B737-300 CREW LOST CABIN PRESSURIZATION AND MADE AN EMER DSCNT IN ZAU CLASS A AIRSPACE.

Narrative

I WAS PF MDW-MHT. WHEN AFTER REACHING CRUISE ALT; APPROX 10-15 MINS; WE LOST CABIN PRESSURIZATION. I WOULD NOT CATEGORIZE IT AS RAPID; BUT MAYBE TOWARDS TOTAL LOSS IN ABOUT 10-15 SECONDS. I HEARD A HISSING NOISE; FELT PRESSURE CHANGE; AND THEN ALT WARNING HORN. THE CAPT TOOK OVER ACFT CTL AND DIRECTED ME TO DON OXYGEN MASK AND TURNED OVER AFT CTL TO ME; AND DIRECTED A DSCNT (EMER). THE CAPT DECLARED EMER TO ATC; WHICH WAS ACKNOWLEDGED WITH CLRNC TO ANY REQUIRED ALT. PF ELECTED 10000 FT. CAPT CONTINUED EMER CHKLIST (BOLD FACE) WHILE I CONTINUED FLYING THE DSCNT PROFILE. CAPT MADE SEVERAL CALLS TO FLT ATTENDANTS AND PAX TO ASCERTAIN THEIR WELFARE. UPON REACHING 10000 FT; REVIEW OF CHKLIST WAS COMPLETED. STATUS REVIEWED OF CREW AND PAX RECONFIRMED NO INJURIES. WE HAD NO REASON TO BELIEVE THE ACFT WAS DAMAGED. WE DETERMINED SUFFICIENT FUEL ON BOARD TO FLY TO MHT. CAPT DISCUSSED ALL ABOVE WITH DISPATCH AND THE DECISION WAS MADE TO CONTINUE TO MHT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR WAS UNSURE OF WHAT THE PROB WAS; BUT THOUGHT MAINT REPLACED THE CABIN OUTFLOW VALVE. HE HAD NO OTHER INFO.