FLT CREW BELIEVED THEY HAD CORRECTLY PROGRAMMED THE FMS BUT LATER LEARNED SELECTING AN ENTRY TO THE TOP LINE OF THE 'DIR INT' PAGE WOULD CAUSE THE FMS TO SKIP TO THE NEXT POINT.

Narrative

ZLC CLRED US TO CROSS EFFTA AT FL270. THE COPLT PROGRAMMED THE FMS AND THE DSCNT WAS STARTED. BOTH OF US VERBALIZED THE RESTR AND POINTED TO THE ALT WINDOW ON THE MODE CTL PANEL TO CONFIRM THE ALT. I XCHKED THE DSCNT AS WE CROSSED BYI VOR AND THINGS LOOKED OK. BOTH OF US WERE DISTRACTED WITH PREPARATORY DUTIES OF LISTENING TO ATIS; REMOVING APCH PLATES; CHKING ARR GATE; AND RE-COMPUTING THE ARR TIME WHEN CTR ASKED IF WE WERE GOING TO MAKE THE RESTR. NOW JUST A FEW MI BEFORE EFFTA; WITH SEVERAL THOUSAND FEET TO GO; IT WAS EVIDENT THAT WE COULDN'T MAKE IT. I ASKED CTR IF WE NEEDED TO MAKE A TURN TO AVOID A CONFLICT AND THEY SAID NO; JUST HURRY DOWN. THEY HAD ALREADY TURNED THE CONFLICTING TFC TO AVOID A CONFLICT. WHEN WE CHKED THE FMS; IT WAS SHOWING THE RESTR PROGRAMMED FOR NORDD; THE NEXT WAYPOINT. THE COPLT WAS SURE HE HAD SELECTED EFFTA AND CONFIRMED IT BEFORE 'EXECUTING' THE CHANGE IN THE FMS. I LISTENED TO HIM READ BACK THE CLRNC TO ME AS HE ENTERED IT IN THE FMS; BUT DIDN'T RECHK IT AFTER HE HAD ENTERED IT. INITIALLY ASSUMING WE HAD MADE A MISTAKE; WE COMPLETED THE FLT UNEVENTFULLY. AS WE LEFT THE PLANE AND RODE TO THE PARKING LOT WE CONTINUED DISCUSSING HOW THIS COULD HAVE HAPPENED. WE REMEMBERED A SIMILAR EVENT EARLIER IN THE FLT. AS WE HAD BEEN RADAR VECTORED LEAVING SEA; DEP CTL HAD CLRED US DIRECT TO SUMMA. SINCE THE COPLT WAS HAND-FLYING; I HAD LINE SELECTED SUMMA AND ENTERED IT ON THE TOP LINE OF THE 'DIR INT' PAGE. AFTER SEEING SUMMA IN THE PROPER PLACE; I HAD EXECUTED THE CHANGE AND SELECTED LNAV ON THE MODE CTL PANEL. THE COPLT STARTED THE TURN THEN NOTED THAT IT WAS GOING TO BKE; WHICH WAS THE NEXT POINT. BAFFLED; I HAD TO RETYPE SUMMA AND REENTER IT. IT SEEMED THAT IN BOTH INCIDENTS THE FMS HAD SKIPPED TO THE NEXT POINT. NOT BELIEVING THAT THAT WAS EVEN A POSSIBILITY; BUT JUST AS SURE AS THE COPLT THAT WE HAD ENTERED THE CHANGES CORRECTLY; I RETURNED TO THE TERMINAL TO TALK TO THE AVIONICS MAINT PERSONNEL. THE AVIONICS MECH ASSURED ME THAT THAT TYPE OF SOFTWARE PROB WAS IN FACT POSSIBLE AND THAT IT HAD HAPPENED ON A B737-800 ABOUT A WK AGO. IN CLOSING; I NEED TO GET INTO THE HABIT OF CHKING THE FMS AFTER EXECUTING A CHANGE INSTEAD OF JUST BEFORE THE EXECUTION. THIS IS A B737-300; NON EFIS; SO THE FMS SCREEN AND THE RAW DATA ARE THE ONLY WAY TO VERIFY ITS ACTION.