Narrative:

Climbing through FL300 to FL310 we heard the cabin alt horn and observed the cabin alt lights illuminated. I did not see the auto fail or altitude light illuminated on the pressurization panel. I confirmed on the cabin altimeter that we were above 10;000 ft cabin altitude. We accomplished the immediate action items. I maintained aircraft control by stopping the climb and going into a shallow descent with vertical speed. The autopilot stayed on. We [advised ATC] and got an initial clearance to FL240. Used level change to descend after the clearance. The copilot ran the cabin altitude warning checklist and was able to recover control of the cabin altitude very quickly with manual control. We continued our descent to a new clearance of 13000 ft. I turned aircraft control over to the copilot to coordinate with everyone. The cabin altitude never exceeded 14000 ft. I think we recovered control at about 12500 ft cabin altitude. We made the decision to divert to [an alternate] since we were almost over it. We continued our descent and vectors for [the alternate]. Not able to communicate with dispatch on VHF because we were already at 10;000 ft by the time we tried. We used ACARS to send a divert report and VHF with ops to coordinate the divert. We took our oxygen masks off sometime after 18;000 ft when the cabin altitude was under 10;000 ft. I took the airplane back from the copilot at around 6000 ft altitude and we landed uneventfully.

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Original NASA ASRS Text

Title: B737 flight crew reported diverting to an alternate after experiencing a loss of cabin pressure.

Narrative: Climbing through FL300 to FL310 we heard the cabin alt horn and observed the Cabin Alt lights illuminated. I did not see the auto fail or altitude light illuminated on the pressurization panel. I confirmed on the cabin altimeter that we were above 10;000 ft cabin altitude. We accomplished the immediate action items. I maintained aircraft control by stopping the climb and going into a shallow descent with vertical speed. The autopilot stayed on. We [advised ATC] and got an initial clearance to FL240. Used level change to descend after the clearance. The copilot ran the cabin altitude warning checklist and was able to recover control of the cabin altitude very quickly with Manual control. We continued our descent to a new clearance of 13000 ft. I turned aircraft control over to the copilot to coordinate with everyone. The cabin altitude never exceeded 14000 ft. I think we recovered control at about 12500 ft cabin altitude. We made the decision to divert to [an alternate] since we were almost over it. We continued our descent and vectors for [the alternate]. Not able to communicate with dispatch on VHF because we were already at 10;000 ft by the time we tried. We used ACARS to send a divert report and VHF with Ops to coordinate the divert. We took our oxygen masks off sometime after 18;000 ft when the cabin altitude was under 10;000 ft. I took the airplane back from the copilot at around 6000 ft altitude and we landed uneventfully.

Data retrieved from NASA's ASRS site 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.