ACR LGT NON ADHERENCE TO ATC CLRNC. FLT CREW FAILED TO START DESCENT AFTER CLRNC READBACK.

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F/O'S 6TH TRIP ON THE LGT; SINCE COMPLETING TRAINING APPROX 1 MONTH EARLIER. CAPT HAD ALSO COMPLETED TRAINING RECENTLY. DETAILS: I WAS THE F/O ON THE FLT. I WAS OPERATING THE ACFT ON AUTOPLT AT THE TIME. THE CAPT WAS MAKING THE REQUIRED IN-RANGE CALL TO WASHINGTON NATIONAL OPERATIONS AT THE TIME. I HAD JUST COMPLETED A VNAV DESCENT TO FL270 WHEN WE WERE GIVEN A VECTOR HEADING FOLLOWED SHORTLY BY A CLRNC TO DESCEND TO FL240. SINCE THE CAPT WAS ON THE OTHER RADIO; I ACKNOWLEDGED THE CLRNC AND RESET THE ALT ALERT ON THE MODE SELECT PANEL (MCP) TO 24000. I THEN PULLED UP THE CRUISE PAGE ON THE FLT MGMNT COMPUTER (FMC) AND ENTERED FL240 INTO IT AND EXECUTED. IN MY MIND THE AUTOPLT/FLT DIRECTOR WAS STILL IN THE VNAV MODE AND IN THAT MODE; EXECUTING THE CRUISE ALT OF 240 SHOULD HAVE STARTED A DESCENT TO THAT ALT. THE ACFT HAD; HOWEVER; LEVELED OFF AT 270 AND TRANSFERRED INTO THE ALT HOLD MODE; WHICH WOULD NOT AUTOMATICALLY RESPOND TO THE SETTING AND EXECUTING OF A NEW; LOWER ALT IN THE FMC. MEANWHILE; THE CAPT HAD TUNED THE ATIS AND I HEARD FROM HIS COCKPIT SPEAKER THAT WASHINGTON NATIONAL HAD SWITCHED FROM THE NORTH OPERATION WE HAD EXPECTED AND HAD SET IN THE FMC TO A SOUTH OPERATION. I PULLED UP THE ARRIVAL PAGE ON THE FMC AND RESET THE COMPUTER TO THE NEW ARRIVAL WHILE THE CAPT WAS COPYING THE ATIS. IN THE MEANTIME; THE ACFT CONTINUED TO CRUISE AT FL270. SHORTLY THEREAFTER; WASHINGTON CENTER CALLED AND ASKED TO VERIFY OUR ALT; AT WHICH TIME I REALIZED WHAT HAD HAPPENED AND STARTED AN IMMEDIATE DESCENT. THERE WAS NO INDICATION FROM CENTER THAT THE FAILURE TO DESCEND HAD JEOPARDIZED SAFETY. OTHER FACTORS: IT WAS APPROX NINE HOURS SINCE I HAD REPORTED FOR DUTY AT ATLANTA THAT MORNING. I HAD BEEN CALLED FOR THE TRIP AT ABOUT XA30 AM FOR AN XD:17 DEP. SINCE I USUALLY ALLOW 3 HOURS BEFORE DEP AS A TIME TO LEAVE HOME; AND BEING UNSURE OF HOW MUCH GAS I HAD AND HOW OFTEN THE CREW BUS RAN FROM THE PARKING LOT IN THE EARLY MORNING; I DID NOT RETURN TO BED AFTER THE CALL. I HAD HAD ABOUT 3 HOURS SLEEP AT THE TIME OF THE PHONE CALL. IN TRAINING THEY EMPHASIZED THAT ONE PLT SHOULD FLY AND THE OTHER SHOULD PROGRAM THE FMC. I UNDERSTOOD AND BELIEVE THAT; HOWEVER; MOST OF THE EXPERIENCE PLTS I HAD BEEN FLYING WITH SINCE TRAINING SEEMED TO DO MOST OF THEIR OWN FMC MGMNT WHILE FLYING; ESPECIALLY IF I WAS OTHERWISE OCCUPIED ON THE OTHER RADIO. FOLLOWING THAT EXAMPLE; WHICH MAY WORK FOR AN EXPERIENCED LGT PLT BUT CERTAINLY NOT FOR ONE AT MY LEVEL; I FELL INTO THE TRAP THEY HAD WARNED ME ABOUT! I PUSHED THE BUTTONS; BUT I DID NOT CHECK THE RESPONSE TO THE INPUT BEFORE GOING ON TO SOMETHING ELSE. NO ONE WAS FLYING THE ACFT. IN THE FUTURE I WILL INITIATE ALL ALT CHANGES ON THE MCP (USING FL LEVEL CHANGE) WHEN THE OTHER PLT IS UNABLE TO ENTER DATA IN THE FMC; AND WILL CHECK THE BASIC ACFT INSTRUMENTS FOR A RESPONSE TO THE INPUTS I MAKE TO THIS COMPLEX; MULTI-FACETED AUTO FLT CTL SYSTEM.