A HVT ACR FREIGHTER ACFT HAD A GROSS NAV ERROR OVER THE ATLANTIC.

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WE DEPARTED STANSTED ENGLAND. MY FO WAS FLYING AND I WAS HANDLING THE RADIOS AND NAV. WE WERE CLBING ABOVE 18000 FT TO FL350 ENRTE TO LANDS END VOR. SINCE WE WERE GOING TO COAST OUT SOON; I TOLD MY FO I WOULD GET THE OCEANIC CLRNC AND THE FE WOULD BACK ME UP ON THE CLRNC AND FOR HIM (FO) TO FLY THE ACFT AND MONITOR LONDON ATC. OUR FLT PLAN CALLED FOR AN OCEANIC XING OF 49N15W 49N20W 49N30W 49N50W YQY (GANDER); BUT THE OCEANIC CLRNC WAS DIFFERENT TO 49N15W 49N20W 49N30W 49N40W 48N50W YYT (TORBAY). ON THE MASTER FLT PLAN I CROSSED OUT YQY (GANDER) AND WROTE IN YYT (TORBAY) AND ENTERED THE COORDINATES INTO MY INS. BUT I DIDN'T CROSS OUT 49N50W TO 48N50W; SO I DIDN'T CHANGE THE COORDINATES IN MY INS. MY FO ASKED ME WHAT THE CLRNC WAS AND I TOLD HIM CLRED AS FILED EXCEPT THEY CHANGED YQY (GANDER) TO YYT (TORBAY) SO MY FO ENTERED THE COORDINATES INTO HIS INS. WE COMPARED COORDINATES AND THE RTING BTWN THE 2 INS'S. SINCE I DIDN'T CHANGE THE COORDINATES FROM 49N502 TO 48N50W; THE DISCREPANCY WASN'T FOUND. I THEN FILLED OUT THE AIREP FORM TO MAKE OUR POS RPTS. I FILLED OUT THE COORDINATES OF THE SCRATCH PAD WHERE I HAD WRITTEN DOWN THE ATC OCEANIC CLRNC WHERE THE CORRECT COORDINATE OF 48N50W WAS WRITTEN DOWN. ALL THE WAY ACROSS THE ATLANTIC I CHKED THE WAYPOINT COORDINATES AGAINST THE COMPUTERIZED FLT PLAN. WHEN I CHKED THE WAYPOINT 49N50W ON THE FLT PLAN; IT MATCHED THE INS. AT ABOUT 10 NM FROM 49N50W; I SAW ON MY SCRATCH PAD WHERE I HAD WRITTEN THE OCEANIC CLRNC 48N50W DIRECT YYT (TORBAY). SINCE WE WEREN'T IN RADAR CONTACT; I IMMEDIATELY INFORMED GANDER CTR THAT WE WERE PROCEEDING DIRECT 49N50W INSTEAD OF 48N50W. GANDER SAID PROCEED DIRECT 48N50W SQUAWK XXXX. I FEEL MY BIGGEST MISTAKE WAS I SHOULD HAVE WRITTEN THE OCEANIC CLRNC ON THE COMPUTER FLT PLAN WHERE I WOULD HAVE PICKED UP THE MISTAKE OF NOT XING OUT THE COORDINATES OF 49N50W FOR 48N50W. THINKING BACK OVER THIS TRIP; NOWHERE DURING THE OCEANIC XING DID MY FO PICK UP THE COMPUTER FLT PLAN OR THE AIREP FORM TO CHK FOR ANY MISTAKES. I FEEL I AM RESPONSIBLE FOR THE NAV ERROR. BUT; IF MY FO HAD LOOKED AT THE AIREP FORM; MY MISTAKE MIGHT HAVE BEEN FOUND LONG BEFORE IT WOULD HAVE BECOME A NAV ERROR.