Narrative:

At FL350 the cabin altitude warning horn sounded as the cabin was climbing through 10000 ft. After donning our oxygen masks, we asked ATC for a descent. ATC stated we had traffic at our 4 O'clock position at FL310. I instructed the first officer to declare an emergency as we began a descent. ATC gave us a turn of approximately 90 degrees to the right of 030 degrees. As we turned, the first officer said he had the traffic in sight. We reviewed the QRH items during our descent. I selected both packs to high and passing approximately FL310 the cabin pressure returned to normal and we leveled off at FL280. ATC had given our traffic a left turn, who had stated they had us in sight. We also had the traffic on TCASII which gave us a TA but no RA. As we passed FL310, TCASII indicated our horizontal separation to be approximately 2 mi. I feel that without declaring an emergency the cabin may have exceeded 14000 ft had we stayed at FL350 for much longer. As a result, the passenger oxygen masks did not deploy and the descent was smooth with the autoplt engaged. After briefing the #1 flight attendant, she stated that the cabin crew as well as the passenger were unaware of any problems. With a stabilized cabin at FL280 we continued to sea. ATC requested that I contact them upon our arrival in sea on a landline, which I did to confirm the nature of our emergency as well as our traffic separation issue. In debriefing this situation with the first officer, he stated that he was unsure what the problem was initially when I instructed him to don his oxygen mask. I think it would have helped to clarify the problem by stating 'we have a loss of cabin pressure.'

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Original NASA ASRS Text

Title: B737-300 FLT CREW HAS LOSS OF CABIN PRESSURIZATION, DECLARES AN EMER AND DSNDS WHILE ENRTE SEA.

Narrative: AT FL350 THE CABIN ALT WARNING HORN SOUNDED AS THE CABIN WAS CLBING THROUGH 10000 FT. AFTER DONNING OUR OXYGEN MASKS, WE ASKED ATC FOR A DSCNT. ATC STATED WE HAD TFC AT OUR 4 O'CLOCK POS AT FL310. I INSTRUCTED THE FO TO DECLARE AN EMER AS WE BEGAN A DSCNT. ATC GAVE US A TURN OF APPROX 90 DEGS TO THE R OF 030 DEGS. AS WE TURNED, THE FO SAID HE HAD THE TFC IN SIGHT. WE REVIEWED THE QRH ITEMS DURING OUR DSCNT. I SELECTED BOTH PACKS TO HIGH AND PASSING APPROX FL310 THE CABIN PRESSURE RETURNED TO NORMAL AND WE LEVELED OFF AT FL280. ATC HAD GIVEN OUR TFC A L TURN, WHO HAD STATED THEY HAD US IN SIGHT. WE ALSO HAD THE TFC ON TCASII WHICH GAVE US A TA BUT NO RA. AS WE PASSED FL310, TCASII INDICATED OUR HORIZ SEPARATION TO BE APPROX 2 MI. I FEEL THAT WITHOUT DECLARING AN EMER THE CABIN MAY HAVE EXCEEDED 14000 FT HAD WE STAYED AT FL350 FOR MUCH LONGER. AS A RESULT, THE PAX OXYGEN MASKS DID NOT DEPLOY AND THE DSCNT WAS SMOOTH WITH THE AUTOPLT ENGAGED. AFTER BRIEFING THE #1 FLT ATTENDANT, SHE STATED THAT THE CABIN CREW AS WELL AS THE PAX WERE UNAWARE OF ANY PROBS. WITH A STABILIZED CABIN AT FL280 WE CONTINUED TO SEA. ATC REQUESTED THAT I CONTACT THEM UPON OUR ARR IN SEA ON A LANDLINE, WHICH I DID TO CONFIRM THE NATURE OF OUR EMER AS WELL AS OUR TFC SEPARATION ISSUE. IN DEBRIEFING THIS SIT WITH THE FO, HE STATED THAT HE WAS UNSURE WHAT THE PROB WAS INITIALLY WHEN I INSTRUCTED HIM TO DON HIS OXYGEN MASK. I THINK IT WOULD HAVE HELPED TO CLARIFY THE PROB BY STATING 'WE HAVE A LOSS OF CABIN PRESSURE.'

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.