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.