FLC OF A B767-200 OVERSHOT DSCNT XING ALT DURING A STAR ARR DUE TO THE CAPT SELECTING THE WRONG FMC MODE CTL.

Narrative

FLT WAS ON BOARD A B767-200 AND WAS CLRED TO DSND VIA THE CIVIT ARR TO LAX. THE CAPT WAS PF. I WAS BUSY WITH COMPANY DUTIES; EG; CALLING COMPANY OPS FOR GATE ASSIGNMENT; ALERTING THE CABIN CREW TO PREPARE FOR ARR. ORIGINALLY; THE CAPT WAS DSNDING USING VNAV AND IT HAD BEEN PERFORMING SATISFACTORY. I RECALL BEING GIVEN AN AIRSPD ASSIGNMENT OF '300 KTS OR GREATER.' I NOTICED THAT HIS AIRSPD BUG DROPPED TO 250 KTS (DUE TO VNAV PREPARING TO DSND BELOW 10000 FT OR POSSIBLY BEING PROGRAMMED TO CROSS BREMR AT 250 KTS AND GREATER THAN OR EQUAL TO 12000 FT. WHEN PAUSING BTWN TASKS; I NOTICED WE WERE 8 NM FROM BREMR AND WERE DSNDING THROUGH APPROX 13300 FT. I NOTICED THAT THE CAPT WAS TRYING TO OVERRIDE THE SPD REDUCTION BY SELECTING 300 KTS ON THE MCP. I ASSUMED THAT HE WAS USING VNAV SPD INTERVENTION. THEREFORE; I ASSUMED THAT THE AUTOPLT WAS GOING TO LEVEL OFF AT 12000 FT TO MAKE THE XING AT BREMR AT OR ABOVE 12000 FT (D40 LAX). AFTER MY INCORRECT ASSUMPTION; I ADVISED THE CAPT THAT WE WERE GOING THROUGH 12000 FT -- A RECHK OF THE SID CONFIRMED THE REQUIREMENT. WE WERE 4 MI FROM BREMR AND DSNDING THROUGH 11700 FT! THE CAPT AND I WERE BOTH PERPLEXED AS TO WHY THE AUTOPLT DID NOT LEVEL OFF AT 12000 FT. IT TURNS OUT THAT THE AUTOPLT WAS DOING EXACTLY AS IT WAS TOLD TO DO. THE CAPT HAD SELECTED FLT LEVEL CHANGE; 10000 FT WAS THE SELECTED ALT -- THE LOWEST ALT ON THE ARR. THE CAPT WAS FIXATING ON HIS AIRSPD; I WAS DISTR BY MISC DUTIES AND FAILED TO XCHK THE AUTOFLT MODE IN USE.