A B747-200 FLC EXPERIENCES A COURSE DEV FROM AN IMPROPERLY MONITORED INS AT MID-ATLANTIC ON FREQ WITH EGGX; FO.

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PRIOR TO OUR DEP FROM ORD; OUR ACFT HAD ARRIVED WITH SOME INS PROBS. 2 OUT OF 3 UNITS HAD BEEN DEFERRED MAINT ITEMS. THE 2 UNITS WERE REPLACED BY MAINT AT ORD. ALL INS UNITS WERE CHKED AND OPERABLE FOR THE NORTH ATLANTIC FLT. OUR FLT WAS UNEVENTFUL EXCEPT FOR A NAV ERROR DISCOVERED AFTER XING 59N/302 FOR 56N/20W. THE ACFT APPEARED TO TRACK CORRECTLY. THE WAYPOINTS HAD BEEN CAREFULLY CHKED PER OUR COMPANY PROCS. THE FO CONFIRMED THAT A POS CHK AFTER LEAVING 30W WAS PLOTTED CORRECTLY. I FELT THAT OUR COURSE WAS MORE S THAN REQUIRED. TO MY SHOCK; I WAS CORRECT. WE WERE INDEED TRACKING MORE S THAN REQUIRED. WE WERE S OF OUR INTENDED TRACK. A CHK OF THE WAYPOINT 'TO' INDICATED AN INCORRECT 'N' LONGITUDE POS; BUT THE 'W' LATITUDE OF 20W WAS CORRECT. SOMEHOW IT HAD CHANGED AFTER IT WAS VERIFIED. THE ONLY PROBABLE CAUSE WAS THE FO HAD INPUTTED ADDITIONAL WAYPOINTS AFTER 30W; AND SOMEHOW MADE A MISTAKE. ADDITIONAL CHKING ALSO VERIFIED THAT THE 'CHK PLOT' AFTER 30W WAS INCORRECT. SOMEHOW ALL THE SAFEGUARDS HAD BEEN OVERLOOKED. WE MADE A CORRECTING COURSE TOWARDS N TO BE BACK ON COURSE BY 20W. MY FO AND I DISCUSSED THE EVENT IN DETAIL AFTER THE TRIP TO PREVENT A RECURRENCE IN THE FUTURE. IN THE FUTURE; KNOW YOUR RVSM; MNPS PROCS. DOUBLECHK; DOUBLECHK; ALWAYS! DO NOT LET YOUR GUARD DOWN. WE WERE LUCKY THIS TIME.