ACR WDB GROSS NAVIGATIONAL ERROR BETWEEN GREENLAND AND NF.

Narrative

WHEN WE RECEIVED OUR OCEANIC CLRNC; AN ADDITIONAL RPTING POINT (58N; 60W) WHICH WAS NOT ON OUR PREPRINTED FLT PLAN HAD BEEN INSERTED BTWN 62N 50W AND NAIN RADIO BEACON. I INSERTED THESE POSITIONS AND THE CAPT CHKED THEM. SHORTLY AFTER PASSING 62N; 50W; I BEGAN TO TUNE IN AND COPY THE WX WHICH DISTRACTED ME FROM DOUBLE CHKING THE NEXT POS. SHORTLY AFTER I BEGAN TO COPY THE WX; THE CAPT BROUGHT TO MY ATTN AN APPARENT MALFUNCTION IN ALL 3 INS'S IN THAT THE LONGITUDE SEEMED TO BE FROZEN AT 50W. IN THAT I HAD JUST RETURNED TO THE COCKPIT; IT TOOK ME A FEW MOMENTS TO ORIENT MYSELF AND TAKE A FEW RADIO BEARINGS. BELIEVING THAT WE WERE S OF COURSE BECAUSE OF THESE BEARINGS; WE INITIATED A SHARP RIGHT TURN AND NOTIFIED ATC THAT OUR POS WAS QUESTIONABLE AND THAT WE BELIEVED WE WERE AS MUCH AS 80 MI S OF COURSE. AFTER CHKING THE COORDINATES IN WAYPOINT POS #4 AND FINDING THEM TO BE 58N; 50W; WE MADE THE CORRECTION AND ATC CLRED US DIRECT TO NAIN. WE HAD BOTH BEEN ON DUTY FOR 10 HRS AND WERE VERY TIRED NOT HAVING SLEPT WELL THE PREVIOUS NIGHT IN MOSCOW DUE TO THE TIME CHANGE. WE BOTH SAW WHAT WE EXPECTED TO SEE.