A WDB LINE CHK AIRMAN GOT 80 MI OFF COURSE WHEN HE SWITCHED THE INS COUPLED TO THE AUTOPLT FROM INS TO HDG.

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I WAS THE FE ON FLT X FROM MILAN; ITALY; TO NEW YORK-JFK ON M/D/92. AS WE APCHED THE FIX-DOTTY-ENDING THE OCEANIC PORTION OF OUR FLT; WE WERE CLRED TO CLB TO FL370 FROM FL350 AND TO PROCEED VIA N AMERICA RTE 144 FROM DOTTY TO EBONY. WE CLBED TO 370 AND INSERTED THE PROPER EBONY COORDINATES IN ALL 3 INS'S. AT ABOUT THIS TIME; THE PF APPARENTLY PUT THE AUTOPLT MODE SELECTOR IN THE HDG MODE. ABOUT 25 MINS LATER; THE GANDER CTLR CALLED US TO HAND US OVER TO MONCTON CTR AND HE ASKED WHERE WE WERE GOING. WE TOLD HIM WE WERE PROCEEDING ON NA-144. HE ADVISED US THAT WE WERE 80 MI N OF COURSE. WE IMMEDIATELY CHKED THE COORDINATES IN THE INS AND FOUND THEM CORRECT. WE THEN SAW THE AUTOPLT MODE SELECTOR WAS STILL IN HDG MODE INSTEAD OF INS MODE. THE REASON FOR NAV ERROR WAS THE AUTOPLT MODE SELECTOR HAD NOT BEEN RETURNED TO INS MODE AFTER PASSING DOTTY. THE CTLR ADVISED US HE WOULD FILE A VIOLATION AGAINST US FOR GROSS NAV ERROR; AND HANDED US OVER TO MONCTON. THE REMAINDER OF THE FLT WAS NORMAL. SOME FACTORS WHICH MAY HAVE CONTRIBUTED TO THIS MISTAKE WERE: ROUTING VIA NA-144 WAS A CHANGE FROM OUR FLT PLAN REQUESTED ROUTING. CREW MEALS FOR PF AND FLT ENGINEER WERE BROUGHT TO COCKPIT ABOUT SAME TIME AS PASSING DOTTY. ALTHOUGH UNDER RADAR CTL; GANDER CTLR DIDN'T QUESTION OUR POS UNTIL WE WERE 80 MI OFF COURSE. SUPPLEMENTAL INFO FROM ACN 211123. OBSERVATIONS IN THE AFTERMATH. MOST COUNTRIES DO NOT PREDICATE ATC ON RADAR AS WE DO IN THE UNITED STATES BUT USE RADAR PRIMARILY AS A MONITOR. MY PRIMARY CONCERN ON NORTH AMERICAN RTES HAS BEEN TO TAKE NAV FIXES TO CONFIRM ADHERENCE TO TRACK. THE PRIMARY CONCERN SHOULD BE CHKING WAYPOINTS AND INS/AUTOPLT STATUS THE SAME AS WE DO IN MHPS AIRSPACE. I HAVE NEVER HEARD OF AN EXCURSION ATTRIBUTED TO INS MALFUNCTION. IT IS ALWAYS A PROGRAMMING ERROR OF SOME SORT; USUALLY INVOLVING A 'RERTE.' I PICKED A POOR TIME TO 'DEBRIEF' A STUDENT. FATIGUE. PUT INS CHKING PROCS AHEAD OF NAV FIX PLOTTING. THE PROBLEM DOES NOT LIE IN THE HARDWARE BUT IN ITS PROGRAMMING AND USE. DO NOT DEBRIEF ON CHKRIDES UNTIL AFTER THE FLT IS OVER. ELIMINATE THE FIFTH DIGIT IN ENRTE FIX COORDINATES. ROUNDING TO THE NEAREST MIN COULD NOT RESULT IN MORE THAN 1/2 MI CHANGE IN POS.