Narrative:

Flight from day to lax. We were approaching lax with a clearance from ZLA to maintain a speed of 270 KTS and descend to FL240. Next we were given a left turn to heading 210 degrees and clearance to intercept the lax 25L localizer, and a speed of 250 KTS. After we began the turn, we were reclred, 'direct civet, cleared profile descent,' whereupon I selected 8000' as the altitude cleared to on the altitude selector. The autoplt was on, automatic throttles engaged, civet altitude restrictions 14000-18000' displayed on legs page of FMC's. Then ATC requested an expedited descent through FL200 and I selected speed descent mode on the descent page of the FMC and a speed of 250 KTS, which no longer affords altitude protection for restrictions on the profile descent. We continued descending until reaching approximately 12100' over norton AFB when ATC issued a climb to 14000'. Realizing we had descended too early, we immediately climbed to 14000', crossing civet at that altitude, then I reselected path mode on the descent page of the FMC's, restoring the altitude restrictions to the vertical navigation mode of the FMC's. We landed at lax west/O further incident. I failed to realize that the altitude restrictions are not in effect during a speed mode descent. This deviation could have been avoided had I set 14000' in the altitude selector window rather than the lowest altitude on the civet SID, as is now recommended by our company altitude awareness program. Setting 14000' is the recommended procedure. Supplemental information from acn 163660: I made a quick visit to the restroom while the captain, who was actively flying the aircraft, reprogrammed the FMC for the profile descent. Upon my return to the cockpit we were passing through FL190 and the captain informed me that the FMC was programmed and set up for the profile descent. I noticed that he selected 8000' in the altitude selector window, which is the bottom at on the STAR. After accomplishing the approach checklist, I focused my attention outside the cockpit for possible VFR traffic conflict. Approximately 8-10 mi east of civet, we were instructed by lax approach to climb and maintain 14000'. From about 12500' we quickly climbed back to 1400' and crossed civet at that altitude. After a quick discussion with the captain we realized that in my absence he had selected the speed mode instead of the path mode on the FMC. In the speed mode, the FMC follows the programmed speeds and disregards all altitude restrictions, which caused us to descend too early. We could have avoided the altitude deviation if I'd rechked the FMC after he programmed it, and 1 of us stayed on the INS throughout the entire descent. Don't blindly trust computers!

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Original NASA ASRS Text

Title: ADVTECH ACR MLG ALT OVERSHOT IN DESCENT INTO LAX.

Narrative: FLT FROM DAY TO LAX. WE WERE APCHING LAX WITH A CLRNC FROM ZLA TO MAINTAIN A SPD OF 270 KTS AND DSND TO FL240. NEXT WE WERE GIVEN A LEFT TURN TO HDG 210 DEGS AND CLRNC TO INTERCEPT THE LAX 25L LOC, AND A SPD OF 250 KTS. AFTER WE BEGAN THE TURN, WE WERE RECLRED, 'DIRECT CIVET, CLRED PROFILE DSNT,' WHEREUPON I SELECTED 8000' AS THE ALT CLRED TO ON THE ALT SELECTOR. THE AUTOPLT WAS ON, AUTO THROTTLES ENGAGED, CIVET ALT RESTRICTIONS 14000-18000' DISPLAYED ON LEGS PAGE OF FMC'S. THEN ATC REQUESTED AN EXPEDITED DSNT THROUGH FL200 AND I SELECTED SPD DSNT MODE ON THE DSNT PAGE OF THE FMC AND A SPD OF 250 KTS, WHICH NO LONGER AFFORDS ALT PROTECTION FOR RESTRICTIONS ON THE PROFILE DSNT. WE CONTINUED DSNDING UNTIL REACHING APPROX 12100' OVER NORTON AFB WHEN ATC ISSUED A CLB TO 14000'. REALIZING WE HAD DSNDED TOO EARLY, WE IMMEDIATELY CLBED TO 14000', XING CIVET AT THAT ALT, THEN I RESELECTED PATH MODE ON THE DSNT PAGE OF THE FMC'S, RESTORING THE ALT RESTRICTIONS TO THE VERT NAV MODE OF THE FMC'S. WE LANDED AT LAX W/O FURTHER INCIDENT. I FAILED TO REALIZE THAT THE ALT RESTRICTIONS ARE NOT IN EFFECT DURING A SPD MODE DSNT. THIS DEVIATION COULD HAVE BEEN AVOIDED HAD I SET 14000' IN THE ALT SELECTOR WINDOW RATHER THAN THE LOWEST ALT ON THE CIVET SID, AS IS NOW RECOMMENDED BY OUR COMPANY ALT AWARENESS PROGRAM. SETTING 14000' IS THE RECOMMENDED PROC. SUPPLEMENTAL INFO FROM ACN 163660: I MADE A QUICK VISIT TO THE RESTROOM WHILE THE CAPT, WHO WAS ACTIVELY FLYING THE ACFT, REPROGRAMMED THE FMC FOR THE PROFILE DSNT. UPON MY RETURN TO THE COCKPIT WE WERE PASSING THROUGH FL190 AND THE CAPT INFORMED ME THAT THE FMC WAS PROGRAMMED AND SET UP FOR THE PROFILE DSNT. I NOTICED THAT HE SELECTED 8000' IN THE ALT SELECTOR WINDOW, WHICH IS THE BOTTOM AT ON THE STAR. AFTER ACCOMPLISHING THE APCH CHKLIST, I FOCUSED MY ATTN OUTSIDE THE COCKPIT FOR POSSIBLE VFR TFC CONFLICT. APPROX 8-10 MI E OF CIVET, WE WERE INSTRUCTED BY LAX APCH TO CLB AND MAINTAIN 14000'. FROM ABOUT 12500' WE QUICKLY CLBED BACK TO 1400' AND CROSSED CIVET AT THAT ALT. AFTER A QUICK DISCUSSION WITH THE CAPT WE REALIZED THAT IN MY ABSENCE HE HAD SELECTED THE SPD MODE INSTEAD OF THE PATH MODE ON THE FMC. IN THE SPD MODE, THE FMC FOLLOWS THE PROGRAMMED SPDS AND DISREGARDS ALL ALT RESTRICTIONS, WHICH CAUSED US TO DSND TOO EARLY. WE COULD HAVE AVOIDED THE ALT DEVIATION IF I'D RECHKED THE FMC AFTER HE PROGRAMMED IT, AND 1 OF US STAYED ON THE INS THROUGHOUT THE ENTIRE DSNT. DON'T BLINDLY TRUST COMPUTERS!

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.