Narrative:

While cruising at FL370; the master caution light came on. The master caution panel said 'air conditioning.' looking over; we saw the 'automatic fail' light on. I noticed the cabin climbing rapidly. I verified that both packs were on and engine bleeds were on. I asked my copilot to switch to 'standby' on the pressurization panel. He did; and cabin was still climbing. I told him to declare an emergency and I manually clicked off the autothrottles and autoplt and stated an emergency descent. I was hoping that we could descend fast enough to catch the cabin. As the cabin approached 10000 ft; I got on the PA and made a brief statement to the people that we had a pressurization problem and were descending. The first officer was running the rapid descent checklist; followed by the loss of pressurization checklist. No help. Cabin was still climbing! We were initially heading direct bucko. Since we were going towards ZZZ1; my initial thought was to go to ZZZ2 (we were heading that way anyway; and it would take a while to get down). With the speed brakes out and idle on the throttles; I indicated approximately 320 KTS. I mentioned to the first officer that maybe we should drop into ZZZ2 instead of ZZZ1. He advocated that we should go to ZZZ instead. He was right; and I agreed -- the first officer did a real nice job in running the checklist and working ATC. After I confirmed with the first officer that all checklists were done; I made another PA to the people that we would be landing in about 10 mins. I confirmed with the 'a' flight attendant how the people were doing. She said everyone was fine and sucking oxygen. She put me at ease with her tone of voice. She should be commended for a job well done. On downwind at ZZZ; I was confident that all checklists were done (including the descent checklist) and the cabin was ready. We were cleared for the visual to runway 24L and landed safely. The passenger were very thankful for getting them safely on the ground. Passenger told of a 'thud' noise that happened before the oxygen masks deployed. They were sitting in the middle of the cabin. The training we received for this scenario was very good. The only thing that we did differently was the 'crew communication' part. We found that when you descend at 320 KTS with the speed brakes out; it gets very; very noisy in the cockpit. I tried talking to my first officer through the interphone. Initially; he didn't hear me. The thing that worked for us was to pull the mask away from our mouths just to speak a few words and then let it go back. That helped our crew communication process immensely! Supplemental information from acn 680647: we were unable to regain the control of the cabin pressure. We decided to divert to ZZZ. The cabin altitude reached a maximum of 15000-16000 ft. In manual; the cabin outflow valve was showing full closed but there was no effect on the cabin rate of climb (still at approximately 3000 FPM) until we descended through the mid 20's. I noticed at about FL200 that we began to re-pressurize and were able to get the cabin altitude to below 10000 ft by the time the aircraft was approximately 13000 ft. Once the cabin altitude descended below 10000 ft; we removed oxygen and proceeded to ZZZ with a normal landing. Callback conversation with reporter acn 680647 revealed the following information: reporter stated that the crew was later advised by maintenance that the outflow valve had failed. He also said that the outflow valve gauge indicated full closed even though they were not initially pressurizing. He operated the pressurization system in manual the remainder of the flight.

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Original NASA ASRS Text

Title: A B737-300'S OUTFLOW VALVE FAILED AT FL370. THE CREW DECLARED AN EMER; DSNDED; AND REGAINED PRESSURIZATION CTL WHILE DIVERTING TO ANOTHER ARPT.

