Narrative:

Cabin altitude warning horn sounded while cruising at FL340; I commanded the first officer to silence the horn and discovered cabin altitude gauge at 10000 ft. I commanded the first officer to switch to manual AC and close the outflow valve. Immediately; the cabin began to descend at 2000 ft per min; verified by rate gauge and decline in cabin altitude. Within seconds cabin pressure was under complete control; fully pressurized and descending to desired cabin altitude of 7000 ft. I considered reaching for my oxygen mask but decided it was not necessary because the cabin was; at all times; below 10000 ft. QRH pressurization checklists were reviewed. As the cabin altitude descended through 8000 ft; I asked the first officer to rotate the cabin mode selector to sby to have finer control of cabin rate of descent. Cabin pressure was fully controllable; verified with cabin rate vsi. A cabin altitude of 7000 ft was selected in the controller window and attained shortly thereafter. After a brief discussion to review altitude options with first officer and check airman; decided to descend to FL280 to lower the cabin to approximately 5000 ft. 3 flight attendants and jump seat flight attendant were briefed on what had just transpired and reassured that cabin pressure was under complete control. Both dispatch and maintenance controller were notified. We landed 1 hour later; uneventfully. In hindsight; I should have put on my oxygen mask the moment I heard the cabin altitude warning horn. After all; it is the first action item depicted in the qrc under 'rapid depressurization/cabin altitude warning horn.' I expected to see a rapid rise in cabin rate; but instead I saw a zero rate of change. This led me to believe the cause of the problem could be solved immediately. I believe that I analyzed and responded correctly.callback conversation with reporter revealed the following information: reporter stated that he very quickly identified the cause of the horn because this aircraft had a similar problem earlier in the day with another crew. He believed the horn sounded for two cycles before they reacted. He stated that he does not believe the cabin went higher than 9500 ft.

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Original NASA ASRS Text

Title: A B737 FLT CREW PERFORMED A RAPID DSCNT FOLLOWING A SLOW CABIN PRESSURIZATION LOSS WITH A WARNING HORN. AN EMER WAS NOT DECLARED.

Narrative: CABIN ALT WARNING HORN SOUNDED WHILE CRUISING AT FL340; I COMMANDED THE FO TO SILENCE THE HORN AND DISCOVERED CABIN ALT GAUGE AT 10000 FT. I COMMANDED THE FO TO SWITCH TO MANUAL AC AND CLOSE THE OUTFLOW VALVE. IMMEDIATELY; THE CABIN BEGAN TO DSND AT 2000 FT PER MIN; VERIFIED BY RATE GAUGE AND DECLINE IN CABIN ALT. WITHIN SECONDS CABIN PRESSURE WAS UNDER COMPLETE CTL; FULLY PRESSURIZED AND DSNDING TO DESIRED CABIN ALT OF 7000 FT. I CONSIDERED REACHING FOR MY OXYGEN MASK BUT DECIDED IT WAS NOT NECESSARY BECAUSE THE CABIN WAS; AT ALL TIMES; BELOW 10000 FT. QRH PRESSURIZATION CHKLISTS WERE REVIEWED. AS THE CABIN ALT DSNDED THROUGH 8000 FT; I ASKED THE FO TO ROTATE THE CABIN MODE SELECTOR TO SBY TO HAVE FINER CTL OF CABIN RATE OF DSCNT. CABIN PRESSURE WAS FULLY CONTROLLABLE; VERIFIED WITH CABIN RATE VSI. A CABIN ALT OF 7000 FT WAS SELECTED IN THE CTLR WINDOW AND ATTAINED SHORTLY THEREAFTER. AFTER A BRIEF DISCUSSION TO REVIEW ALT OPTIONS WITH FO AND CHK AIRMAN; DECIDED TO DSND TO FL280 TO LOWER THE CABIN TO APPROX 5000 FT. 3 FLT ATTENDANTS AND JUMP SEAT FLT ATTENDANT WERE BRIEFED ON WHAT HAD JUST TRANSPIRED AND REASSURED THAT CABIN PRESSURE WAS UNDER COMPLETE CTL. BOTH DISPATCH AND MAINT CTLR WERE NOTIFIED. WE LANDED 1 HR LATER; UNEVENTFULLY. IN HINDSIGHT; I SHOULD HAVE PUT ON MY OXYGEN MASK THE MOMENT I HEARD THE CABIN ALT WARNING HORN. AFTER ALL; IT IS THE FIRST ACTION ITEM DEPICTED IN THE QRC UNDER 'RAPID DEPRESSURIZATION/CABIN ALT WARNING HORN.' I EXPECTED TO SEE A RAPID RISE IN CABIN RATE; BUT INSTEAD I SAW A ZERO RATE OF CHANGE. THIS LED ME TO BELIEVE THE CAUSE OF THE PROB COULD BE SOLVED IMMEDIATELY. I BELIEVE THAT I ANALYZED AND RESPONDED CORRECTLY.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE VERY QUICKLY IDENTIFIED THE CAUSE OF THE HORN BECAUSE THIS ACFT HAD A SIMILAR PROBLEM EARLIER IN THE DAY WITH ANOTHER CREW. HE BELIEVED THE HORN SOUNDED FOR TWO CYCLES BEFORE THEY REACTED. HE STATED THAT HE DOES NOT BELIEVE THE CABIN WENT HIGHER THAN 9500 FT.

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.