AN ACR B767-200 FLC DSNDED BELOW THEIR ASSIGNED ALT WHILE ATTEMPTING TO COMPLY WITH ATC SPD CHANGES AND LOADING NEW WAYPOINTS IN THE FMC.

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WE WERE FLYING THE NEW PDZ 3 ARR AS FILED ON THE FLT PLAN TO LAX. AN INITIAL DSCNT CLRNC VIA THE PDZ 3 WAS RECEIVED AND CANCELED AS WE STEPPED DOWN TO 17000 FT; SLOWED TO 260 KTS AS REQUESTED; AND THEN DIRECTED TO ACCELERATE BACK TO 280 KTS BY THE NEXT CTLR. AS A RESULT; WE WERE NEARLY PAST TRTLE BEFORE RECEIVING CLRNC TO CONTINUE OUR DSCNT. ALTHOUGH WE HAD BEEN EXPECTING AN APCH TO RWY 25R WITH A SIDESTEP TO RWY 25L AS INDICATED ON THE ATIS; ANOTHER ACFT HAD HIS CLRNC CHANGED TO THE MUDDE 4 AFTER ARRIVING ON THE PDZ 3; SO THE SPECIFIC APCH WAS STILL IN DOUBT. THE BOTTOM LINE IS WE DIDN'T RECEIVE FINAL CONFIRMATION OF A RWY 25R APCH UNTIL VERY CLOSE TO PDZ. AS LAST MIN CHANGES OF STARS AND RWYS ARE QUITE COMMON AT LAX; WE DID HAVE THE MITTS 1 SETUP IN RTE 2 OF THE FMS. IN ADDITION; I HAD THE CIVIT 4; MITTS 1; MUDDE 4 AND PDZ 3 CHARTS OUT AND AVAILABLE; AS ATC ALSO ROUTINELY DELAYS STAR ASSIGNMENTS TO WITHIN 10 OR 15 NM OF TRANSITION NAVAIDS TO SEQUENCE ARR TFC. I ALSO HAD ILS APCH CHARTS OUT FOR RWYS 25L/R AND RWY 24R; AS LAST MIN CHANGES TO ANY ONE OF THE 3 ARE ALWAYS POSSIBLE AT LAX. ASSUMING AN APCH TO RWY 25R; HOWEVER; WE DID PROGRAM THE FMS FOR THE PDZ 3 PRIOR TO PDZ WITH THE FOLLOWING RESULTS: 1) NO TRANSITION FIX BEYOND TEJAY; 2) A DISCONNECT BTWN TEJAY AND CASIT; AND 3) NO INDICATION OF TRESE. WITH THIS AS OUR FIRST INDICATION OF A POTENTIAL PROB; A PLAN TO FLY THE PDZ 277 DEG RADIAL OUTBOUND WAS BRIEFED. IMMEDIATELY AFTER RECEIVING CONFIRMATION OF A CLRNC TO RWY 25R; THE PNF MADE SEVERAL UNSUCCESSFUL ATTEMPTS TO ENTER TRESE IN THE FMS. AT THAT POINT WE ELECTED TO GO RAW DATA; TRANSITIONED TO THE PDZ 277 DEG RADIAL; AND DSNDED SO AS TO CROSS TEJAY AT OR ABOVE 12000 FT. WHEN IT BECAME APPARENT WE WOULD CROSS TEJAY CONSIDERABLY ABOVE 12000 FT; THE ALT WINDOW WAS RESET TO 10000 FT. ALTHOUGH APCH NEVER SPECIFICALLY DIRECTED A SPD REDUCTION FROM 280 KTS; I SLOWED TO 250 KTS ON MY OWN SOON AFTER PDZ. SOCAL REQUESTED A REDUCTION TO 210 KTS ALMOST IMMEDIATELY THEREAFTER. DUE TO OUR SPD; ALT AND THE RELATIVELY SHORT DISTANCE BTWN PDZ AND ARNES; I THEN ELECTED TO HAND FLY THE ACFT TO INSURE AN AGGRESSIVE INTERCEPT OF THE PDZ 277 DEG RADIAL AND DSND TO 10000 FT. I REGRETTED THAT DECISION ALMOST IMMEDIATELY; AS TRANSITION FROM FMS TO RAW DATA; GIVEN COMPLICATING FACTORS OF HIGH AIRSPD AND ALT; AND SUCH A SHORT DISTANCE TO THE TURN-IN POINT; WAS MADE CONSIDERABLY MORE DIFFICULT. A DEG OF TASK SATURATION FOLLOWED. WITH BOTH AUTOPLTS IN THE RAW DATA MODES; THE RWY 25R APCH UNEXTENDED; AND THE PNF IN THE PROCESS OF SATING THE FLT ATTENDANTS FOR THE LNDG; I REALIZED I WAS INADVERTENTLY DSNDING THROUGH 10000 FT. ARRESTING THE DSCNT BY 9700 FT; I FOCUSED ON GETTING THE ACFT BACK TO 10000 FT AS SMOOTHLY AS POSSIBLE. ALTHOUGH APCH MENTIONED NO ALTDEV; THE PNF'S ALTIMETER WAS 50 FT LOWER THAN MINE AND THE ALT WARNING HORN AND LIGHT DID GO OFF. ABOUT THIS TIME WE WERE SWITCHED TO A SECOND CTLR WHO GAVE US A FINAL CLRNC FOR AN ILS TO RWY 25R; SIDESTEP TO RWY 25L; AND ASKED FOR A SPD REDUCTION TO 180 KTS. AFTER LESS THAN A BAR-WIDTH OVERSHOOT OF THE LOC; THE REMAINDER OF THE APCH WAS UNEVENTFUL. PROBABLE CAUSES: 1) LAST MIN CLRNC FOR A NEW ARR PROC THAT HAD NOT BEEN PROGRAMMED IN THE FMS DATABASE. 2) LACK OF SUFFICIENT APCH CHART STUDY DUE TO UNCERTAINTY AS TO WHICH APCH WOULD ACTUALLY BE FLOWN. 3) POOR TRANSITION FROM FMS TO RAW DATA PROCS DUE TO HIGHER THAN NORMAL PROFILE ALTS AND AIRSPDS; AND A DEG OF TASK SATURATION ON THE PART OF BOTH PLTS. 4) HIGH SPD IN RELATION TO THE SHORT DISTANCE BTWN PDZ AND THE FINAL APCH GATES. 5) DECISION TO HAND FLY THE ACFT PRECIPITATED AN ABNORMALLY HIGH WORKLOAD FOR BOTH THE PNF AND THE PF; AS WELL AS AN ACCOMPANYING LOSS OF ALT PROTECTION. 6) ATC'S POSSIBLE USE OF THE PDZ 3 AS AN 'OVERFLOW' APCH THAT LEAVES THE ACTUAL RWY IN DOUBT UNTIL THE LAST MIN. 7) A SEEMING RELUCTANCE ON ATC'S PART TO USE HOLDING AS A ROUTINE TFC SEQUENCING TECHNIQUE AT LAX.