pressurization panel (or any other panel for that matter). The QRH followed with no results and the outflow valve was noted stuck in the near closed position. An emergency was declared; a descent to 12000 ft (assigned by ATC for terrain clearance) was commenced and left turn toward ZZZ was started as assigned ATC. At about 5000 ft the cabin altitude was noted to descend below 0 ft. Cycling the pressurization controller between standby and manual relieved the pressure; the packs were secured as the cabin reached the aircraft altitude at about 3000 ft and an unpressurized uneventful landing was made at ZZZ. On long final; the 'a' flight attendant called to inform us one pregnant passenger was experiencing some pain so paramedics were requested through the tower. The cabin altitude never exceeded 12000 ft throughout the flight. It was not realized until block in that manual dc had not been tried in-flight. After block in; the flight attendant crew was debriefed and although all 3 reported they felt ok; they were released from duty.supplemental information from acn 687193: the fwd lav smoke alarm went off at some time during this event; causing a very high pitch squeal that lasted until I shut it off after block in. During the flight; this was a huge distraction and I thought it may have been a cabin/door seal leak. Overspd during descent. Rapid depressurization/cabin altitude warning horn qrc/QRH says to put pressurization controller in manual mode. Once on the ground; we realized we had not tried the manual dc mode. QRH does not delineate either. Once the event occurred I should have tried to use not only manual AC mode; but also manual dc mode.

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Original NASA ASRS Text

Title: B737-400 EXPERIENCES SLOW DECOMPRESSION REQUIRING AN EMER DSCNT FROM FL280 AND A RETURN TO DEP ARPT.

Narrative: A NORMAL PREFLT; TAXI; TKOF; AND CLB WERE CONDUCTED AND ALL CHKLISTS COMPLETED THROUGH FL180. AT ABOUT FL280 AND AROUND XA05Z; THE ALT WARNING HORN SOUNDED. OXYGEN MASKS WERE DONNED; COMMUNICATIONS ESTABLISHED; AND THE CABIN ALT WAS NOTED AT JUST ABOVE 10000 FT AND SLOWLY CLBING. NO WARNING OR ABNORMAL LIGHTS WERE NOTED ON THE PRESSURIZATION PANEL (OR ANY OTHER PANEL FOR THAT MATTER). THE QRH FOLLOWED WITH NO RESULTS AND THE OUTFLOW VALVE WAS NOTED STUCK IN THE NEAR CLOSED POSITION. AN EMER WAS DECLARED; A DSCNT TO 12000 FT (ASSIGNED BY ATC FOR TERRAIN CLRNC) WAS COMMENCED AND L TURN TOWARD ZZZ WAS STARTED AS ASSIGNED ATC. AT ABOUT 5000 FT THE CABIN ALT WAS NOTED TO DSND BELOW 0 FT. CYCLING THE PRESSURIZATION CTLR BETWEEN STANDBY AND MANUAL RELIEVED THE PRESSURE; THE PACKS WERE SECURED AS THE CABIN REACHED THE ACFT ALT AT ABOUT 3000 FT AND AN UNPRESSURIZED UNEVENTFUL LNDG WAS MADE AT ZZZ. ON LONG FINAL; THE 'A' FLT ATTENDANT CALLED TO INFORM US ONE PREGNANT PAX WAS EXPERIENCING SOME PAIN SO PARAMEDICS WERE REQUESTED THROUGH THE TWR. THE CABIN ALT NEVER EXCEEDED 12000 FT THROUGHOUT THE FLT. IT WAS NOT REALIZED UNTIL BLOCK IN THAT MANUAL DC HAD NOT BEEN TRIED INFLT. AFTER BLOCK IN; THE FLT ATTENDANT CREW WAS DEBRIEFED AND ALTHOUGH ALL 3 RPTED THEY FELT OK; THEY WERE RELEASED FROM DUTY.SUPPLEMENTAL INFO FROM ACN 687193: THE FWD LAV SMOKE ALARM WENT OFF AT SOME TIME DURING THIS EVENT; CAUSING A VERY HIGH PITCH SQUEAL THAT LASTED UNTIL I SHUT IT OFF AFTER BLOCK IN. DURING THE FLT; THIS WAS A HUGE DISTRACTION AND I THOUGHT IT MAY HAVE BEEN A CABIN/DOOR SEAL LEAK. OVERSPD DURING DSCNT. RAPID DEPRESSURIZATION/CABIN ALT WARNING HORN QRC/QRH SAYS TO PUT PRESSURIZATION CTLR IN MANUAL MODE. ONCE ON THE GND; WE REALIZED WE HAD NOT TRIED THE MANUAL DC MODE. QRH DOES NOT DELINEATE EITHER. ONCE THE EVENT OCCURRED I SHOULD HAVE TRIED TO USE NOT ONLY MANUAL AC MODE; BUT ALSO MANUAL DC MODE.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.