FLT CREW OF B744 DSNDED BELOW CLRED ALT AT VHHH WHEN CAPT FAILED TO CHANGE FROM QNE TO QNH AT TRANSITION ALT. CREW FATIGUE DUE TO BACK SIDE OF CLOCK OP AND DISTR DUE TO INOP INTERPHONE CONTRIBUTED.

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DSNDING INTO HKG; WE WERE CLRED TO 4100 FT ON A VECTOR FOR THE ILS RWY 7L. THIS FLT IS NOTORIOUS FOR FATIGUE DUE TO REVERSED SLEEP CYCLE. WE ALSO HAD A PROB WITH THE CABIN TO COCKPIT INTERPHONE WHERE THE FLT ATTENDANTS WOULD TRY TO CALL THE COCKPIT; AND WE WOULD GET MULTIPLE DINGS; BUT WERE NOT ABLE TO TALK TO THEM. THIS EVENTUALLY LED TO 3 DIFFERENT FLT ATTENDANTS COMING TO THE DOOR TO SEE WHAT WAS WRONG; AND I HAD TO KEEP GETTING UP TO OPEN THE DOOR. WE WERE BOTH FIGHTING TO KEEP OUR EYES OPEN DURING THE INITIAL PART OF THE DSCNT; AND AFTER LEVELING AT FL 110 (TRANSITION ALT) WE DROVE IN FOR SEVERAL MINS. I WANTED TO GIVE THE PREP FOR LNDG CALL AS SOON AS WE LEFT THAT ALT; AND WAS WONDERING IF THEY WOULD EVEN HEAR IT. THEN WE WERE TOLD TO DSND TO 4100 FT. THE COPLT SET HIS ALTIMETER; WHILE I MADE THE PREP FOR LNDG PA; AND WHEN DONE; I WAS DISTR BY ADDITIONAL CALLS FROM APCH CTL AND FORGOT TO SET MINE. THE FO WAS FLYING ON AUTOPLT; AND WHEN THE ACFT DSNDED THROUGH 4100 FT AND LEVELED AT 3900 FT; HE CAUGHT IT AND WAS TRYING TO GET IT BACK TO 4100 FT. AS SOON AS HE SAID 'IT ISN'T LEVELING;' I KNEW WHY; AND HIT MY ALTIMETER BUTTON; AND HE HIT FLT LEVEL CHANGE TO GET US BACK. APCH CALLED AND REMINDED US OF THE ALT; AND GAVE US THE ALTIMETER SETTING AGAIN. THERE WERE SEVERAL DISTRS HERE; BUT THE FATIGUE WAS THE BIGGEST FACTOR IN THIS.