EMER DSCNT PERFORMED BY A DC9-30 PIC WITH A FAILURE OF THE PRESSURIZATION SYS. PIC NOTED THAT THE OXYGEN MASKS DID NOT ENABLE THE CREW TO COM WITH FLT ATTENDANT OR USE PA SYS.

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DURING CRUISE AT FL310 EXPERIENCED A CABIN PRESSURE ABNORMALITY. CABIN ROSE RAPIDLY TO 10000 FT. MASTER WARNING AND RED CABIN PRESSURE HIGH ANNUNCIATOR LIGHT ILLUMINATED. PERFORMED EMER MEMORY ITEMS AND INITIATED EMER RAPID DSCNT. DECLARED EMER. ELECTED TO MAKE PRECAUTIONARY LNDG AT TLH. AUTOPRESSURIZATION SYS HAD BEEN WRITTEN UP IN MAINT ACFT LOGBOOK MANY TIMES DURING A 3 WK PERIOD. IN ADDITION AUTOPRESSURIZATION IRREGULARITY OCCURRED AFTER TKOF ON THIS FLT; BUT AFTER COMPLETING ABNORMALITY CHKLIST; SYS RESET AND FLT CONTINUED UNTIL EMER. NORMALLY; ON THE COMPANY DC9 ACFT; WITH OXYGEN MASKS ON; DIRECT COMS ARE ONLY POSSIBLE BTWN PLTS (INTERCOM SWITCH) AND ATC (VHF 1 OR 2 SWITCH). THE MD82 SERIES ACFT WHICH THE COMPANY ALSO HAS; HAVE A DIRECT OXYGEN MASK PA AND FLT ATTENDANT 'INT' SWITCH. DURING THE RAPID DSCNT; I WAS UNABLE TO TALK TO THE FLT ATTENDANTS OR PAX. IN PART; I WAS TOO BUSY WITH ACFT; MY COPLT AND THE CHKLIST/EMER. I ELECTED TO DSND TO 10000 FT AND THEN REMOVE MY OXYGEN MASK AND THEN SPEAK TO FLT ATTENDANT AND PAX. COMS UNDER THIS PROC ARE VERY POOR VIA HAND SET INTERPHONE AND REQUIRE OXYGEN MASK REMOVAL. A REAL PROB IS SMOKE/FIRE IS PRESENT. POSTFLT INSPECTION AND TRIALS INDICATED DC9-30 LACK OF DIRECT COM SWITCH. EVERYONE THOUGHT COM WAS FEASIBLE AND STRAIGHT FORWARD BUT AFTER MY EMER IT WAS OBVIOUSLY NOT THE CASE. MIXED ACFT TYPES; LACK OF SPECIFIC TRAINING; LAX PLANNING AND SAFETY PROCS AND POOR CONSIDERATION ON THE COMPANY'S BEHALF LED TO A CHAIN OF EVENTS THAT LED TO INABILITY OF FLT CREW TO COM WITH FLT ATTENDANTS VIA INTERPHONE ON THESE ACFT. THE COMPANY IS NOW GOING TO WIRE A SWITCH TO ENABLE 2-WAY COM IN SUCH CIRCUMSTANCES; AND HAS IMPLEMENTED AN INTERIM MEASURE TO FACILITATE 2-WAY COM; ALBEIT CLUMSY AND INVASIVE; INVOLVING XMITTING INTO PA HEADSET VIA 'INT' SPEAKER IN THE FLT DECK. THE UNSCHEDULED LNDG WAS UNEVENTFUL. WE ALL LEARNED A VALUABLE LESSON; THE HARD WAY.