A B737 CREW MISSED A LAX CIVET ALT CONSTRAINT AFTER BEING DISTRACTED BY FMC WORK FOR A LATE RWY ASSIGNMENT; SUN POSITION AND LOSS OF SITUATIONAL AWARENESS.

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THIS FLT WAS FILED FOR THE CIVET 5 ARR; HEC TRANSITION. WE WERE ASSIGNED FL190 AND TOLD TO CONTACT SOCAL APCH JUST PRIOR TO GRAMM INTXN. WE CROSSED GRAMM AT 280 KTS PER THE ARR. UPON CHANGING TO APCH CTL FREQ; APCH WAS BUSY TALKING TO OTHER TFC. WHEN WE CHKED IN; WE WERE TOLD; 'DSND VIA THE CIVET ARR FOR RWY 24R.' WE WERE AT OR PAST RUSTT INTXN WHEN THIS WAS ASSIGNED. THE FO WAS FLYING. HE STARTED THE DSCNT WHILE I WENT 'HEADS DOWN' TO RELOAD THE ASSIGNED ARR AND RWY INTO THE FMC AND VERIFY THE WAYPOINTS. AT OR NEAR CIVET; WE WERE ASSIGNED 250 KTS. AS THE PNF; I BECAME TOO INVOLVED IN VERIFYING THE WAYPOINTS ALONG THE ARR AND SETTING THE RADIOS TO THE ILS FREQ AND LOST SITUATIONAL AWARENESS. ADDING TO THE DIFFICULTIES; THE SUN WAS LOW ON THE HORIZON AND DIRECTLY IN OUR EYES. I LOOKED UP TO REGAIN MY SITUATIONAL AWARENESS AND DISCOVERED WE WERE NOT GOING TO MAKE THE XING RESTR AT DECOR (9800-9000 FT). I TOLD THE FO TO 'GET DOWN' BUT IT WAS TOO LATE. THE SPD BRAKES WERE EXTENDED AND WE WERE AT 250 KTS. WE ENDED UP XING DECOR ABOUT 10300 FT. APCH CTL FREQ WAS TOO CONGESTED FOR US TO COM WITH SOCAL OUR MISSING THE XING RESTR. WE CONTINUED THE ARR; APCH AND LNDG WITHOUT FURTHER INCIDENT. A LATE CHANGE IN THE ARR RWY; A LATE DSCNT; RADIO CONGESTION AND THE SUN CAUSED A LOSS OF SITUATIONAL AWARENESS AND THIS ALTDEV.