Narrative:

I was plting the aircraft on a flight from lax to las. The route took us from daggett on the skebr 1 arrival. Center cleared us to cross misen at FL240. We were at FL290 at the time. I had programmed the 'box' for the arrival, but missed the fact that there was an 'a' preprogrammed into the route at misen, ie, misen 240A. So, about 6 mi from misen, we got asked to start down, so we did. We crossed misen at about FL260. Needless to say, a closer examination/preflight of the arrival would have solved the missed altitude restr request by ZLA. Supplemental information from acn 541161: I had a feeling that something was not right with the FMC setup, but was unable to figure out what it was. It seems to me that I have read about this problem in the past, but could not remember the specifics. I tried to look at the legs page to see the altitude that the box was showing at misen. I saw '240A' -- unfortunately, I didn't really understand the significance of the 'a.' I went off the ATIS and upon returning, ZLA asked if we had started down for misen yet. I said no and then noticed we were only 1.5 mi from misen. We started down and were handed off to the next sector. A misunderstanding of the preprogrammed altitude and the effect of the 'a' next to it led us to believe the descent page would keep us where we needed to be. A better understanding of the system from this experience should preclude it from happening again at least to this crew.

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Original NASA ASRS Text

Title: A B737-300 CREW, DSNDING INTO LAS, UNDERSHOT PUBLISHED STAR ATC ASSIGNED ALT.

Narrative: I WAS PLTING THE ACFT ON A FLT FROM LAX TO LAS. THE RTE TOOK US FROM DAGGETT ON THE SKEBR 1 ARR. CTR CLRED US TO CROSS MISEN AT FL240. WE WERE AT FL290 AT THE TIME. I HAD PROGRAMMED THE 'BOX' FOR THE ARR, BUT MISSED THE FACT THAT THERE WAS AN 'A' PREPROGRAMMED INTO THE RTE AT MISEN, IE, MISEN 240A. SO, ABOUT 6 MI FROM MISEN, WE GOT ASKED TO START DOWN, SO WE DID. WE CROSSED MISEN AT ABOUT FL260. NEEDLESS TO SAY, A CLOSER EXAMINATION/PREFLT OF THE ARR WOULD HAVE SOLVED THE MISSED ALT RESTR REQUEST BY ZLA. SUPPLEMENTAL INFO FROM ACN 541161: I HAD A FEELING THAT SOMETHING WAS NOT RIGHT WITH THE FMC SETUP, BUT WAS UNABLE TO FIGURE OUT WHAT IT WAS. IT SEEMS TO ME THAT I HAVE READ ABOUT THIS PROB IN THE PAST, BUT COULD NOT REMEMBER THE SPECIFICS. I TRIED TO LOOK AT THE LEGS PAGE TO SEE THE ALT THAT THE BOX WAS SHOWING AT MISEN. I SAW '240A' -- UNFORTUNATELY, I DIDN'T REALLY UNDERSTAND THE SIGNIFICANCE OF THE 'A.' I WENT OFF THE ATIS AND UPON RETURNING, ZLA ASKED IF WE HAD STARTED DOWN FOR MISEN YET. I SAID NO AND THEN NOTICED WE WERE ONLY 1.5 MI FROM MISEN. WE STARTED DOWN AND WERE HANDED OFF TO THE NEXT SECTOR. A MISUNDERSTANDING OF THE PREPROGRAMMED ALT AND THE EFFECT OF THE 'A' NEXT TO IT LED US TO BELIEVE THE DSCNT PAGE WOULD KEEP US WHERE WE NEEDED TO BE. A BETTER UNDERSTANDING OF THE SYS FROM THIS EXPERIENCE SHOULD PRECLUDE IT FROM HAPPENING AGAIN AT LEAST TO THIS CREW.

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.