A320 FO OVERSHOT DSCNT ALT DUE TO MISPROGRAMMING THE AUTO FCU AND CORRECTED BACK IN TIME TO MAKE THE STAR FIX.

Narrative

THE FO WAS FLYING FROM PHX TO LAX. DURING THE PDZ 4 ARR TO RWY 25L AT LAX; SOCAL APCH CLRED 'DSND ON THE PDZ 4 ARR.' THE DSCNT WAS 'MANAGED' ON AUTOPLT WITH 8000 FT (THE ALT LIMIT) AND 'SPD' ON THE FCU. AFTER PASSING KONZL AT 17000 FT; APCH CTL RECLRED 'TO MAINTAIN MAX FORWARD SPD AND TO CHANGE RWY TO RWY 24R. AS THE FO MADE THE RWY CHANGE IN THE MCDU; THE FCU DEFAULTED TO 'OPEN DSCNT.' THE FO ALSO CHANGED THE FCU TO A SELECTED HIGHER SPD (ABOUT 340 KTS). THE ACFT DSNDED TO 15000 FT ABOUT 2 NM E OF TRTLE. THE ARR LIMITS AT TRTLE ARE 'CROSS AT OR BELOW 17000 FT; CROSS AT OR ABOVE 16000 FT.' THE CAPT AND FO; UPON RECOGNIZING THAT ACTIONS ON THE FCU HAD CAUSED THE FCU TO DEFAULT TO 'OPEN DSCNT;' AND ALLOWED A DSCNT 1000 FT LOW JUST PRIOR TO TRTLE; IMMEDIATELY CORRECTED THE ACFT BACK TO 16000 FT ON AUTOPLT AT TRTLE AND THE FO FLEW THE REMAINDER OF THE ARR AND APCH TO RWY 24R IN 'SELECT' SPDS AND ALTS ON THE FCU. AS THE FO WAS FLYING THE ACFT ON AUTOPLT AND MAKING CLRNC CHANGES IN THE MCDU; THE CAPT WAS RESPONDING TO ATC'S CLRNC CHANGES AND MONITORING CHANGES BEING ENTERED IN THE MCDU. THE COCKPIT WORKLOAD WAS HIGH TRYING TO ACCOMMODATE THE CLRNC CHANGES AND ACFT POSITIONING MONITORING WAS MOMENTARILY NEGLECTED; LEADING TO THE ALTDEV JUST SHORT OF THE NEXT FIX. RECLRNCS SUCH AS THIS OCCASIONALLY EXCEED CAPABILITIES OF ACFT'S AUTOMATION TO COMPLY; LEADING TO SUCH A DEFAULT. ALTHOUGH THESE RECLRNCS ARE UNDERSTANDABLY NECESSARY; AN ATTEMPT TO LIMIT THEM; OR GIVE THEM AT A MORE TIMELY POINT SO THAT AUTOMATION CAN BE PROGRAMMED (IE; PRIOR TO INITIAL DSCNT CLRNC) MAY PREVENT THIS FROM RECURRING IN THE FUTURE.