Narrative:

En route towards dag VOR on loop 9 departure from lax at FL270 to join the creso 3 arrival into las at dag, first officer flying with B autoplt engaged, received instructions from ZLA (125.72) to cross jokur intersection at FL240. 24000 ft was selected in the altitude window of the mode control panel. Pitch mode was in altitude hold with vertical speed armed. The FMC computed top of descent point was still ahead of us, and the descent was delayed to get closer to an idle descent location. At this altitude, the airplane would make an idle descent of 3000 ft in a little over 1 min, so the descent would have started about 8 mi from the intersection. Clear day, watching for traffic, planning the runway arrival and turnoff, determining gate to be used, filling out the logbook, we were only about 2 mi from jokur when we realized we had missed the top of descent point. As first officer initiated descent, I called center knowing it was too late to get lt relief. They instructed me to contact center on 134.65, to whom I reported out of FL270 for FL240. They said they needed us at FL230 now, then to cross whigg at 12000 ft. I do not believe there was any traffic conflict. Contributing factors: only mins from top of descent, one of us should have been paying attention full time to the flight progress and specifically to the next task -- the top of descent point. Being in altitude hold with vertical speed armed, if VNAV had been engaged, the aircraft would have started down on its own at the proper time and this altitude deviation would not have occurred.

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Original NASA ASRS Text

Title: ALTDEV DURING DSCNT.

Narrative: ENRTE TOWARDS DAG VOR ON LOOP 9 DEP FROM LAX AT FL270 TO JOIN THE CRESO 3 ARR INTO LAS AT DAG, FO FLYING WITH B AUTOPLT ENGAGED, RECEIVED INSTRUCTIONS FROM ZLA (125.72) TO CROSS JOKUR INTXN AT FL240. 24000 FT WAS SELECTED IN THE ALT WINDOW OF THE MODE CTL PANEL. PITCH MODE WAS IN ALT HOLD WITH VERT SPD ARMED. THE FMC COMPUTED TOP OF DSCNT POINT WAS STILL AHEAD OF US, AND THE DSCNT WAS DELAYED TO GET CLOSER TO AN IDLE DSCNT LOCATION. AT THIS ALT, THE AIRPLANE WOULD MAKE AN IDLE DSCNT OF 3000 FT IN A LITTLE OVER 1 MIN, SO THE DSCNT WOULD HAVE STARTED ABOUT 8 MI FROM THE INTXN. CLR DAY, WATCHING FOR TFC, PLANNING THE RWY ARR AND TURNOFF, DETERMINING GATE TO BE USED, FILLING OUT THE LOGBOOK, WE WERE ONLY ABOUT 2 MI FROM JOKUR WHEN WE REALIZED WE HAD MISSED THE TOP OF DSCNT POINT. AS FO INITIATED DSCNT, I CALLED CTR KNOWING IT WAS TOO LATE TO GET LT RELIEF. THEY INSTRUCTED ME TO CONTACT CTR ON 134.65, TO WHOM I RPTED OUT OF FL270 FOR FL240. THEY SAID THEY NEEDED US AT FL230 NOW, THEN TO CROSS WHIGG AT 12000 FT. I DO NOT BELIEVE THERE WAS ANY TFC CONFLICT. CONTRIBUTING FACTORS: ONLY MINS FROM TOP OF DSCNT, ONE OF US SHOULD HAVE BEEN PAYING ATTN FULL TIME TO THE FLT PROGRESS AND SPECIFICALLY TO THE NEXT TASK -- THE TOP OF DSCNT POINT. BEING IN ALT HOLD WITH VERT SPD ARMED, IF VNAV HAD BEEN ENGAGED, THE ACFT WOULD HAVE STARTED DOWN ON ITS OWN AT THE PROPER TIME AND THIS ALTDEV WOULD NOT HAVE OCCURRED.

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.