B767-300 CREW HAD ALT AND TRACK DEV ON THE PDZ 4 ARR TO LAX.

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THE CAPT BRIEFED THE PDZ 4 ARR USING A VNAV DSCNT TO THE RWY 25L ILS. THE ALTS IN THE FMS AT EACH WAYPOINT WERE VERIFIED AND VNAV AND LNAV WERE ENGAGED. LAX AND SLI VOR WERE SELECTED IN THE CAPT'S FMC AS BRIEFED TO PREVENT MAP SHIFT. WE WERE CLRED FOR THE ARR AND 8000 FT WAS SET IN THE ALT WINDOW. SOMEWHERE IN THE VICINITY OF TEJAY THE CAPT CALLED FOR THE NO SMOKING LIGHT TO BE CYCLED. AFTER CYCLING THE LIGHT AND COMPLETING THE CHKLIST I WAS LOOKING FOR SOME INFO IN MY CHARTS; I DON'T REMEMBER HOW LONG OR WHAT I WAS LOOKING FOR THERE. DURING THIS TIME I REMEMBER THE CAPT SAYING SOMETHING ABOUT A MAP SHIFT AND BEING 800 FT HIGH AND THEN 200 FT LOW ON THE DSCNT PROFILE. AS I LOOKED BACK TO THE INSTS I FIRST NOTICED AND STATED TO THE CAPT THAT WE WERE AT 300 KIAS AND BELOW 10000 FT. I LOOKED BACK TO CHK THE ARR PLATE ALT AND ALTHOUGH I DON'T REMEMBER LOOKING AT THE HSI; I MUST HAVE; BECAUSE I NEXT SAID; WE NEED TO CLB; WE NEED TO BE AT 10000 FT; AT ARNES. THE CAPT BEGAN A PULL UP AND WE BOTTOMED OUT AT 6300 FT. THE CAPT SAID TO CONTACT APCH; BUT APCH CAME UP RIGHT THEN AND ASKED US TO VERIFY OUR ALT. I RESPONDED 6300 FT AND CLBING BACK UP TO 10000 FT AND THAT WE THOUGHT WE MIGHT HAVE HAD A MAP SHIFT AND ASKED IF HE COULD VERIFY OUR POS. HE REPLIED THAT WE WERE RIGHT OVER ARNES AND TO MAINTAIN PRESENT ALT AND THAT WE WERE CLRED FOR THE RWY 25L ILS APCH. AT THAT TIME THE HSI SHOWED US OVER ARNES AS WELL. I BELIEVE WE WERE SET UP PROPERLY FOR THE APCH. WHY THE ACFT DSNDED THROUGH A 10000 FT RESTR AT ARNES AND THROUGH THE MCP ALT OF 8000 FT I CANNOT EXPLAIN. MORE IMPORTANTLY IS WHY WE DIDN'T CATCH THE DEV SOONER? POSSIBLE CONTRIBUTING FACTORS: FATIGUE; IT WAS XA00 BODY TIME FOR ME. COMPLACENCY; MY CONFIDENCE IN VNAV WAS EXTREMELY HIGH. DISTR; FINISHING THE CHKLIST AT A CRITICAL TIME (LEVELOFF) TOOK ME OFF THE INSTS AND WHY I WENT LOOKING IN MY PUBS AFTER THE CHKLIST I AM NOT SURE. CLRLY MORE VIGILANCE TO BACKING UP THE PF IS REQUIRED IN THIS CRITICAL PHASE OF FLT. CONTINUAL ALT AWARENESS MUST BE IN THE SCAN REGARDLESS OF WHAT ADDITIONAL TASK WE MAY BE DOING UNTIL ALT CAPTURE AND HOLD ARE VERIFIED ON THE ADI. THIS MAY REQUIRE A PAUSE IN THE CHKLIST OR ANY OTHER TASK TO COME BACK TO THE ALTIMETER AND VSI EVERY FEW SECONDS UNTIL ALT HOLD IS CONFIRMED.