Narrative:

Cleared from 10000 ft to 16000 ft. While climbing through approximately 12000 ft, we heard a horn. Both pilots immediately idented horn as takeoff warning horn. Unfortunately, I continued climbing while we tried to identify the source of the alarm. I concluded the ground/air compressor switch had malfunctioned, hence, the takeoff warning horn. The first officer was scanning the overhead and I saw his hand reach for the cabin altitude horn cutout. That was when I realized that cabin altitude horn was sounding (both system use the same intermittent horn). That meant that cabin altitude was above 10000 ft. I immediately leveled off, but it was too late. The #1 flight attendant entered the cockpit and reported that the masks had dropped. I put on my mask, declared an emergency and dove back to 10000 ft. Cabin control was recovered and we returned to ewr airport. I regret costing the company 35 oxygen canisters. We know the cabin altitude horn exists, but we so rarely hear it in training, that it came back to bite me. The takeoff warning horn is always heard sometime during training. It was the first time I had heard it in-flight and the first time since training. 3 aircraft types ago, had I immediately idented it as the cabin horn, I would have leveled off much earlier. The manufacturers should also try to differentiate the sound. The problem was tentatively traced to a sticking outflow valve. However, that may not have been the only problem. I had not closely monitored the new first officer's use of the pressurization panel (his third flight). I should have more closely supervised him on the after start check and the before takeoff check. Even though we did not feel pressure changes in our ears as we climbed, there is still a question of whether the packs were on for takeoff.

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Original NASA ASRS Text

Title: B737-500 CREW HAD THE PAX OXYGEN MASKS DEPLOY BECAUSE OF MISSET PRESSURIZATION CTLS.

Narrative: CLRED FROM 10000 FT TO 16000 FT. WHILE CLBING THROUGH APPROX 12000 FT, WE HEARD A HORN. BOTH PLTS IMMEDIATELY IDENTED HORN AS TKOF WARNING HORN. UNFORTUNATELY, I CONTINUED CLBING WHILE WE TRIED TO IDENT THE SOURCE OF THE ALARM. I CONCLUDED THE GND/AIR COMPRESSOR SWITCH HAD MALFUNCTIONED, HENCE, THE TKOF WARNING HORN. THE FO WAS SCANNING THE OVERHEAD AND I SAW HIS HAND REACH FOR THE CABIN ALT HORN CUTOUT. THAT WAS WHEN I REALIZED THAT CABIN ALT HORN WAS SOUNDING (BOTH SYS USE THE SAME INTERMITTENT HORN). THAT MEANT THAT CABIN ALT WAS ABOVE 10000 FT. I IMMEDIATELY LEVELED OFF, BUT IT WAS TOO LATE. THE #1 FLT ATTENDANT ENTERED THE COCKPIT AND RPTED THAT THE MASKS HAD DROPPED. I PUT ON MY MASK, DECLARED AN EMER AND DOVE BACK TO 10000 FT. CABIN CTL WAS RECOVERED AND WE RETURNED TO EWR ARPT. I REGRET COSTING THE COMPANY 35 OXYGEN CANISTERS. WE KNOW THE CABIN ALT HORN EXISTS, BUT WE SO RARELY HEAR IT IN TRAINING, THAT IT CAME BACK TO BITE ME. THE TKOF WARNING HORN IS ALWAYS HEARD SOMETIME DURING TRAINING. IT WAS THE FIRST TIME I HAD HEARD IT INFLT AND THE FIRST TIME SINCE TRAINING. 3 ACFT TYPES AGO, HAD I IMMEDIATELY IDENTED IT AS THE CABIN HORN, I WOULD HAVE LEVELED OFF MUCH EARLIER. THE MANUFACTURERS SHOULD ALSO TRY TO DIFFERENTIATE THE SOUND. THE PROB WAS TENTATIVELY TRACED TO A STICKING OUTFLOW VALVE. HOWEVER, THAT MAY NOT HAVE BEEN THE ONLY PROB. I HAD NOT CLOSELY MONITORED THE NEW FO'S USE OF THE PRESSURIZATION PANEL (HIS THIRD FLT). I SHOULD HAVE MORE CLOSELY SUPERVISED HIM ON THE AFTER START CHK AND THE BEFORE TKOF CHK. EVEN THOUGH WE DID NOT FEEL PRESSURE CHANGES IN OUR EARS AS WE CLBED, THERE IS STILL A QUESTION OF WHETHER THE PACKS WERE ON FOR TKOF.

Data retrieved from NASA's ASRS site as of July 2007 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.