Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Aircraft
We had already begun the descent for the RNAV arrival and were at FL310 when we heard the cabin altitude warning horn. We (including the jumpseater) immediately donned our O2 masks; ran the immediate action checklist and went to the QRH. As the First Officer started on the pressurization checklist; I asked the jumpseater (737 CA) to coordinate with the flight attendants to tell them of our situation (TEST). We told ATC we had a pressurization issue if we could commence our arrival descent. They gave us clearance to descend immediately so we performed an emergency descent to raise the cabin altitude at the same time the First Officer and the jumpseater where closing the outflow valve to raise the cabin manually. The cabin altitude never went above 10;500. Since we were already in the descent portion of our flight and since ATC gave us the normal clearance to descend and since we only lost some pressurization; I opted not to declare an emergency. We knew that if our condition deteriorated we could declare an emergency at some point in the remaining flight. Also; the jumpseater said everything was fine in the cabin. We felt no pressure discomfort whatsoever. The First Officer and the jumpseater continued to manage the manual pressure controller to set us up for landing differential. Told ATC we would not need any other assistance and landed normally.
B737 Captain experiences a cabin altitude warning horn at FL310 during the descent phase.  ATC is advised and the descent rate increased while the First Officer and a jump seating Captain manage the outflow valve manually. The cabin altitude does not exceed 10;500 FT and a normal landing ensues.