DA2000 FLT CREW IS ISSUED NEW RWY ASSIGNMENT DURING CIVET 5 TO LAX AND FMC DUMPS ALL WAYPOINTS DURING REPROGRAMMING. FLT CREW QUICKLY REQUESTS VECTORS AND ALTITUDE ASSIGNMENT AND NO DEVIATIONS OCCUR.

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ON ASSIGNED ARR OF CIVET 5 (JUST OUTSIDE OF CIVET) TO RWY 25L WE WERE GIVEN A LAST-MINUTE CHANGE TO INITIATE 'DSND VIA' CIVET 5 ARR AND THEN A RWY CHANGE TO RWY 24R. FO WAS FLYING AND HE REENTERED THE RWY 24R AND ARR INTO THE HONEYWELL EASY FMS AND ALL THE WAYPOINTS DUMPED LEAVING US WITH NO VERT OR HORIZ DATA TO NAV WITH. I IMMEDIATELY ADVISED ATC AND THEY ISSUED A HDG AND ALT TO INTERCEPT THE RWY 24R ILS AND THE REMAINDER OF THE APCH AND LNDG WAS UNEVENTFUL. LAX APCH IS NOTORIOUS FOR ISSUING VERY LAST-MINUTE CHANGES TO THESE SOMEWHAT COMPLEX ARR PROCS WHICH REQUIRE REPROGRAMMING MODERN DAY FMS SYS AND READILY SET UP PLTS FOR DEVIATING FROM THESE CLRNCS. EVEN ON DEP FROM LAX RWY 25R; WE WERE GIVEN A DIFFERENT DEP CLRNC WHILE BEGINNING OUR TKOF ROLL THAT REQUIRED A VOR REF THAT WAS DIFFERENT FROM OUR ORIGINAL DEP CLRNC. HAVING FLOWN INTO LAX IN BOTH A MAJOR AIRLINE POS AND A CPR FLYING POS I'VE FOUND THAT THIS KIND OF RAPID CHANGE CLRNCS TO BE THE WORST IN THE COUNTRY. MODERN DAY FMS SYS REQUIRE SUBSTANTIAL ENTRIES AND OVERSIGHT TO PROPERLY LOAD THE APPROPRIATE ROUTING AND ALT RESTRS SO WHEN THESE TYPES OF RAPID-FIRE CHANGES OCCUR DURING AN APCH THAT CONSIST OF MANY WAYPOINTS AND XING RESTRS; THE CHANCES OF DATA ERROR IS ENORMOUS. IN MY OPINION; IT'S ONLY A MATTER OF TIME BEFORE 2 ACFT ARE IN THE SAME PLACE; AT THE SAME TIME; BECAUSE OF THIS TYPE OF ATC HANDLING.