Narrative:

At cruise at FL220 cabin altitude warning horn sounded. I know; and I have read many times; that crews often mistake it for the takeoff warning horn. Never thought it would happen to me. My initial thought was; 'what is causing the takeoff warning horn to go off.' it took just a few seconds thought to realize that it was the cabin altitude warning horn. We performed the procedure and received an immediate descent from center. No emergency was declared and we were able to hold the cabin in packs high. At some point in the descent though; my first officer and I both realized that we had not actually completed the procedure. We had not put on our oxygen masks. Supplemental information from acn 723002: at cruise altitude just prior to top of descent we got the takeoff confign/altitude warning horn. The captain requested a descent and we started running the abnormal pressurization checklist. The cabin altitude was in a slow climb through 10000 ft and the cabin emergency oxygen masks had not deployed. We verified the pressurization switches and in the descent put both packs to high. This got the pressurization under control and put the cabin pressure back into a descent. We sent an ACARS message to dispatch that we would need maintenance for the pressurization problem. We did not declare an emergency with ATC because we were at our descent and we were able to keep the pressurization under control. While analyzing the malfunction and then executing checklist we did omit the first 2 steps of the checklist where we should have put on the oxygen mask. The surprise of the takeoff confign warning horn at altitude with no rapid decompression or apparent pressurization problem leads to trying to analyze the problem before executing the first 2 steps of the checklist. Additionally the pressurization check at 10000 ft appeared normal but we did not catch that the pressurization stayed in a slow climb.

Related reports:

Google
 

Original NASA ASRS Text

Title: B737-300 FLT CREW HAS PRESSURIZATION MALFUNCTION; EXECUTE DSCNT.

Narrative: AT CRUISE AT FL220 CABIN ALT WARNING HORN SOUNDED. I KNOW; AND I HAVE READ MANY TIMES; THAT CREWS OFTEN MISTAKE IT FOR THE TKOF WARNING HORN. NEVER THOUGHT IT WOULD HAPPEN TO ME. MY INITIAL THOUGHT WAS; 'WHAT IS CAUSING THE TKOF WARNING HORN TO GO OFF.' IT TOOK JUST A FEW SECONDS THOUGHT TO REALIZE THAT IT WAS THE CABIN ALT WARNING HORN. WE PERFORMED THE PROC AND RECEIVED AN IMMEDIATE DSCNT FROM CENTER. NO EMER WAS DECLARED AND WE WERE ABLE TO HOLD THE CABIN IN PACKS HIGH. AT SOME POINT IN THE DSCNT THOUGH; MY FO AND I BOTH REALIZED THAT WE HAD NOT ACTUALLY COMPLETED THE PROC. WE HAD NOT PUT ON OUR OXYGEN MASKS. SUPPLEMENTAL INFO FROM ACN 723002: AT CRUISE ALT JUST PRIOR TO TOP OF DSCNT WE GOT THE TKOF CONFIGN/ALT WARNING HORN. THE CAPT REQUESTED A DSCNT AND WE STARTED RUNNING THE ABNORMAL PRESSURIZATION CHKLIST. THE CABIN ALT WAS IN A SLOW CLB THROUGH 10000 FT AND THE CABIN EMER OXYGEN MASKS HAD NOT DEPLOYED. WE VERIFIED THE PRESSURIZATION SWITCHES AND IN THE DSCNT PUT BOTH PACKS TO HIGH. THIS GOT THE PRESSURIZATION UNDER CTL AND PUT THE CABIN PRESSURE BACK INTO A DSCNT. WE SENT AN ACARS MESSAGE TO DISPATCH THAT WE WOULD NEED MAINT FOR THE PRESSURIZATION PROB. WE DID NOT DECLARE AN EMER WITH ATC BECAUSE WE WERE AT OUR DSCNT AND WE WERE ABLE TO KEEP THE PRESSURIZATION UNDER CTL. WHILE ANALYZING THE MALFUNCTION AND THEN EXECUTING CHKLIST WE DID OMIT THE FIRST 2 STEPS OF THE CHKLIST WHERE WE SHOULD HAVE PUT ON THE OXYGEN MASK. THE SURPRISE OF THE TKOF CONFIGN WARNING HORN AT ALT WITH NO RAPID DECOMPRESSION OR APPARENT PRESSURIZATION PROB LEADS TO TRYING TO ANALYZE THE PROB BEFORE EXECUTING THE FIRST 2 STEPS OF THE CHKLIST. ADDITIONALLY THE PRESSURIZATION CHK AT 10000 FT APPEARED NORMAL BUT WE DID NOT CATCH THAT THE PRESSURIZATION STAYED IN A SLOW CLB.

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.