AN A300 FLT CREW DSND BELOW ASSIGNED ALT WHEN THEY FORGET TO SET THE CORRECT ALTIMETER SETTING ON APCH INTO VHHH.

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ON OUR ARR TO VHHH FIR WE WERE INITIALLY CLRED ELATO 1B; RWY 25L MAINTAIN FL280. WE WERE LATER CLRED TO DSND TO FL130 'WHEN READY;' NO SPD RESTRS 'ON THE ELATO 1B ARR.' AFTER THE FO BRIEFED THE APCH AND WE STARTED THE DSCNT TO FL130; WE WERE CLRED TO FURTHER DSND TO 5000 FT QNH 0994 (AT FL150) AND FLY DIRECT TO LOTUS. THE 'DSCNT CHKLIST' HAD BEEN CALLED FOR BY THE FO AT FL180 AND WAS BEING HELD AT 'ALTIMETERS.' AT FL120 (TRANSITION LEVEL 110) WE WERE ASKED TO SLOW TO 230 KTS FROM 310 KTS. THE FO LEVELED OFF AT FL115; EXTENDED THE SPD BRAKE; SLOWED TO BELOW 250 KTS; THEN CALLED FOR 'SLATS EXTEND.' THE CAPT EXTENDED THE SLATS. WHEN THE ACFT SLOWED TO 230 KTS THE DSCNT WAS CONTINUED. PRIOR TO REACHING 5000 FT WE WERE CLRED TO DSND TO 4500 FT AND CLRED FOR THE ILS DME RWY 25L APCH. THE RADIOS WERE TUNED AND SET FOR THE APCH AND THE 'APCH CHKLIST' CALLED FOR AT 7 NM FROM LOTUS ON A HDG OF 270 DEGS AT 4500 FT. AT 1 NM FROM LOTUS THE APCH CTLR CALLED AND SAID: 'DSND TO 4300 FT ON QNH 994.' THIS ALERTED US TO THE FACT THAT OUR ALTIMETERS WERE STILL SET ON 1013 MB; WHICH PUT OUR ALT AT 4000 FT MSL. THE FO IMMEDIATELY CLBED BACK TO 4300 FT AND WE CONTINUED THE APCH. ONCE ESTABLISHED ON THE ILS RWY 25L; WE INFORMED APCH CTL AND HE TURNED US OVER TO TWR. NEITHER THE CAPT NOR FO ASKED IF THE 'DSCNT CHKLIST' HAD BEEN COMPLETED BEFORE CALLING FOR THE 'APCH CHKLIST;' AND ALL 3 CREW MEMBERS MISSED THE 'TRANSITION LEVEL' CALL. I HAVE NO EXPLANATION WHY ALL 3 CREWMEN MISSED THE CALL. WE WERE NOT FATIGUED AND THE WORKLOAD WAS NORMAL. PERHAPS THE CALL TO SLOW TO 230 KTS; AND REQUIRED ACTIONS TO ACHIEVE THIS SPD THAT CAME JUST BEFORE TRANSITION LEVEL 110 DIVERTED OUR ATTN LONG ENOUGH TO CAUSE US TO FORGET THE CALL AND COMPLETE THE 'DSCNT CHKLIST.'