Narrative:

During normal cruise at 31000 ft, the cabin altitude horn sounded, the cabin pressurization showed 10000 ft, and the cabin rate of climb showed 500-700 FPM climb. The horn was silenced, the packs were put in high, and a descent was initiated. The autofail or unscheduled pressurization change checklist was accomplished. A request for a lower altitude had been requested and the clearance was issued within seconds. Upon initiating the descent, the cabin rate of climb began to show a positive increase in pressurization, and no further complications were noted. The discrepancy was recorded in the logbook, and corrective action was taken. The cause of the problem is unknown at this time. However, my guess is that it was a simple, momentary failure of the automatic pressurization system, which corrected itself as soon as a descent was begun.

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Original NASA ASRS Text

Title: CABIN PRESSURIZATION PROB IN B737-300.

Narrative: DURING NORMAL CRUISE AT 31000 FT, THE CABIN ALT HORN SOUNDED, THE CABIN PRESSURIZATION SHOWED 10000 FT, AND THE CABIN RATE OF CLB SHOWED 500-700 FPM CLB. THE HORN WAS SILENCED, THE PACKS WERE PUT IN HIGH, AND A DSCNT WAS INITIATED. THE AUTOFAIL OR UNSCHEDULED PRESSURIZATION CHANGE CHKLIST WAS ACCOMPLISHED. A REQUEST FOR A LOWER ALT HAD BEEN REQUESTED AND THE CLRNC WAS ISSUED WITHIN SECONDS. UPON INITIATING THE DSCNT, THE CABIN RATE OF CLB BEGAN TO SHOW A POSITIVE INCREASE IN PRESSURIZATION, AND NO FURTHER COMPLICATIONS WERE NOTED. THE DISCREPANCY WAS RECORDED IN THE LOGBOOK, AND CORRECTIVE ACTION WAS TAKEN. THE CAUSE OF THE PROB IS UNKNOWN AT THIS TIME. HOWEVER, MY GUESS IS THAT IT WAS A SIMPLE, MOMENTARY FAILURE OF THE AUTOMATIC PRESSURIZATION SYS, WHICH CORRECTED ITSELF AS SOON AS A DSCNT WAS BEGUN.

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.