B737-300 CREW LOST CTL OF CABIN PRESSURIZATION; AND MADE AN EMER DSCNT AS A RESULT.

Narrative

ACFT WAS IN SMOOTH AIR AT FL370 APPROX 100 NM NW OF DSM AND N OF OMA ON A DIRECT ROUTING TO DBQ VOR. THE ACFT WAS NORMALLY CONFIGURED FOR FLT AND INDICATING .74 MACH. ACFT WT WAS APPROX 104000 LBS. FO WAS THE PF ON AUTOPLT 'B' WHILE I WAS WORKING ATC. EARLIER IN THE FLT; THE AFT FLT ATTENDANT INQUIRED IF ALL PACKS WERE ON BECAUSE SHE FELT THERE WAS LIMITED AIR FLOW IN THE AFT END OF THE CABIN. BOTH PACKS WERE IN AUTO AND BOTH BLEEDS WERE ON. CABIN INDICATED APPROX 8000 FT AND THE DIFFERENTIAL WAS APPROX 7.65. THE CABIN WAS ABOUT 2/3 FULL. NO OTHER CONCERNS WERE EXPRESSED TO THE COCKPIT AFTER I ASSURED THE FLT ATTENDANT EVERYTHING WAS ON AND ALL SYS CHKED NORMALLY. THE 'A' FLT ATTENDANT HAD NO INFLT CONCERNS BROUGHT TO MY ATTN. NW OF DSM AT FL370; THE 10000 FT ALT HORN ALERTED US TO A CLBING CABIN. THE CABIN READ 10000 FT AND THE DIFFERENTIAL WAS APPROX 7.00 - 7.20 PSI. I IMMEDIATELY REACHED UP AND TURNED THE SEATBELT SIGN ON AND SWITCHED THE L PACK TO HIGH. THE FO PUSHED THE ALT HORN CUTOUT BUTTON. THE CABIN IMMEDIATELY BEGAN A DSCNT OF 500 FPM AND THE DIFFERENTIAL ROSE TO APPROX 7.65 - 7.7. WE DISCUSSED THE ISSUE AS I CALLED TO GET THE BOOK OUT. I INSTRUCTED THAT I HAD THE ACFT AND THE RADIOS WHILE FO WOULD USE THE FLT MANUAL. ALTHOUGH THE CABIN APPEARED TO STABILIZE; I REQUESTED FL330 AND I BELIEVE FL290. ATC INSTRUCTED THERE WAS TFC AND IT WOULD BE A FEW MINS. THERE IS NO IRREGULAR ONLY AN EMER QRC AND FOLLOW-UP CHKLIST. AS I WAS ABOUT TO CALL THE FLT ATTENDANTS WITH A FOLLOW-UP CABIN ANNOUNCEMENT; THE 10000 FT ALT HORN STARTED SOUNDING INTERMITTENTLY AGAIN. THEN THE L PACK TRIP LIGHT ILLUMINATED AND THE CABIN BEGAN TO CLB. FO AND I DONNED OUR OXYGEN MASKS; INITIATED CREW COMS; AND I INFORMED ATC WE WERE DECLARING AN EMER AND BEGINNING AN EMER DSCNT. ATC ASKED THE NATURE OF OUR EMER AND INTENTIONS. I STATED WE HAD A PRESSURIZATION PROB AND NEEDED TO DSND TO 10000 FT. WHILE FO WAS WORKING THE QRC FOR CABIN ALT WARNING/RAPID DEPRESSURIZATION; EMER DSCNT; AND IRREGULAR PACK TRIP OFF LIGHT ON; I INSTRUCTED HIM TO CALL THE FLT ATTENDANTS TO TAKE THEIR SEATS AND MAKE AN ANNOUNCEMENT TO THE PAX. AFTER COMPLETING THAT WORK I ASKED FOR THE APCH DSCNT CHKLIST AND TO SEND A MESSAGE TO ACR DISPATCH TO 'CALL ME.' I HAD ASKED FO IF HE CONCURRED WITH LNDG AT DSM; AND HE DID. I FURTHER ASKED ATC TO DSND TO 8000 FT WITH A DIRECT ROUTING TO DSM; WHICH WAS APPROVED. THE EMER DSCNT WAS SMOOTH AND CONSTANT WITH THE AIRSPD AND PITCH WELL WITHIN NORMAL PROCS. I DISCUSSED THE SIT WITH DISPATCH AND MAINT CTLR. DISPATCH WAS HELPFUL BRINGING MAINT CTLR ON THE LINE; BRIEFING WX IN DSM; AND NOTIFYING THE STATION. I WAS THEN ABLE TO CALL THE FLT ATTENDANTS AND TELL THEM WHAT WAS HAPPENING AND THAT WE HAD 15 MINS AND IT WOULD BE A NORMAL LNDG. THEY TOLD ME THEY UNDERSTOOD AND THE CABIN WAS READY FOR LNDG; THE PAX WERE CALM ALTHOUGH CONCERNED ABOUT THEIR CONNECTIONS IN ORD. I TOLD THEM WE WOULD HAVE UPDATED INFO ON THE GND. I THEN MADE AN ANNOUNCEMENT TO THE PAX ABOUT OUR SIT AND OFFERED TO TALK TO ANY OF THEM ON THE GND AND WE WOULD HAVE AN UNEVENTFUL LNDG. WX IN DSM WAS 700 FT OVCST; 3 MI; FOG. THE LNDG WAS UNEVENTFUL ON RWY 13 WITHOUT EMER EQUIP. THE DSM STATION WAS WONDERFUL AND THE CUSTOMER SVC AND RAMP PERSONNEL WERE OUTSTANDING IN SWITCHING ACFT IN AN ATTEMPT TO MAKE A 35 MIN TURN TO ORD. PRIOR TO SHUTTING DOWN THE ACFT; THE LOGBOOK WAS FILLED OUT. FO PERFORMED AT THE HIGHEST STANDARDS OF PROFESSIONALISM DURING THIS EMER. AFTER THE UNEVENTFUL FLT FROM DSM-ORD; THE PAX APPEARED CALM AND APPRECIATIVE OF THE INFO AND HOW THE PLTS AND FLT ATTENDANTS HANDLED THE EMER.