Narrative:

Planned flight to ZZZ with an alternate of ZZZ1. Turbulence plots and PIREPS dictated that we fly to ZZZ2 at FL230 with a cruise climb to FL350 from ZZZ2 to ZZZ. Planned arrival fuel in ZZZ of 8.9; with alternate fuel of 2.9. Initial leveloff at FL230 with enroute winds entered into the computer showed arrival fuel at ZZZ of 7.9. A few minutes later ATC assigned us FL210 for crossing traffic. During the descent to FL210 the 'off schedule' descent light illuminated. We accomplished the quick reference handbook; reset the flight altitude to FL210; and the light extinguished. Once the checklist was completed; the flight altitude was reset to our anticipated final cruise altitude of FL350 arrival fuel at FL210 now indicated 7.7. As we approached abc VOR; several other aircraft were beginning climbs to higher altitudes. We checked on the rides and requested a climb to FL350. We were initially assigned FL280; then FL330. While climbing through FL310; the cabin altitude warning horn sounded. We immediately performed the memory items for 'cabin altitude warning/abnormal pressurization' and donned our oxygen masks. The cabin pressure was noted to be 10;000 ft and climbing at 1;000 FPM. We requested an immediate descent and ATC assigned us FL290. The first officer continued with the checklist while I began our descent. Approaching FL290; I twice requested lower and ATC told us to maintain FL290. The cabin pressure was still at 10;000 ft and climbing. I declared an emergency. We were assigned 17;000 ft and given the local altimeter setting. The descent was accomplished per the 'emergency descent' checklist to FL190 where we were able to stabilize the cabin. Neither the 'automatic fail' nor 'standby' lights illuminated on the control panel and the first officer stated the outflow valve indicated closed. He followed the checklist items and was able to re-pressurize the cabin in manual. At FL190 we would have been unable to arrive in ZZZ with sufficient fuel; so I coordinated with dispatch for a diversion to ZZZ2. The first officer did an excellent job and we accomplished the checklist items from the quick reference handbook; descent checklist; and got the ZZZ2 weather while I flew the aircraft. Dispatch concurred and we diverted to ZZZ2. Once the situation was under control; I notified the flight attendants of our diversion and made an announcement to the passengers of the same. Descent and arrival into ZZZ2 was uneventful. Because the cabin altitude warning horn sounds exactly like the takeoff warning horn; we experienced several seconds of confusion before we realized what our problem really was. The first officer also had some moments of confusion before we realized with the cabin pressure gauge when the cabin over pressurized as we continued our descent. We corrected the error; but this might need to be stressed more in training. Another warning horn for cabin pressurization would be nice also.

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Original NASA ASRS Text

Title: A B737-300 flight crew experienced loss of cabin pressure. They declared an emergency; descended; coordinated with dispatch; and diverted to a nearby airport. The crew experienced confusion about the warning horn's meaning.

Narrative: Planned flight to ZZZ with an alternate of ZZZ1. Turbulence plots and PIREPS dictated that we fly to ZZZ2 at FL230 with a cruise climb to FL350 from ZZZ2 to ZZZ. Planned arrival fuel in ZZZ of 8.9; with alternate fuel of 2.9. Initial leveloff at FL230 with enroute winds entered into the computer showed arrival fuel at ZZZ of 7.9. A few minutes later ATC assigned us FL210 for crossing traffic. During the descent to FL210 the 'Off Schedule' descent light illuminated. We accomplished the Quick Reference Handbook; reset the flight altitude to FL210; and the light extinguished. Once the checklist was completed; the flight altitude was reset to our anticipated final cruise altitude of FL350 arrival fuel at FL210 now indicated 7.7. As we approached ABC VOR; several other aircraft were beginning climbs to higher altitudes. We checked on the rides and requested a climb to FL350. We were initially assigned FL280; then FL330. While climbing through FL310; the cabin altitude warning horn sounded. We immediately performed the memory items for 'Cabin Altitude Warning/Abnormal Pressurization' and donned our oxygen masks. The cabin pressure was noted to be 10;000 FT and climbing at 1;000 FPM. We requested an immediate descent and ATC assigned us FL290. The First Officer continued with the checklist while I began our descent. Approaching FL290; I twice requested lower and ATC told us to maintain FL290. The cabin pressure was still at 10;000 FT and climbing. I declared an emergency. We were assigned 17;000 FT and given the local altimeter setting. The descent was accomplished per the 'Emergency Descent' checklist to FL190 where we were able to stabilize the cabin. Neither the 'Automatic Fail' nor 'Standby' lights illuminated on the control panel and the First Officer stated the outflow valve indicated closed. He followed the checklist items and was able to re-pressurize the cabin in manual. At FL190 we would have been unable to arrive in ZZZ with sufficient fuel; so I coordinated with Dispatch for a diversion to ZZZ2. The First Officer did an excellent job and we accomplished the checklist items from the Quick Reference Handbook; Descent Checklist; and got the ZZZ2 weather while I flew the aircraft. Dispatch concurred and we diverted to ZZZ2. Once the situation was under control; I notified the Flight Attendants of our diversion and made an announcement to the passengers of the same. Descent and arrival into ZZZ2 was uneventful. Because the cabin altitude warning horn sounds exactly like the takeoff warning horn; we experienced several seconds of confusion before we realized what our problem really was. The First Officer also had some moments of confusion before we realized with the cabin pressure gauge when the cabin over pressurized as we continued our descent. We corrected the error; but this might need to be stressed more in training. Another warning horn for cabin pressurization would be nice also.

Data retrieved from NASA's ASRS site as of April 2012 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.