Narrative: WHILE CRUISING AT FL370; THE MASTER CAUTION LIGHT CAME ON. THE MASTER CAUTION PANEL SAID 'AIR CONDITIONING.' LOOKING OVER; WE SAW THE 'AUTO FAIL' LIGHT ON. I NOTICED THE CABIN CLBING RAPIDLY. I VERIFIED THAT BOTH PACKS WERE ON AND ENG BLEEDS WERE ON. I ASKED MY COPLT TO SWITCH TO 'STANDBY' ON THE PRESSURIZATION PANEL. HE DID; AND CABIN WAS STILL CLBING. I TOLD HIM TO DECLARE AN EMER AND I MANUALLY CLICKED OFF THE AUTOTHROTTLES AND AUTOPLT AND STATED AN EMER DSCNT. I WAS HOPING THAT WE COULD DSND FAST ENOUGH TO CATCH THE CABIN. AS THE CABIN APCHED 10000 FT; I GOT ON THE PA AND MADE A BRIEF STATEMENT TO THE PEOPLE THAT WE HAD A PRESSURIZATION PROB AND WERE DSNDING. THE FO WAS RUNNING THE RAPID DSCNT CHKLIST; FOLLOWED BY THE LOSS OF PRESSURIZATION CHKLIST. NO HELP. CABIN WAS STILL CLBING! WE WERE INITIALLY HEADING DIRECT BUCKO. SINCE WE WERE GOING TOWARDS ZZZ1; MY INITIAL THOUGHT WAS TO GO TO ZZZ2 (WE WERE HEADING THAT WAY ANYWAY; AND IT WOULD TAKE A WHILE TO GET DOWN). WITH THE SPD BRAKES OUT AND IDLE ON THE THROTTLES; I INDICATED APPROX 320 KTS. I MENTIONED TO THE FO THAT MAYBE WE SHOULD DROP INTO ZZZ2 INSTEAD OF ZZZ1. HE ADVOCATED THAT WE SHOULD GO TO ZZZ INSTEAD. HE WAS RIGHT; AND I AGREED -- THE FO DID A REAL NICE JOB IN RUNNING THE CHKLIST AND WORKING ATC. AFTER I CONFIRMED WITH THE FO THAT ALL CHKLISTS WERE DONE; I MADE ANOTHER PA TO THE PEOPLE THAT WE WOULD BE LNDG IN ABOUT 10 MINS. I CONFIRMED WITH THE 'A' FLT ATTENDANT HOW THE PEOPLE WERE DOING. SHE SAID EVERYONE WAS FINE AND SUCKING OXYGEN. SHE PUT ME AT EASE WITH HER TONE OF VOICE. SHE SHOULD BE COMMENDED FOR A JOB WELL DONE. ON DOWNWIND AT ZZZ; I WAS CONFIDENT THAT ALL CHKLISTS WERE DONE (INCLUDING THE DSCNT CHKLIST) AND THE CABIN WAS READY. WE WERE CLRED FOR THE VISUAL TO RWY 24L AND LANDED SAFELY. THE PAX WERE VERY THANKFUL FOR GETTING THEM SAFELY ON THE GND. PAX TOLD OF A 'THUD' NOISE THAT HAPPENED BEFORE THE OXYGEN MASKS DEPLOYED. THEY WERE SITTING IN THE MIDDLE OF THE CABIN. THE TRAINING WE RECEIVED FOR THIS SCENARIO WAS VERY GOOD. THE ONLY THING THAT WE DID DIFFERENTLY WAS THE 'CREW COM' PART. WE FOUND THAT WHEN YOU DSND AT 320 KTS WITH THE SPD BRAKES OUT; IT GETS VERY; VERY NOISY IN THE COCKPIT. I TRIED TALKING TO MY FO THROUGH THE INTERPHONE. INITIALLY; HE DIDN'T HEAR ME. THE THING THAT WORKED FOR US WAS TO PULL THE MASK AWAY FROM OUR MOUTHS JUST TO SPEAK A FEW WORDS AND THEN LET IT GO BACK. THAT HELPED OUR CREW COM PROCESS IMMENSELY! SUPPLEMENTAL INFO FROM ACN 680647: WE WERE UNABLE TO REGAIN THE CTL OF THE CABIN PRESSURE. WE DECIDED TO DIVERT TO ZZZ. THE CABIN ALT REACHED A MAX OF 15000-16000 FT. IN MANUAL; THE CABIN OUTFLOW VALVE WAS SHOWING FULL CLOSED BUT THERE WAS NO EFFECT ON THE CABIN RATE OF CLB (STILL AT APPROX 3000 FPM) UNTIL WE DSNDED THROUGH THE MID 20'S. I NOTICED AT ABOUT FL200 THAT WE BEGAN TO RE-PRESSURIZE AND WERE ABLE TO GET THE CABIN ALT TO BELOW 10000 FT BY THE TIME THE ACFT WAS APPROX 13000 FT. ONCE THE CABIN ALT DSNDED BELOW 10000 FT; WE REMOVED OXYGEN AND PROCEEDED TO ZZZ WITH A NORMAL LNDG. CALLBACK CONVERSATION WITH RPTR ACN 680647 REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE CREW WAS LATER ADVISED BY MAINT THAT THE OUTFLOW VALVE HAD FAILED. HE ALSO SAID THAT THE OUTFLOW VALVE GAUGE INDICATED FULL CLOSED EVEN THOUGH THEY WERE NOT INITIALLY PRESSURIZING. HE OPERATED THE PRESSURIZATION SYS IN MANUAL THE REMAINDER OF THE FLT.

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.