A B737-500'S CABIN ALT WARNING HORN SOUNDED IN CRUISE AT FL330. AFTER MANUAL CABIN OUTFLOW VALVE SELECTION PRESSURIZATION CONTROL WAS REGAINED.

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AFTER BEING IN STEADY CRUISE FOR APPROX 15 MINS; THE TKOF WARNING HORN BEGAN TO SOUND INTERMITTENTLY. THE CABIN ALT WAS AT APPROX 10500 FT AND CLBING VERY SLOWLY. THE CABIN PRESSURE CONTROLLER WAS QUICKLY SWITCHED FROM AUTO TO STANDBY WITH NO EFFECT. MASKS WERE DONNED AND THE QRC RUN. WITH ACFT PACKS SWITCHED TO HIGH AND THE CABIN OUTFLOW VALVE IN MANUAL; THE CTL OF CABIN ALT WAS REGAINED. WE COORDINATED WITH ATC AND RECEIVED CLRNC TO DSND TO FL240 TO GIVE OURSELVES A GREATER MARGIN OF SAFETY. AFTER CONFERRING WITH DISPATCH; WE ELECTED TO CONTINUE TO ZZZ. FLYING AN INST APCH IN THIS CONFIGN IS DIFFICULT; AS THE PNF IS QUITE FULLY ENGAGED KEEPING TRACK OF THE CABIN DSCNT RATE AS THE PWR IS CHANGED. ATC WAS AGREEABLE AND HELPFUL ALLOWING US A GRADUAL DSCNT. THE ADVENT OF AUTOMATIC CONTROLLERS HAS MADE THIS ONCE-COMMON PROC A LOST ART. THERE IS VERY LIMITED INFO OR PROCEDURAL ADVICE IN THE FLT MANUAL. I WOULD SUGGEST THAT THE CABIN BE DSNDED TO ABOUT 4000 FT AFL THEN REQUEST A LEVEL SEGMENT FROM ATC AT THAT ALT. DSND SLOWLY TO THAT ALT; CHASING THE OUTFLOW VALVE ALL THE WAY. ONCE LEVEL; THE OUTFLOW VALVE SHOULD BE FULLY OPENED BEFORE ENTERING THE FINAL ENVIRONMENT. WE MADE THE MISTAKE OF DSNDING THE CABIN TO ABOUT 1000 FT INITIALLY AND THEN WERE VERY BUSY CHASING THE DSCNT RATE DOWN TO THAT ACFT ALT; REDUCING THE PNF'S SITUATIONAL AWARENESS OF THE APCH. THE FAILURE WAS VERY INSIDIOUS AND WOULD HAVE GONE UNNOTICED MUCH LONGER IF NOT FOR THE WARNING HORN. THIS IS THE SECOND SUCH FAILURE I HAVE EXPERIENCED IN THE LAST FEW MONTHS.