Narrative:

Cleared civet 4 arrival, descent profile, cross bremr at or above 12000 ft. 3 mi prior to bremr, aircraft passed 300-400 ft below 12000 ft. Approach controller caught the altitude error at same time crew noticed. Immediately climbed aircraft to 12000 ft and crossed bremr at 12000 ft. Crew was flying a VNAV descent and apparently VNAV descent was not captured. Captain was reading approach check at time of altitude deviation and looking at approach chart. Prior to approach, descent was briefed and all altitude restrs discussed. Civet 4 arrival selected in FMS and all altitudes confirmed. This is a very high workload arrival. It requires total crew concentration and any distraction can result in an altitude deviation, especially if 1 crew member is inexperienced. I don't know the reason for all the altitude restrs, but it's very difficult to fly and should be modified with an enhanced GS that can be captured 40-50 mi out. Supplemental information from acn 485462: approaching 12000 ft, it was apparent that the aircraft was not going to level off at 12000 ft. At that point, I intervened using the vertical speed mode to stop the descent. Seconds later, I determined that the vertical speed mode wasn't gong to be aggressive enough to stop the descent in a timely manner, so I disengaged the autoplt, stopped the descent and climbed back to 12000 ft. The controller noted no conflicts and gave us a frequency change.

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Original NASA ASRS Text

Title: A B757-200 FLC DSNDS 400 FT PAST THEIR ASSIGNED ALT ON THE CIVET 4 ARR 40 MI NE OF LAX, CA.

Narrative: CLRED CIVET 4 ARR, DSCNT PROFILE, CROSS BREMR AT OR ABOVE 12000 FT. 3 MI PRIOR TO BREMR, ACFT PASSED 300-400 FT BELOW 12000 FT. APCH CTLR CAUGHT THE ALT ERROR AT SAME TIME CREW NOTICED. IMMEDIATELY CLBED ACFT TO 12000 FT AND CROSSED BREMR AT 12000 FT. CREW WAS FLYING A VNAV DSCNT AND APPARENTLY VNAV DSCNT WAS NOT CAPTURED. CAPT WAS READING APCH CHK AT TIME OF ALTDEV AND LOOKING AT APCH CHART. PRIOR TO APCH, DSCNT WAS BRIEFED AND ALL ALT RESTRS DISCUSSED. CIVET 4 ARR SELECTED IN FMS AND ALL ALTS CONFIRMED. THIS IS A VERY HIGH WORKLOAD ARR. IT REQUIRES TOTAL CREW CONCENTRATION AND ANY DISTR CAN RESULT IN AN ALTDEV, ESPECIALLY IF 1 CREW MEMBER IS INEXPERIENCED. I DON'T KNOW THE REASON FOR ALL THE ALT RESTRS, BUT IT'S VERY DIFFICULT TO FLY AND SHOULD BE MODIFIED WITH AN ENHANCED GS THAT CAN BE CAPTURED 40-50 MI OUT. SUPPLEMENTAL INFO FROM ACN 485462: APCHING 12000 FT, IT WAS APPARENT THAT THE ACFT WAS NOT GOING TO LEVEL OFF AT 12000 FT. AT THAT POINT, I INTERVENED USING THE VERT SPD MODE TO STOP THE DSCNT. SECONDS LATER, I DETERMINED THAT THE VERT SPD MODE WASN'T GONG TO BE AGGRESSIVE ENOUGH TO STOP THE DSCNT IN A TIMELY MANNER, SO I DISENGAGED THE AUTOPLT, STOPPED THE DSCNT AND CLBED BACK TO 12000 FT. THE CTLR NOTED NO CONFLICTS AND GAVE US A FREQ CHANGE.

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.