A320 FLT CREW ON APCH TO LAS BECOMES PREOCCUPIED WITH A POTENTIAL OVERWT LNDG; BOTH PLTS ARE HEADS DOWN AS THE CTLR ADVISES OF A LOW ALT ALERT.

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I WAS AN FO ON A FLT FROM ABC TO DEF. THE ACFT HAD A FUEL LOAD ABOVE THE RELEASE QUANTITY DUE TO TANKERING FROM THE INBOUND FLT. THE LOADING WAS COMPLETE AND I COMPLETED THE WT AND BAL WHICH SHOWED US 600 LBS OVER MAX LNDG WT. WE DISCUSSED THE SITUATION AND MADE THE DECISION TO BURN THE EXCESS OFF INFLT. TKOF AND CLB WERE NORMAL. AT CRUISE; OUR CALCULATIONS SHOWED US 1200 LBS OVER MAX LNDG WT. WE REQUESTED AND RECEIVED AN ALT 2000 FT LOWER TO INCREASE BURN. THE LOWER ALT AND HIGH CRUISE SPD LOWERED OUR WT TO +800 LBS. WE CONTINUED FMS STAR INTO DEF WITH LIBERAL USE OF SPD BRAKE TO INCREASE DRAG AND EXPEDITE DSCNTS TO LOWER ALTS. UPON CHK-IN WITH APCH; WE INFORMED THEM OF OUR SITUATION AND REQUESTED A 360 DEG TURN ON THE ARR TO ELIMINATE EXCESS FUEL. APCH AUTH AND WE PERFORMED 1 360 DEG TURN WHICH DROPPED FUEL TO 600-400 LBS OVER. AFTER COMPLETING THE 360 DEG TURN; WE CONTINUED INBOUND ON THE ARR WITH APCH STATING HE WOULD HAVE DIFFICULTY WITH ANY ADDITIONAL MANEUVERING DUE TO TFC. WE MADE THE DECISION TO CONTINUE AND CONFIGURE EARLY TO BURN OFF EXCESS FUEL. I WAS PF AND BEGAN TO CONFIGURE EARLY WHILE COMPLYING WITH THE STAR ALT AND SPD CONSTRAINTS. ONCE ESTABLISHED ON DOWNWIND IT BECAME EVIDENT THAT WITH THE VOLUME OF TFC WE WOULD BE GETTING A SHORTENED LNDG PATTERN. APCH CLRED US TO 7000 FT WHILE MAINTAINING 210 KTS WHICH LIMITS THE ACFT TO SLAT EXTENSION ONLY. THE NEXT CLRNC WAS A BASE TURN AND 5000 FT FOLLOWED BY A REQUEST TO RPT AND FOLLOW TFC TO THE RWY. WHAT FOLLOWED WAS FAST PACED HEADS DOWN ACTION THAT COULD HAVE LED TO A CFIT. THE CAPT CALLED THE TFC AND RWY IN SIGHT. APCH CLRED US FOR THE VISUAL WITH OUR EXPECTATIONS BEING THAT WE GET DOWN; SLOW DOWN; AND BURN OFF THE EXCESS FUEL. AS THE PF; I WAS ATTEMPTING TO CONTINUE THE TURN TOWARDS THE RWY; COMMAND THE CONFIGURING OF THE ACFT AND SEQUENCE THE FMS FLT PLAN IN EVENT OF A MISSED APCH. THE CAPT WAS HEADS DOWN CALCULATING IF WE WOULD BURN OFF THE EXCESS 400 LBS PRIOR TO LNDG. AT SOME POINT; 2000 FT WAS SET IN THE ALT SELECT WINDOW; BY WHICH OF US I DON'T KNOW; FOR A REASON I DON'T KNOW. THE ALT MAY HAVE GOTTEN SET WHEN THE AUTOPLT CAPTURED 5000 FT AND WE REALIZED WE NEEDED LOWER TO INTERCEPT THE GS. THE SITUATION EVOLVED AS THE ACFT WAS FULLY CONFIGURED AND I WAS DRIVING THE ACFT WITH THRUST IDLE; OPEN DSCNT TOWARD THIS ARBITRARY ALT OF 2000 FT WHILE PROGRAMMING THE FMS. IN MY MIND I WAS THINKING; GET BELOW THE GS INTO LEVEL FLT AND DRIVE INTO INTERCEPT THE GS FROM BELOW WHILE BURNING OFF THE EXCESS FUEL DOWN LOW AND DIRTY. 2 THINGS OCCURRED AT ONCE. I LOOKED UP AND THOUGHT THAT WE WERE LOW BASED ON MY VIEW OF THE RWY AND APCH CALLED AND STATED THEY HAD A LOW ALT ALERT ON US. THE CAPT ACKNOWLEDGED THE CALL AS I STOPPED THE DSCNT AND CONTINUED TOWARDS THE RWY. APCH SWITCHED US TO TWR AND BASED ON OUR 400 LB EXCESS WT WE MADE A NORMAL GAR AND RETURNED FOR A NORMAL PATTERN. I ASK MYSELF; HOW COULD 2 EXPERIENCED PLTS DRIVE A PERFECTLY GOOD ACFT INTO THE GND ON A CLR DAY? I BELIEVE WE FIXATED ON THE FUEL AND LOST THE BIG PICTURE AND SITUATIONAL AWARENESS. THE ACFT IS VERY CUMBERSOME TO FLY A VISUAL APCH WITH THE AUTOPLT ON DUE ITS LEVEL OF AUTOMATION. I USUALLY HAND FLY ALL VISUAL APCHS FOR THIS REASON; BUT IN THIS CASE I ALLOWED AUTOMATION TO REMAIN ON WHILE I TRIED TO DO MULTIPLE THINGS AT ONCE. AS A CREW WE VIOLATED THE RULE BELOW 10000 FT WHERE ALL HEADS DOWN WORK IS DONE BY THE PNF. SINCE THE EXCESS WT WAS ONLY 400 LBS WE ALLOWED OURSELVES TO GET SO WRAPPED UP IN BURNING OFF THE EXCESS THAT WE LOST SITUATIONAL AWARENESS; HAD THE EXCESS BEEN GREATER I DON'T BELIEVE THE SITUATION WOULD HAVE OCCURRED. PLTS AS A GROUP ARE GENERALLY 'CAN' DO' PEOPLE AND IF IT'S POSSIBLE; WE WILL ATTEMPT TO ACHIEVE IT. IT'S THIS MINDSET THAT MAY HAVE ALLOWED US TO FIXATE ON THE EXCESS FUEL.