A CPR HS25 FLC GOT OFF COURSE WHEN THEIR FMC DATABASE WAS IN ERROR AS TO THE WAYPOINT COORDINATES. THE ARTCC CTLR POINTED OUT THE ERROR AND THE FLC WAS ABLE TO FIND THE ERROR LATER.

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THE PROB -- ON MAY/FRI/98; AT APPROX XA00Z; UPON DEPARTING CZQX AIRSPACE; I WAS INFORMED BY MY PICKUP RADAR CTLR THAT I WAS 50 NM N OF MY ASSIGNED EXIT POINT. COCKPIT INDICATIONS -- I INFORMED THE CTLR THAT ALL OF MY COCKPIT INDICATIONS SHOWED ME EXACTLY ON COURSE. THE CTLR ASSURED ME THAT I WAS NOT AT THE ASSIGNED POS AND THEN HE HANDED ME OFF TO GANDER APCH CTL FOR TERMINAL GUIDANCE. DETERMINING THE PROB -- AFTER BEING ASSURED THAT WE WERE OFF COURSE; WE BEGAN TO EVALUATE OUR LONG RANGE NAV SYS. AS WE HAD DONE SEVERAL TIMES THROUGHOUT THE FLT WE CHKED BOTH INERTIAL NAV SYS AGAINST THE GPS AND BOTH FLT MGMNT COMPUTERS. WE COULD NOT FIND ANY PROBS WITH THE NAV SYS. THE ENTIRE FLT WAS IN DAYLIGHT AND IN VMC CONDITIONS. WE HAD MADE NOTE OF SEVERAL COMMERCIAL ACFT PASSING DIRECTLY OVERHEAD ON OUR SAME TRACK AND HOW ACCURATE OUR NAV SYS WERE. HOWEVER; FOR SOME REASON THE ACFT HAD VEERED OFF COURSE AT THE LAST WAYPOINT. I DECIDED TO LOOK AT THE DATABASE WHERE THE WAYPOINTS WERE DEFINED. WHAT HAPPENED -- WITHIN OUR NAV DATABASE; LATITUDE/LONGITUDE COORDINATES ARE REPRESENTED BY 5 CHARACTER IDENTIFIERS. FOR EXAMPLE: THE LATITUDE/LONGITUDE COORDINATES (N 48 DEGS 00.0 FT W050 DEGS 00.0 FT) IS REPRESENTED BY (4850N). THE 5 CHARACTER IDENTIFIER THAT WE HAD ENTERED AND DOUBLE CHKED FOR THE LAST WAYPOINT WAS CORRECT. HOWEVER UPON CHKING THE NAV DATABASE; WE FOUND THE PROB. INSTEAD OF (N 48 DEGS 00.0 FT W050 DEGS 00.0 FT); WE FOUND THAT THE ACTUAL NUMBERS WERE (N 48 DEGS 50.0 FT W050 DEGS 00.0 FT). THIS ACCOUNTED FOR THE 50 NM ERROR ON OUR LAST WAYPOINT. WE IN THE FLT DEPT HAVE CHKED AND VERIFIED THIS DATABASE ON NUMEROUS OCCASIONS. IN ADDITION; WE HAVE MADE MORE THAN 30 NORTH ATLANTIC XINGS USING THIS DATABASE WITH NO PRIOR ERRORS OR INCIDENTS. WE HAVE NO IDEA HOW THE DATABASE BECAME CORRUPTED; HOWEVER; WE ARE SURE THAT THIS WAS THE PROB. THE FIX -- A NEW PREFLT AND INFLT VERIFICATION PROC. A) BECAUSE IT IS POSSIBLE FOR THE DATABASE TO BE ALTERED; THE ONLY WAY TO ASSURE THAT THE 5 CHARACTER IDENTIFIER CORRESPONDS TO CORRECT LATITUDE/LONGITUDE COORDINATES IS TO CHK THE DATABASE PRIOR TO FLT FOR EACH WAYPOINT ASSIGNED. B) WHEN A WAYPOINT IS SELECTED INFLT; AS SOON AS POSSIBLE RE- VERIFY THAT THE ACTUAL COORDINATES OF THAT WAYPOINT CORRESPOND TO THE 5 CHARACTER WAYPOINT SELECTED. C) MONITOR RAW DATA FROM THE 3 LONG RANGE NAV SYS TO VERIFY THAT THE ACFT IS IN FACT MOVING TOWARD THE INTENDED WAYPOINT. THESE PROCS HAVE BEEN ADOPTED BY OUR FLT AS OF THIS DAY.