ALTDEV.

Narrative

WE WERE APCHING LAX FROM SEAL BEACH VOR AT 7000 FT. LAX APCH WAS VECTORING US ACROSS THE APCH PATHS FOR RWYS 25L AND R FOR OUR APCH TO RWY 24R. THE APCH CTLR CLRED AN ACFT APCHING THE RWY 25 COMPLEX AT LAX OUT OF 6000 FT FOR 5000 FT; AND IMMEDIATELY CLRED US OUT OF 7000 FT FOR 6000 FT. THE CTLR POINTED OUT THE TFC; WHICH WE IMMEDIATELY ACQUIRED VISUALLY. OUR TCASII THEN BEGAN GIVING US A TA FOR THE ACFT APCHING RWYS 25. DISTRACTED BY THE AURAL TCASII WARNING; I FAILED TO NOTE THAT THE AUTOPLT'S LEVEL-OFF ALT OF 6000 FT HAD BECOME DISARMED. I IMMEDIATELY ENGAGED THE ALT HOLD DSNDING THROUGH 5800 FT. THE AUTOPLT LOST AN ADDITIONAL 100 FT; DSNDING TO 5700 FT BEFORE LEVELING OFF. THE CTLR NOTICED THIS; AND CLRED US DOWN TO 5500 FT. BECAUSE OF THE AURAL WARNING FROM THE TCASII; I ONLY HEARD 'CLRED TO 5000 FT' AND BEGAN A DSCNT TO 5000. SUPPLEMENTAL INFO FROM ACN 216615: WE BOTH CONFIRMED THE TFC AND I TOLD APCH WE HAD THE TFC IN SIGHT. I THEN TURNED MY HEAD TO THE L TO DEPRESS THE PA XMITTER BUTTON AND ASKED THE FLT ATTENDANTS TO BE SEATED FOR LNDG. WHEN I TURNED BACK; I NOTICED THE ACFT AT 5800 FT AND DSNDING. I CALLED OUT THE DSCNT TO THE FO WHO WAS FLYING. THE ACFT CONTINUED TO DSND SO I SAID STOP YOUR DSCNT. THE COPLT DID NOT REACT SO I TOOK CTL AND STOPPED DSCNT. CTLR THEN TOLD US TO DSND AND MAINTAIN 5500 AND SHORTLY AFTER CLRED US FOR THE APCH TO RWY 24R. THE COPLT WAS CONFUSED AND DID NOT REACT BECAUSE HE FROZE UP FOR A COUPLE OF SECONDS. HE IS NEW TO FAR 121 OPS; NEW TO THE AIRLINE INDUSTRY AND NEW TO MODERN TECHNOLOGY. I BELIEVE THIS ALL CONTRIBUTED TO OVERWHELMING HIM TO THE POINT THAT HE JUST FROZE UP. I'M NOT SURE HOW WE CORRECT THIS PROBLEM; BUT I CAN SAY THAT THE INITIAL APCH SEGMENT IS VERY BUSY IN A 2-MAN COCKPIT AND IT IS VERY DIFFICULT TO PERFORM YOUR OWN DUTIES; SUPERVISE A NEW PLT AND REACT QUICKLY ENOUGH TO PREVENT PROBLEMS; ALL AT THE SAME TIME.