LGT ACR ACFT IS CLRED FROM ONE STAR TO ANOTHER; THEN WITHIN 10 MI OF LNDG; IS AGAIN RECLRED FOR ANOTHER APCH; LNDG ON A PARALLEL RWY. PLT CONCERNED WITH INCREASED COCKPIT WORKLOAD CAUSED BY RWY CHANGES AND THE ACCOMPANYING AUTOMATION INPUTS.

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ENRTE FROM EWR TO LAX; ZLA CHANGED ENRTE CLRNC 'AFTER PGS VORTAC; J128 TO LAX.' SET UP IN CDU; THE CLRNC CHANGED BACK TO 'DIRECT CIVET; CIVET ARR.' PRIOR TO CIVET; RECLRED FOR 'MITTS ARR; CLRED DIRECT SNRKL INTXN; SNRKL AT OR ABOVE 12000 FT.' HAD TO TYPE IN SNRKL TO CDU; AS IT DID NOT COME UP WHEN SELECTING MITTS ARR; ALSO HAD TO PUT IN ALT RESTR AT SNRKL. NOT SURE IF THEY CLRED US TO DSND VIA THE MITTS ARR (15 MI FROM MITTS) AND FINALLY GOT CONFIRMATION TO DSND VIA ARR 3 MI FROM MITTS AND WE HAD TO USE SPD BRAKES AND VERT SPD MODE TO GET DOWN IN TIME. CLRED INITIALLY FOR ILS RWY 24R; SET IN CDU AND ILS FREQS. FO HAD BEEN ADVISED TO HAVE APCH PLATE READY FOR RWY 24L. REBRIEFED ILS FOR RWY 24R VERSUS RWY 25L; THE INITIAL RWY FOR THE ORIGINAL CIVET ARR. ABOUT 6 MI FROM LAX; APCH ASKED IF WE HAD THE FIELD IN SIGHT. WE REPLIED 'AFFIRMATIVE' AND HE RECLRED US FOR THE 'VISUAL APCH TO RWY 24L (VERSUS RWY 24R) AND TO CONTACT LAX TWR.' WHEN FO SET IN TWR FREQ; HE SET 135.9 VERSUS 133.9 AND DID NOT CALL THEM ASAP. AS THE CAPT HAD ASKED FOR THE ILS FREQ TO BE SET IN FOR RWY 24L; FO COULD NOT CHANGE THE ILS FREQ IN THE HEAD; AS 'APCH' MODE HAD ALREADY BEEN SELECTED FOR RWY 24R. FO RESET RWY 24L IN CDU AND WAS THEN ABLE TO DIAL IN ILS FREQ FOR RWY 24L. CAPT WAS HAND FLYING AFTER THE CLRNC WAS CHANGED FROM RWY 24R TO RWY 24L; USING RAW DATA ON ADI AND HSI. AFTER FO RETUNED ILS TO RWY 24L; NEITHER PLT HAD FLT DIRECTOR DATA. CAPT CONTINUED TO HAND FLY THE APCH UNDER VISUAL CONDITIONS; AND AT 700-800 FT AGL; WE SPOTTED AN ACFT ON RWY 24L. THE FO ASKED THE CAPT IF WE WERE CLRED TO LAND AND THE CAPT REPLIED AFFIRMATIVE (HEARING CLRED FOR THE APCH; AND SWITCH TO TWR; AND WITH ALL OF THE OTHER DISTRS; FOCUSED ON THE APCH AND THOUGHT WE HAD BEEN CLRED TO LAND). ON THE LNDG ROLL; WE COMMENTED ON HOW QUIET IT WAS; AND AFTER CLRING THE RWY AND SWITCHING TO GND CTL; THE FO NOTICED 135.9 HAD BEEN SET VERSUS 133.9. GND GAVE US TAXI INSTRUCTIONS TO THE GATE WITH NO COMMENTS. WE CALLED THE TWR SUPVR TO CLR UP THE SIT. HIS REACTION TO THE STATEMENT WAS THAT IT WAS A COORD PROB BTWN TRACON AND THE TWR; ADDING THAT HE DID NOT CONTACT US UNTIL 2 NM FROM THE RWY; AS IF HE WAS UNAWARE THAT WE WERE NOT ON FREQ. WE DID NOT ADMIT TO HIM THAT WE HAD NOT HEARD THE LNDG CLRNC; AND HE MADE NO FURTHER COMMENTS ON THE MATTER. WE BRIEFLY DISCUSSED WHY LAX ALWAYS SEEMS TO GIVE MULTIPLE APCH AND RWY CHANGES; AND HE ANSWERED THAT THEY LAND ACFT ON THE INSIDE RWYS -- OCCASIONALLY; TO HELP OUT WITH SPACING OF ACFT FOR APCH CTL. 2 EXPERIENCED CREW MEMBERS LANDED WITHOUT A CLRNC FROM THE TWR DUE TO ALL OF THE WORKLOAD AND DISTRS LISTED ABOVE. AUTOMATION IS GOOD ONLY TO A POINT. WHEN IT BECOMES CUMBERSOME; THEN RAW DATA AND BASIC AVIATION SKILLS TAKE PREFERENCE. RECOMMENDATIONS: KEEP APCH AND RWY CHANGES TO A MINIMUM. ENSURE AT 1000 FT AGL (500 FT AGL MINIMUM) THAT LNDG CLRNC HAS BEEN RECEIVED. IT MIGHT NOT BE A BAD IDEA TO PUT IT ON THE LNDG CHKLIST; AS BASIC AS THAT MAY SEEM (IN THIS CASE IT MAY HAVE HELPED). AVOID CDU CHANGES AT LOW ALTS THAT COULD PROVE TO BE DISTRACTING.