GROSS NAV ERROR ON THE NAT.

Narrative

IN PREPARATION FOR FLT ACROSS ATLANTIC (WBOUND); THE INS'S WERE LOADED AND XCHKED AGAINST THE FLT PLAN AS PER COMPANY POLICY. AFTER DEP; OUR N ATLANTIC TRACK WAS CHANGED AND WE RELOADED THE NEW TRACK IN EACH INS SEPARATELY; AFTER LOADING IN EACH TRACK POS BY 'NAME;' IE: 5640N; WE CONTINUED ON OUR WAY. I DID NOT CHK THAT THE CORRESPONDING COORDINATES FOR EACH FIX WERE CORRECT. THE COORDINATES ARE PRELOADED. AFTER XING A FIX POS; 57N40W; THE ACFT TURNED TO ITS NEXT; BUT INCORRECT FIX POS. INSTEAD OF MY INS -- INS #2 -- DRIVING THE ACFT TO 56N5OW WHICH WAS THE PRELOADED LIBRARY FIX WHICH WE WANTED TO GO TO AND WHICH WAS NAMED AS NEXT WAYPOINT; IT DROVE US TOWARDS 5ON56W; THE COORDINATES WHICH WERE ACTUALLY LOADED UNDER 565ON. THE CAPT'S; OR #1 INS; WAS LOADED CORRECTLY; SO WHEN HE CHKED HDG; DISTANCE AND ETA TO NEXT POINT; IT ALL CHKED OK. THE #2 INS; AS THE ONE DRIVING THE ACFT; WAS DISPLAYING COORDINATES. WE PROCEEDED ALONG WRONG COURSE UNTIL THE CAPT ACCOMPLISHED THE POST FIX CHK AND NOTED THAT WE WERE OFF INTENDED COURSE ON PLOTTING CHART. WE WERE APPROX 40 MI S OF INTENDED COURSE BEFORE CORRECTING BACK. FURTHER EXAMINATION SHOWED THAT ONLY #2 INS HAD INCORRECT POS LOADED. THIS INCIDENT COULD HAVE BEEN PRECLUDED IF I HAD CHKED THE COORDINATES OF EACH NEW POS THAT WAS LOADED INSTEAD OF ASSUMING ITS CORRECTNESS.