ALT BUST IN AN A-320.

Narrative

WE WERE CLRED FOR THE CIVET 1 ARR; ILS TO RWY 25L. WE REQUESTED THE ILS TO RWY 24R APCHING BRAEMR INTXN (CROSS AT 12). NEXT POINT WAS ARNES (CROSS AT 10000 FT) WHICH WAS SET IN FCU. AFTER MY REQUEST FOR RWY 24R; THE CTLR GAVE US TURN R TO 270 DEGS; INTERCEPT THE 24R LOC MAINTAIN 12000 FT. I ACKNOWLEDGED THE CLRNC R 270 DEGS; INTERCEPT THE LOC RWY 24R; MAINTAIN 12000 FT. AS THE PNF; I WAS TO BRING UP THE SECONDARY FLT PLAN IN THE MDCU; WHICH WE HAD PUT THE ILS RWY 4 APCH INTO AS A BACKUP. WE MUST HAVE BEEN PAST BRAEMR INTXN; WHEN THE CAPT STARTED HIS TURN TO 270 DEGS. I WAS CHANGING THE FLT PLAN IN THE MDCU WHEN THE CAPT ASKED WHAT ALT SHOULD WE BE AT. I REPLIED THAT THE CTLR SAID MAINTAIN 12000 FT AND LOOKED AT THE ALTIMETER; DSNDING TO 11400 FT. CAPT REQUESTED I VERIFY WITH THE CTLR. BEFORE I COULD; THE CTLR SAID TO MAINTAIN 9000 FT. NOTHING ELSE WAS SAID ABOUT ALT FROM THE CTLR. BEFORE UPDATING THE MDCU IN SUCH A BUSY ENVIRONMENT AS LAX; I SHOULD HAVE MADE SURE THE CAPT HAD SET THE REQUESTED ALT IN THE FCU; INSTEAD OF HURRYING THE ACTUATION OF THE SECONDARY APCH. WE WANTED THE APCH IN THE MCDU QUICKLY SINCE WE WEREN'T TOO FAR FROM THE RWY 24R LOC. THE CAPT AND I HAVE DISCUSSED THIS THOROUGHLY. CONTRIBUTING FACTOR WAS THAT IN SUCH AN AUTOMATED ACFT THE NEXT ALT HAD BEEN SET IN AND THE AUTOPLT WAS ALREADY DSNDING US TO THAT ALT; WHEN GIVEN THE AMENDED CLRNC FROM THE CTLR. IT MAY ALSO BE BETTER FOR THE PNF TO SET THE ALT ALERT EVEN WHEN THE PF IS FLYING THE ACFT WITH AUTOPLT ENGAGED. CURRENTLY THE PF DOES THIS WITH AUTOPLT ENGAGED AND THE PNF SETS THE ALT WHEN THE PF IS HAND FLYING THE ACFT. SUPPLEMENTAL INFO FROM ACN 294240: I NEVER HEARD THE CLRNC. LAX IS A VERY BUSY ARPT AND THESE SIDESTEP MANEUVERS CAN BE TRICKY. WE BRIEFED WELL IN ADVANCE. MY NOT HEARING THE ALT TO MAINTAIN AND TURNING BEFORE I CLARIFIED THE COMPLETE CLRNC PROBABLY CONTRIBUTED TO THE SIT. IN THESE NEW ACFT WHEN YOU HAVE MULTIPLE ALT CONSTRAINTS ENTERED INTO THE MCDU YOU PUT THE LOWEST ALT IN THE FCU ALT WINDOW WHEN CLRED FOR AN ARR THEN CHK ALL CONSTRAINTS AS YOU ARRIVE OVER EACH ONE. HOWEVER; IF YOU GO HDG SELECT THE ACFT GOES INTO HDG AND VERT SPD. IN OUR CASE 10000 FT BEING SET; THE ACFT CONTINUED IN VERT SPD. THE FO WAS BRIEFED TO REPEAT THE CLRNC AND CONFIRM THAT I HAVE HEARD AND SET THE PROPER ALT IN THE FCU IN THE FUTURE.