Narrative:

A few mins after leveling at FL340; the cabin altitude warning horn sounded. I saw that the cabin was at 10000 ft and climbing at 500 FPM. I also saw that the pack and bleed valves were in their correct position; and the automatic fail light was on. We performed the cabin altitude warning horn checklist. When it called for mode switch to manual; I paused with the mode switch in the standby position to observe the result. The automatic fail light immediately extinguished and the green standby light illuminated. The cabin rate of climb immediately reversed to a 500 FPM descent rate. Not wishing to change this ideal condition; I continued to observe the cabin altitude descend to below 10000 ft (it had reached about 10200 ft). We accomplished the automatic fail checklist to be sure nothing was missed. I checked with the flight attendants to be sure everything was normal in the cabin; and contacted maintenance control who asked us to return to ZZZ. The pressurization system operated normally in standby for the remainder of the flight.

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Original NASA ASRS Text

Title: AN ACR FLT CREW EXPERIENCED A CABIN PRESSURE AUTO FAIL. THEY ENGAGED THE STANDBY SYSTEM AND RETURNED TO THEIR DEP ARPT.

Narrative: A FEW MINS AFTER LEVELING AT FL340; THE CABIN ALT WARNING HORN SOUNDED. I SAW THAT THE CABIN WAS AT 10000 FT AND CLBING AT 500 FPM. I ALSO SAW THAT THE PACK AND BLEED VALVES WERE IN THEIR CORRECT POS; AND THE AUTO FAIL LIGHT WAS ON. WE PERFORMED THE CABIN ALT WARNING HORN CHKLIST. WHEN IT CALLED FOR MODE SWITCH TO MANUAL; I PAUSED WITH THE MODE SWITCH IN THE STANDBY POS TO OBSERVE THE RESULT. THE AUTO FAIL LIGHT IMMEDIATELY EXTINGUISHED AND THE GREEN STANDBY LIGHT ILLUMINATED. THE CABIN RATE OF CLB IMMEDIATELY REVERSED TO A 500 FPM DSCNT RATE. NOT WISHING TO CHANGE THIS IDEAL CONDITION; I CONTINUED TO OBSERVE THE CABIN ALT DSND TO BELOW 10000 FT (IT HAD REACHED ABOUT 10200 FT). WE ACCOMPLISHED THE AUTO FAIL CHKLIST TO BE SURE NOTHING WAS MISSED. I CHKED WITH THE FLT ATTENDANTS TO BE SURE EVERYTHING WAS NORMAL IN THE CABIN; AND CONTACTED MAINT CTL WHO ASKED US TO RETURN TO ZZZ. THE PRESSURIZATION SYS OPERATED NORMALLY IN STANDBY FOR THE REMAINDER OF THE FLT.

Data retrieved from NASA's ASRS site as of May 2009 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.