AN A320 CREW; PREOCCUPIED WITH REPROGRAMMING THE FMS; MISSED THEIR ASSIGNED XING RESTR.

Narrative

WE WERE CLRED TO DSND ON THE PDZ 4 ARR TO LAX; THE CLRNC LIMIT WAS 8000 FT. ACFT WAS IN A 'MANAGED' DSCNT; SO THAT THE AUTOPLT WOULD AUTOMATICALLY LEVEL OFF AT THE FMC PROGRAMMED XING RESTRS ON THE ARR. ABOUT 3 MI E OF TRTLE WAYPOINT; WE RECEIVED A RWY CHANGE FROM RWY 25L TO RWY 24R. WHEN THE CAPT (PF) INSERTED THE NEW RWY INTO THE FMC; THE WAYPOINTS WERE MISSEQUENCED AND THE ACFT REVERTED TO OPEN DSCNT WHICH RESULTED IN US XING TRTLE WAYPOINT AT 15400 FT AS OPPOSED TO THE CHARTED MINIMUM XING ALT OF 16000 FT. ALTHOUGH I WAS MONITORING OUR FLT PATH USING RAW DATA; IN A BRIEF MOMENT OF MISCOMS I COULD NOT IMPRESS ON THE PF THE ALT TREND/SIT. IN ADDITION; MY WORKLOAD INCREASED AT THE TIME HE INSERTED THE NEW RWY AND UNEXPECTED WAYPOINTS CAME UP; AS HE ASKED ME FOR ASSISTANCE. WE DID EVENTUALLY STOP THE DSCNT. WE WOULD HAVE PROBABLY MISSED THE XING ALT BY A GREATER MARGIN HAD I NOT USED RAW DATA TO BACK UP THE FMC. ATC DID NOT SEEM TO NOTICE. SOME RECOMMENDATIONS: 1) ALTER THE A320 FMC PROGRAMMING TO ALLOW AN EASIER METHOD OF SWITCHING RWYS WHEN ON A STAR (THIS IS NOT THE FIRST TIME WAYPOINT DROPPED FLT PLAN PROB HAS OCCURRED FOR ME ON THE PDZ 4 ARR AS WELL AS OTHERS). 2) ALTER COMPANY SOP'S REQUIRING THE USE OF OPEN DSCNT AND HARD ALTS IN THE FCU INSTEAD OF A 'MANAGED' AUTOMATIC DSCNT WHICH WOULD ELIMINATED THE OPEN DSCNT REVERSION PROB. SUPPLEMENTAL INFO FROM ACN 526644: ATC DID NOT CALL OUT ANY ALTDEV; BUT THE FO WHO WAS THE PNF THOUGHT WE MAY HAVE CROSSED TRTLE INTXN TOO LOW.