Narrative:

Uneventful departure from islip, various vectors from ZBW in the climb for separation. Passing approximately 14000 ft the cabin altitude warning horn sounded. The aircraft was leveled and a descent to 10000 ft was requested and approved. Level at 10000 ft the QRH for cabin pressurization was completed. The decision was made to divert to bwi for maintenance. We changed aircraft and continued on to fll and returned to bwi uneventfully. I was contacted oct/sat/04 and notified that subsequently the aircraft flight recorder data had been reviewed and it appears that we may have departed islip unpressurized. Supplemental information from acn 635043: looking up, I noticed the bleed switches in the off position, but I thought this was normal as the checklist calls for turning them on one at a time. In hindsight, I believe they were off the entire time indicating an unpressurized takeoff from isp. After recently discussing the matter with my first officer, he relayed to me that he thought I had positioned the engine bleeds to the off position when we were at altitude. It's clear now, that we took off unpressurized. Though we established crew communications after donning the masks and calling for the checklist, I needed to ask him my question concerning the bleeds in the off position. He needed to ask if I had placed them there. I truly believed something was amiss with the pneumatic system. Now I know the engine bleeds were off for takeoff.

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Original NASA ASRS Text

Title: B737-300 CREW HAD THE CABIN ALT WARNING HORN GO OFF. THE ACFT WAS NOT PRESSURIZING. BOTH PNEUMATIC SWITCHES WERE FOUND IN THE OFF POS.

Narrative: UNEVENTFUL DEP FROM ISLIP, VARIOUS VECTORS FROM ZBW IN THE CLB FOR SEPARATION. PASSING APPROX 14000 FT THE CABIN ALT WARNING HORN SOUNDED. THE ACFT WAS LEVELED AND A DSCNT TO 10000 FT WAS REQUESTED AND APPROVED. LEVEL AT 10000 FT THE QRH FOR CABIN PRESSURIZATION WAS COMPLETED. THE DECISION WAS MADE TO DIVERT TO BWI FOR MAINT. WE CHANGED ACFT AND CONTINUED ON TO FLL AND RETURNED TO BWI UNEVENTFULLY. I WAS CONTACTED OCT/SAT/04 AND NOTIFIED THAT SUBSEQUENTLY THE ACFT FLT RECORDER DATA HAD BEEN REVIEWED AND IT APPEARS THAT WE MAY HAVE DEPARTED ISLIP UNPRESSURIZED. SUPPLEMENTAL INFO FROM ACN 635043: LOOKING UP, I NOTICED THE BLEED SWITCHES IN THE OFF POS, BUT I THOUGHT THIS WAS NORMAL AS THE CHKLIST CALLS FOR TURNING THEM ON ONE AT A TIME. IN HINDSIGHT, I BELIEVE THEY WERE OFF THE ENTIRE TIME INDICATING AN UNPRESSURIZED TKOF FROM ISP. AFTER RECENTLY DISCUSSING THE MATTER WITH MY FO, HE RELAYED TO ME THAT HE THOUGHT I HAD POSITIONED THE ENG BLEEDS TO THE OFF POS WHEN WE WERE AT ALT. IT'S CLR NOW, THAT WE TOOK OFF UNPRESSURIZED. THOUGH WE ESTABLISHED CREW COMS AFTER DONNING THE MASKS AND CALLING FOR THE CHKLIST, I NEEDED TO ASK HIM MY QUESTION CONCERNING THE BLEEDS IN THE OFF POS. HE NEEDED TO ASK IF I HAD PLACED THEM THERE. I TRULY BELIEVED SOMETHING WAS AMISS WITH THE PNEUMATIC SYS. NOW I KNOW THE ENG BLEEDS WERE OFF 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.