AN MD82 ON APCH INTO LAS; NV; DSNDS THROUGH ASSIGNED ALT.

Narrative

WHILE WE WERE IN VISUAL CONDITIONS DSNDING INTO LAS ARPT; APCH CTL GAVE US A DSCNT OUT OF 8000 FOR 6000 FT. WITH THE AUTOPLT ON; 6000 FT WAS SET IN THE ALT SELECTOR OF THE AUTOPLT; CONFIRMED BY BOTH PLTS BY COCKPIT XREF AND A DSCNT BEGUN. WE WERE ON A L DOWNWIND TO RWY 25L. APCHING 6000 FT; APCH CTL GAVE US TFC TO FOLLOW AT OUR 9 O'CLOCK POS; WHICH WE CONFIRMED WE HAD IN SIGHT AND WERE THEN TOLD TO FOLLOW. WE WERE IN THE BASE TURN TO FOLLOW THE TFC WHEN APCH CTL TOLD US TO CHK ALT; AS WE WERE BELOW 6000 FT. WE LOOKED UP AT THE ALT SELECTOR AND IT WAS AT 5000 FT AND WE WERE DSNDING OUT OF 5700 FT. WE IMMEDIATELY CLBED BACK TO 6000 FT; AT WHICH TIME APCH CTL CLRED US FOR THE VISUAL APCH TO RWY 25L. NEITHER ONE OF US; THE COPLT NOR MYSELF; HAD SET 5000 FT IN THE ALT SELECTOR. THE MAJOR FACTOR WAS GETTING DISTR BY FOLLOWING THE 9 O'CLOCK TFC AND NOT WATCHING THE LEVELOFF AT 6000 FT WITH THE AUTOPLT ON. THE VISUAL CONDITIONS ALSO COULD HAVE BEEN A FACTOR. TO CORRECT THE SIT; CLOSE ATTN TO THE LEVELOFF SHOULD BE ADHERED TO -- EVEN WITH THE AUTOPLT ON.