B767-300 IS DISPATCHED WITH ONE PACK INOP. AFTER LEVELOFF; CABIN ALT WARNING ACTIVATED AND FLT CREW INITIATED EMER DESCENT.

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DEPARTED WITH L PACK DEFERRED DUE TEMP CONTROLLER ISSUES (DID WE REALLY NEED TO LOSE ENTIRE PACK FOR THAT PROB? I THINK MAINT MANUAL DID NOT PUT MUCH THOUGHT INTO THAT; ESPECIALLY AS MAINT CTL AFTER SATCOM DISCUSSION SENT NOTE TO PRINTER ASKING SAME QUESTION). WE WERE RESTR TO FL340 MAX; AND ABOUT 20 MINS AFTER LEVELOFF; THE CABIN ALT WARNING WENT OFF. BOTH PLTS IMMEDIATELY DONNED O2 MASKS; BUT ALTHOUGH I CONFIRMED AND RECONFIRMED SWITCHOLOGY; WE WERE NOT ABLE TO TALK TO EACH OTHER WITH MASKS ON. I ATTEMPTED TO RUN QRC BUT CAPT COULD NOT HEAR ME; AND HE; THEREFORE; WISELY DECLARED EMER WITH ATC SEEKING LOWER ALT. (I COULD HEAR HIS XMISSIONS; BUT MY SPEAKER WAS VERY CRACKLY.) HE BEGAN EMER DSCNT WHILE I CONTINUED CHKING ITEMS ON QRC FOR FIRSTLY CABIN ALT; THEN EMER DSCNT. I WAS COMMUNICATING MOSTLY USING SEMAPHORE AND; THEREFORE; IT WAS OBVIOUS THAT THE SOONER WE COULD GET MASKS OFF THE BETTER. WE ELECTED TO TURN DEFERRED PACK BACK ON USING OUR EMER AUTH. THE L PACK IMMEDIATELY CAUGHT THE CABIN; AND THE CABIN ALT DSNDED QUICKLY BELOW 10000 FT; AT WHICH POINT I REMOVED MY MASK AND TOLD THE CAPT HE COULD DO SAME. HE CONTINUED DSCNT AND WE BOTH WORKED ON CLEAN-UP ITEMS; SUCH AS COMMUNICATING WITH THE PAX AND FLT ATTENDANTS. I COMPUTED DISTANCE/FUEL FLOW ISSUES AND WE DISCUSSED OUR OPTIONS AND DECIDED IT WAS NOT AN ISSUE TO CONTINUE TO ZZZ WITH PERFECT VFR WX AND A PLANNED LNDG FUEL OF ABOUT 55 MINS ENDURANCE REMAINING AT 10000 FT HOLDING SPD. I TALKED WITH DISPATCH AND MAINT ON SATCOM AND ALL PARTIES WERE COMFORTABLE WITH CONTINUING. I CANCELED EMER AND WE CONTINUED TO ZZZ WITH NO FURTHER ISSUES. EDITORIAL UNRESOLVED QUESTIONS/CONCERNS: WHY SINGLE PACK DID NOT HOLD (WE CHKED IT SEVERAL TIMES IN CLB AND INITIAL LEVELOFF; A CREEPER FOR SURE). THIS WILL MAKE ME THINK HARD BEFORE ACCEPTING AN ACFT WITH SINGLE PACK OP IN FUTURE. I WAS SURPRISED THAT MEL LET US FLY SO HIGH; WHEN OTHER ACFT ARE RESTR TO FL250. WHY PACK WAS DEFERRED; ESPECIALLY AS MAINT HAD SAME CONCERN DURING SATCOM CONFERENCE: I THINK MAINT WAS IN TOO BIG A RUSH TO DEFER WITHOUT REALLY THINKING THIS THROUGH. (BENEFIT OF HINDSIGHT; ETC.) WHY WE OPERATE WITH INTERIOR INTRACOCKPIT COMS ON THIS FLEET. CAN WE NOT GET BETTER MASKS AND MIKES/SPEAKERS? ATC HAD GREAT DIFFICULTY UNDERSTANDING US; AS WELL AS OUR OWN COMS PROBS. A FULL FACE MASK WOULD HAVE BEEN OF GREAT HELP (I LOST MY GLASSES INITIALLY PUTTING MASK ON). AND NOW THE HIGH POINTS: GREAT CRM AND OVERALL LEADERSHIP BY THE CAPT. GREAT AND IMMEDIATE HELP FROM DISPATCH AND MAINT AND ATC.