FLC OF A B757 UNDERSHOT DSCNT ALT RESULTING IN NOT MEETING A XING RESTR. THE FLC'S INATTN TO THE ACFT AUTOPLT ACTION; OF WHICH THEY THOUGHT WAS PREPROGRAMMED TO DSND THE ACFT IN TIME TO MEET THE XING RESTR; CAUSED THE ACFT TO BE 2000 FT HIGH AT XING FIX.

Narrative

WHILE 70 NM E OF DEWITT; WE WERE GIVEN A CLRNC TO CROSS DEWITT AT FL310. THE MCA ALT WAS SET TO FL310 AND THE RESTR ENTERED INTO THE FMC. 5 NM FROM THE PROGRAMMED TOP OF DSCNT; 'DSND NOW' WAS ACTIVATED. THE THROTTLES CAME BACK AND THE DSCNT BEGAN. AFTER BEGINNING THE DSCNT; I BRIEFED THE UPCOMING APCH. I EXPECTED THE ACFT TO AUTOMATICALLY FLY THE RESTR LIKE IT IS SUPPOSED TO DO. THE CTR ASKED IF WE HAD A XING RESTR. WE LOOKED UP FROM OUR BRIEFING AND SAW THAT WE HAD CROSSED DEWITT AND WERE STILL DSNDING THROUGH FL330 AT ABOUT 1000 FPM. WE CHKED AND WE WERE STILL IN VNAV BUT THE THROTTLES WERE NOT AT IDLE. WE QUICKLY IDLED THE THROTTLES AND EXPEDITED DSCNT. CTR HAD TFC AT FL330. CONFUSED AT WHY THE ACFT DID NOT MAKE ITS RESTR; WE SET UP A SIMILAR SIT BUT THIS TIME THE ACFT MET THE RESTR. WE DON'T KNOW WHY THE ACFT DIDN'T MEET THE FIRST RESTR BUT WE CAN ONLY GUESS WE DID SOMETHING WRONG. PERHAPS THE EARLY PICKUP (XA15 HOME TIME) CONTRIBUTED. I NORMALLY GO TO SLEEP AROUND XB00 AT HOME AND DIDN'T EVEN COME CLOSE TO A FULL NIGHT'S SLEEP PRIOR TO THE INCIDENT.