ACFT HAS LARGE NAVIGATIONAL ERROR AS IT FLIES ACROSS THE ATLANTIC. ERROR IS CAUSED BY WRONG INSERTION OF LATITUDE AND LONGITUDE COORDINATES INTO FLT MGMNT COMPUTER.

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NORMAL CREW IS 2 PLTS AND 1 FLT ATTENDANT. WE LAUNCHED AS AN AUGMENTED CREW (3 PLTS) TO CONFORM WITH DISPATCH REQUIREMENTS FOR EXTENDED DUTY HRS. THIS PARTICULAR FLT WOULD REQUIRE APPROX 17.5 BLOCK HRS BEGINNING THE AFTERNOON OF JUL/XX/94. ENRTE ON THE 1ST LEG; SUBSEQUENTLY RTES WERE ENTERED INTO THE FMS'S BY THE RELIEF AIR CREW. SEVERAL WAYPOINTS REQUIRED MANUAL ENTRY OF LA9/LONG POSITIONS. THE 3RD AND FINAL LEG WAS EXTENDED; OVERWATER ROUTING WHICH I ELECTED TO FLY. UPON RECEIPT OF OUR CLRNC; WE VERIFIED THE ROUTING AS CORRECT IN THE ACTIVE PLAN OF THE FMS. ENRTE THOUGH OUR TIME AN DISTANCE TO 42NO40W DID NOT MATCH THE COMPUTER PLAN. NEW YORK ARINC ALSO ASKED US TO REPEAT OUR TIME AND TO ALSO RPT OUR TIME OVER 045W. IT WAS THEN I DISCOVERED THAT WAYPOINT 3950 WAS NAMED CORRECTLY BUT ENTERED WITH THE WRONG COORDINATES. WE WERE OFF COURSE BUT CORRECTING TO 42N040W. CLASSIC ERROR! I ASSUMED THE FLT PLAN; AS ENTERED PREVIOUSLY WAS CORRECT. (ONLY THE DESIGNATORS WERE ONE SET OF COORDINATES WAS WRONG.) WE ARE REVIEWING AUGMENTED CREW REQUIREMENTS. THE IMPACT OF FATIGUE AND CREW COORD MUST BE STRESSED; THE NEED FOR ACCURACY IN DATA LOADING VIA A 'CHALLENGE -- RESPOND' METHOD IS POLICY. NOTHING IS ASSUMED. SUPPLEMENTAL INFO FROM ACN 277423: WE FLEW TO MEXICO CITY TO PICK UP OUR PAX. THE WRITER WAS IN THE L SEAT FOR THIS LEG AND DURING THE FLT I INSERTED THE NEXT 2 LEGS OF THE FLT INTO THE FLT MGMNT SYS; WHICH WAS MEXICO CITY TO BERMUDA AND BERMUDA TO MADRID. UNTIL RECENTLY THE SYS SOFTWARE ALLOWED OVERWATER LATITUDE/LONGITUDE TO BEN ENTERED AS; FOR INSTANCE; 3950 N FOR 39 DEGS N/50 DEGS W. A RECENT SOFTWARE UPDATE REMOVED THAT CAPABILITY AND REQUIRED THAT THE ENTIRE N 3900.0/W5000.0 BE TYPED INTO THE FLT PLAN. I TYPED IN THE VARIOUS LATITUDE/LONGITUDES FOR THE OVERWATER PHASE IDENTING EACH WAYPOINT IN THE PREVIOUS MANNER; EG; 3560N; 3950N; 4240N; ETC. IN ACTUALLY TYPING IN THE LATITUDE/LONGITUDES; I ERRONEOUSLY TYPED IN 35 DEGS N LATITUDE AT 50 W LONGITUDE; INSTEAD OF 39 DEGS N LATITUDE AT 50 W LONGITUDE. AFTER REFUELING IN BERMUDA AT ABOUT AM LCL I CAME FROM A CREW REST STATION AND TOOK THE R SEAT FOR THE LEG FROM BERMUDA TO MADRID. THE FLT PLAN WAS CALLED UP AND DISPLAYED. A DEV IN THE PATH WAS NOTED; BUT WAS NOT CONSIDERED UNUSUAL. THE WAYPOINTS HAD BEEN PREVIOUSLY DOUBLE CHKED; BUT SINCE THE LEGEND ASSIGNED WAS 3950 N IT WAS ASSUMED IT REPRESENTED THE CORRECT LATITUDE/LONGITUDE. DURING THE FLT NEW YORK ARINC QUESTIONED OUR ESTIMATE FOR 4240 N AND SUBSEQUENTLY ASKED FOR OUR ESTIMATE AT 45 DEGS W. CREW FATIGUE WAS A DEFINITE FACTOR. NEITHER PLT NOTED ANY PROB UNTIL THE SECOND NEW YORK CALL; EVEN THOUGH PRESENT POS WAS READ OUT CONTINUOUSLY. I DOUBT IF THERE WAS ANOTHER AIRPLANE WITHIN 500 MI AT OUR ALT. OUR TCASII NEVER SHOWED ANYTHING.