AN ACR CREW SUSPECTS THAT A TRACK DEV AFTER XING THE LAST FIX ON THE IDALE RNAV 1 DEP FROM LAS; NV; IS CAUSED BY THEIR FAULTY FMS PROGRAMMING AND MONITORING.

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WE WERE CLRED FROM LAS TO ONT VIA THE IDALE 1 DEP; HEC TRANSITION; ZIGGY ARR; ONT. THE FMC WAS PROGRAMMED. THE ACFT WAS SET UP FOR THE RNAV DEP. AFTER TKOF; LNAV WAS ENGAGED AND THEN THE AUTOPLT WAS ENGAGED. THE ACFT PROCEEDED NORMALLY ON THE DEP. WE WERE CLRED DIRECT BOACH AND THE FMC WAS PROGRAMMED AS SUCH. THE ACFT PROCEEDED DIRECT BOACH AND CONTINUED TO FLY THE DEP NORMALLY; WHEN THE ACFT APCHED HECTOR (HEC VOR); THE LAST FIX OF THE SID; I NOTICED THAT THE COURSE NEEDLE WAS POINTING TO THE REAR OF THE ACFT AT HEC; THE ACFT BEGAN A TURN TO THE R APPEARING TO REVERSE COURSE. I RECOGNIZED THIS RIGHT AWAY; DIALED IN THE HEC VOR AND SET THE COURSE NEEDLE ON 211 DEGS; THE PROPER OUTBOUND COURSE AND THE BEGINNING OF THE ZIGGY ARR. THE ACFT TURNED 30 DEGS BEFORE THE CAPT DISENGAGED THE LNAV AND TURNED BACK ON COURSE. ATC QUERIED US ABOUT OUR COURSE DEV. WE ADVISED THEM THAT WE HAD A NAV EQUIP PROB AND THAT WE WERE RETURNING TO PROPER COURSE. I BELIEVE THAT OUR PROB WAS DUE TO IMPROPER PROGRAMMING OF THE FMC. THIS COULD HAVE BEEN DONE EITHER WHEN I ORIGINALLY SET UP THE FMC AT LAS OR WHEN THE CAPT PROGRAMMED AN ALT RESTRAINT DURING FLT. AFTER XING HEC; THE FMC SHOWED LAS AS THE DEST AND RWY 25R AS THE RWY. CONTRIBUTING FACTORS WERE: THE FMC WAS PROGRAMMED BY THE PREVIOUS CREW AND I MAY HAVE NOT CHKED IT THOROUGHLY. I DID NOT CHK THE FMC AFTER THE CAPT MADE HIS INPUT BECAUSE I WAS DOING AN ENG LOG AT THAT TIME. WE ALLOWED THE ACFT TO VEER OFF COURSE TOO LONG WITHOUT MAKING A CORRECTION. TO PREVENT THIS PROB FROM OCCURRING AGAIN; I: 1) WILL BE MORE VIGILANT WHEN SETTING UP THE FMC. I ALWAYS PLACE A 'DOT' UNDER THE FIX ON THE RELEASE WHEN I VERIFY THE FLT PLAN ON THE RELEASE WITH THAT ON THE FMC. (THIS PARTICULAR TIME I HAD DONE SO.) 2) AVOID DOING ENG LOGS ON SUCH SHORT FLTS. 3) PAY PARTICULAR ATTN TO THE FMC WHEN XING FIXES TO ENSURE THAT THE NEXT FIX IS THE PROPER FIX. 4) PAY PARTICULAR ATTN TO THE FMC WHEN XING FIXES TO ENSURE THAT THE NEXT FIX IS THE PROPER FIX. 5) CHK THE FMC CLOSELY WHEN IT HAS BEEN PROGRAMMED BY THE PREVIOUS CREW. (I WILL NOT PROGRAM THE FMC FOR SUBSEQUENT CREWS.) SUPPLEMENTAL INFO FROM ACN 565651: FROM NOW ON; I WILL CHK AND DOUBLECHK THE ARR ROUTING; RESTRS; AND RWY. I WILL ALSO ALWAYS MAKE SURE THE PNF CHKS THE PF INPUTS AND INSIST THE PNF MONITORS ALL COURSE CHANGES/TURNS. 1 CONTRIBUTING FACTOR TO OUR LACK OF ALERTNESS WAS 2 HRS OF ARPT APPRECIATION TIME FOLLOWED BY A VERY SHORT; BUSY; FAST; AND LAST LEG OF THE DAY.