MLG ACFT CLRED FOR XING RESTR; FO PROGRAMS FMC FOR VNAV DSCNT. CAPT LEAVES COCKPIT. ACFT FAILS TO DSND AND XING RESTR MISSED. FMC PROGRAMMING ERROR.

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WE WERE GIVEN CLRNC TO CROSS COFAX AT FL250 (WE WERE AT FL310 AT THE TIME). I WAS THE PF. I PROGRAMMED COFAX AT FL250 INTO THE FMC AND VERIFIED THAT I WAS ON VNAV. A FEW MINS LATER THE CAPT STATED HE WAS GOING TO GO TO THE BATHROOM AND GOT UP. JUST AS HE STARTED OUT THE COCKPIT DOOR; I LOOKED DOWN AND REALIZED WE WERE 1 MI FROM COFAX. FOR WHATEVER REASON; THE FMC/AUTOPLT DID NOT START THE DSCNT (STILL WAS ANNUNCIATING VNAV) AND I FAILED TO BACK UP THE MACHINE WITH BASIC AIRMANSHIP. I IMMEDIATELY STARTED DOWN MANUALLY; NOTIFIED ATC THAT I WAS STARTING OUT OF FL310 AT THIS TIME AND APOLOGIZED FOR MISSING THE XING RESTR. HE SAID NO PROB; DSND AND MAINTAIN FL250; AND CONTACT NEW YORK ON 132.2. UPON CONTACT; THEY CLRED US FOR DIRECT LRP AND A FURTHER DSCNT WITH NO XING RESTRS. CONTRIBUTING FACTORS: 1) WE HAD EARLIER BEEN SLOWED DOWN BY ATC TO 250 KTS WHICH I PROGRAMMED INTO THE FMC. THEN WHEN WE WERE GIVEN NORMAL SPD; I COULDN'T GET IT OUT. CAPT SAID LOOK AT CRUISE DSCNT PAGE -- THERE IT WAS. I TOOK IT OUT; PUT BACK ECONOMY CRUISE DSCNT AND THOUGHT EVERYTHING WAS OK. I THINK; IN RETROSPECT; I TOOK IT OUT OF PLANNED DSCNT MODE BY DOING THAT. 2) MY NOT BACKING UP BEGINNING OF DSCNT POINT WITH MANUAL CALCULATIONS. NO EXCUSE EXCEPT PERHAPS FATIGUE. (THIS WAS A RED-EYE FLT FROM SFO-PHL THAT WAS DELAYED IN SFO DUE TO LATE ARR OF ACFT. DEPARTED SFO AT XX38 AM LCL TIME.) 3) FAILURE OF CAPT TO BACK ME UP. DUE ALSO; I THINK; TO BACK SIDE OF THE CLOCK FLYING. ACTION: VOW NOT TO BE LULLED INTO A SENSE OF COMPLACENCY WITH FMC. SOMETHING I NORMALLY DON'T DO. BUT FOR WHATEVER REASON; IT HAPPENED THIS TIME.