AN A320 DSNDS ON THE LAX CIVET ARR AND DSNDS BELOW AN ASSIGNED ALT BECAUSE OF MGMNT OF THE AUTOFLT SYS.

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THE FLT WAS PART OF AN OE TRIP FOR A PLT NEW TO THE A320. WE HAD BEEN CLRED FOR THE PARADISE 2 ARR TO LAX AND WERE SWITCHED TO THE CIVET 1 ABOUT 50 MI FROM THE ARPT. DUE TO A LATE DSCNT CLRNC CAUSED BY TFC CONGESTION AND WX DEVS WE WERE ABOVE THE FMGC COMPUTED VERT PATH WHEN CLRED TO DSND VIA THE ARR. IT WAS NECESSARY TO USE OPEN DSCNT MODE AND SPD BRAKES TO RETURN TO THE VERT PATH. JUST PRIOR TO LEVELING AT 10000 FT WE WERE GIVEN AN AIRSPD RESTR TO 210 KTS AND THE OE STUDENT PULLED THE FCU ALT KNOB (I WAS UNAWARE THAT HE HAD) AND PUT THE AIRPLANE BACK TO OPEN DSCNT MODE. BECAUSE THE LOWEST ALT ON THE CIVET 1 IS 8000 FT; IT IMMEDIATELY PUT THE AIRPLANE IN IDLE/OPEN DSCNT CAUSING US TO BEGIN DSNDING PRIOR TO THE RESTR ON THE ARR. TO RETURN IMMEDIATELY TO THE ASSIGNED ALT IT WAS NECESSARY TO DISCONNECT THE AUTOPLT AND AUTOTHROTTLES SINCE THE FMGC WOULD HAVE COMMANDED A MUCH MORE GRADUAL RETURN TO ALT. THE ARRS TO LAX FROM THE E ARE BUSY ONES AND THERE ARE MANY SHORT NOTICE ALTERATIONS TO RTES; RWYS AND SPDS THAT CAN BE DIFFICULT TO KEEP UP WITH IN AN AUTOMATED AIRPLANE. THE CIVET 1 AND THE ILS TO RWY 25L HAVE TOO MANY ALT RESTRS THAT MUST BE MANUALLY COMPLIED WITH. IT WOULD BE MUCH BETTER IF THE ILS GS COULD BE USED OUT TO ARNES INTXN RATHER THAN THE COMBINATION OF THE CIVET 1 AND THE ILS RWY 25L.