Narrative:

Executing civet 1 arrival to lax. Cleared for ILS runway 25L after passing arnes. Approach flown in autoflt, approach mode, localizer and GS captured. At about (?) 8000 ft, socal instructed 'turn right 10 degrees, intercept ILS runway 24R, descend to 7000 ft.' during the confusion of trying to locate the runway 24R approach plate, the turn was initiated but the autoplt flight director system still had localizer and GS capture, commanding a descent on the flight directors. While trying to set up to intercept runway 24L, descended below cleared altitude. Reclred to 6000 ft, and while still following autoplt flight director system commands for runway 25L, descended 300 ft below cleared altitude. In my belief, this incident occurred because of the delayed abandonment of autoflt commands upon receipt of a drastic clearance change and the resultant confusion and increased workload. Also, since we had been cleared for the runway 25L approach, our mindset was to track glide path, even though only cleared to intercept runway 24R. In any event, this event dictates the immediate abandonment of autoflt upon receipt of a clearance change this drastic. With the time involved in reprogramming the FMC and retuning the ILS, autoflt descent on the old approach must be checked. Supplemental information from acn 372334: the controller then cleared us to maintain 6000 ft MSL. At this time an aircraft was spotted approximately 5 mi ahead. While I was distraction with the airplane and the other pilot reprogramming the FMC and selecting the localizer frequency, the aircraft descended to approximately 5700 ft MSL when a climb was immediately initiated back to 6000 ft MSL. We were then cleared for the approach well above sgs for runway 24R ILS.

Related reports:

Google
 

Original NASA ASRS Text

Title: WDB ACFT CLRED FOR ILS APCH AFTER PASSING INTXN WAS RECLRED TO ANOTHER RWY AFTER FMS AND LOC ILS CAPTURE. FLC DISTR BY REPROGRAMMING FMC AND HDG CHANGE DSNDED THROUGH 7000 FT AND WAS RECLRED TO 6000 FT. THEN WHILE DSNDING TO 6000 FT AND BEING DISTR BY TFC, FLC DSNDED THROUGH 6000 FT.

Narrative: EXECUTING CIVET 1 ARR TO LAX. CLRED FOR ILS RWY 25L AFTER PASSING ARNES. APCH FLOWN IN AUTOFLT, APCH MODE, LOC AND GS CAPTURED. AT ABOUT (?) 8000 FT, SOCAL INSTRUCTED 'TURN R 10 DEGS, INTERCEPT ILS RWY 24R, DSND TO 7000 FT.' DURING THE CONFUSION OF TRYING TO LOCATE THE RWY 24R APCH PLATE, THE TURN WAS INITIATED BUT THE AUTOPLT FLT DIRECTOR SYS STILL HAD LOC AND GS CAPTURE, COMMANDING A DSCNT ON THE FLT DIRECTORS. WHILE TRYING TO SET UP TO INTERCEPT RWY 24L, DSNDED BELOW CLRED ALT. RECLRED TO 6000 FT, AND WHILE STILL FOLLOWING AUTOPLT FLT DIRECTOR SYS COMMANDS FOR RWY 25L, DSNDED 300 FT BELOW CLRED ALT. IN MY BELIEF, THIS INCIDENT OCCURRED BECAUSE OF THE DELAYED ABANDONMENT OF AUTOFLT COMMANDS UPON RECEIPT OF A DRASTIC CLRNC CHANGE AND THE RESULTANT CONFUSION AND INCREASED WORKLOAD. ALSO, SINCE WE HAD BEEN CLRED FOR THE RWY 25L APCH, OUR MINDSET WAS TO TRACK GLIDE PATH, EVEN THOUGH ONLY CLRED TO INTERCEPT RWY 24R. IN ANY EVENT, THIS EVENT DICTATES THE IMMEDIATE ABANDONMENT OF AUTOFLT UPON RECEIPT OF A CLRNC CHANGE THIS DRASTIC. WITH THE TIME INVOLVED IN REPROGRAMMING THE FMC AND RETUNING THE ILS, AUTOFLT DSCNT ON THE OLD APCH MUST BE CHKED. SUPPLEMENTAL INFO FROM ACN 372334: THE CTLR THEN CLRED US TO MAINTAIN 6000 FT MSL. AT THIS TIME AN ACFT WAS SPOTTED APPROX 5 MI AHEAD. WHILE I WAS DISTR WITH THE AIRPLANE AND THE OTHER PLT REPROGRAMMING THE FMC AND SELECTING THE LOC FREQ, THE ACFT DSNDED TO APPROX 5700 FT MSL WHEN A CLB WAS IMMEDIATELY INITIATED BACK TO 6000 FT MSL. WE WERE THEN CLRED FOR THE APCH WELL ABOVE SGS FOR RWY 24R ILS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.