AN A-320 FO RELIED ON FMC PROGRAMMING TO LEVEL AT AN INTERMEDIATE DSCNT ALT; WHILE THE FCU WAS SET FOR THE NEXT STEPDOWN. THE FLC WAS LATE TO DISCOVER THE DEP FROM THE NORM; AND ATC ISSUED LOWER.

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TRIP LAS TO LAX. CLRED TO 10 MI E OF CIVIT; THEN DIRECT SMO FOR APCH TO RWY 6R LAX. AFTER MAKING TURN AT CIVIT (10 MI E) CLRED TO CROSS SMO AT 8000 FT; THEN DSND TO 4000 FT; FLY 250 DEG HDG AFTER SMO. I WAS THE PF. I SET 8000 FT IN THE FCU WINDOW INITIALLY; THEN PROGRAMMED THE FMC TO CROSS SMO AT 8000 FT. AFTER PUTTING THE 8000 FT RESTR IN THE FMC; I SET THE 4000 FT IN THE FCU ALT WINDOW. ASSUMING THAT THE ACFT WOULD LEVEL OFF AT THE FMC RESTR. I BEGAN THINKING ABOUT THE APCH AND LOOKING AT THE OTHER TFC DIRECTED BY SOCAL APCH WHEN THE CAPT BROUGHT TO MY ATTN THAT WE WERE AT 7500 FT. I IMMEDIATELY BEGAN TO CORRECT UP TO 8000 FT WHEN SOCAL CALLED TO POINT OUT OUR ERROR; WHICH WE ACKNOWLEDGED. HE THEN CLRED US TO MAINTAIN 7000 FT. THE REST OF THE DSCNT AND APCH WERE UNEVENTFUL. IT HAD BEEN A LONG DAY. SHOW TIME AT PHX FOR THE TRIP WAS XA55 FOR A SAN DIEGO TURN; A 3 HR LAYOVER; 1 LEG TO LAS VEGAS; A 2.5 HR LAYOVER; THEN THE LEG TO LAX. I WAS PRETTY TIRED BY XM30 AM. I THINK THAT IN THE FUTURE WHEN GIVEN A RESTR TO CROSS A FIX AT OR ABOVE THEN DSND TO; THAT I WILL PUT THE HIGHEST CLRNC ALT IN THE FCU WINDOW UNTIL THAT CLRNC IS ATTAINED THEN ALLOW THE ACFT TO DSND. I RELY HEAVILY ON AUTOMATION TO HELP OUT WHEN FLYING LATE AND TIRED; BUT I STILL NEED TO PAY ATTN AND XCHK. SUPPLEMENTAL INFO FROM ACN 304217: MY ATTN WAS DIVERTED IN ORDER TO MAKE THE REQUIRED PA ANNOUNCEMENT AS WE APCHED 10000 FT. THERE WERE ALSO 2 ACFT IN THE VICINITY WHICH KEPT MY ATTN FOR SEVERAL MOMENTS. UPON MY RETURN TO THE FLT INSTS I NOTICED WE WERE DSNDING THROUGH 7500 FT. SOCAL CALLED TO POINT OUT OUR ERROR. THE CTLR THEN GAVE US A CLRNC TO 7000 FT. CONTRIBUTING FACTORS: 1) COMPLACENCY. LAST LEG OF THE DAY; BEAUTIFUL WX; LIGHT TFC. 2) EXHAUSTION. RPT TIME FOR THIS DAY WAS AT XA55 THE PREVIOUS AFTERNOON. FO'S DAY HAD BEGUN MUCH EARLIER; AS HE HAD TO COMMUTE IN FOR HIS RPT. 3) REQUIRED PNF DUTIES. THE 10000 FT CALL CAN SOMETIMES REMOVE ONE OF THE PLTS OUT OF THE LOOP.