A GROSS NAV ERROR ON THE NORTH ATLANTIC TRACKS DUE TO AN UNRECOGNIZED AUTOPLT MODE.

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I WAS OPERATING AS THE CAPT (PIC) OF AN AUGMENTED CREW (1 CAPT; 2 FO'S; 2 FE'S) OF A B747 FLT OPERATING IN THE NORTH ATLANTIC TRACK SYS. I DECIDED TO TAKE REST AFTER WE RECEIVED OUR OCEANIC CLRNC AND WAS RELIEVED BY A TYPE RATED RELIEF OFFICER. APPROX 40 MINS AFTER BEING RELIEVED; I WAS CALLED TO THE COCKPIT AND TOLD THAT THE ACFT WAS NOT RESPONDING TO THE INS. BY THIS TIME WE WERE DISPLACED APPROX 30 NM N OF OUR INTENDED TRACK AND STILL ON A DIVERGING COURSE. I DETERMINED THAT THE AUTOPLT WAS NOT RESPONDING BECAUSE THE AUTOPLT WAS ACCIDENTALLY ENGAGED IN CTL WHEEL STEERING. THE ACFT APPARENTLY CROSSED THE OCEANIC ENTRY POINT IN CTL WHEEL STEERING AND BECAUSE CTL WHEEL STEERING IS A BASIC AUTOPLT MODE; IT CONTINUED ON THE PRIOR HDG INSTEAD OF MAKING AN APPROX 8 DEG HDG CHANGE. THE PNF WAS PLOTTING THE FLT ON A NORTH ATLANTIC PLOTTING CHART AND CAUGHT THE NAV ERROR WHEN HE MADE HIS 2 DEG PLOT PER THE COMPANY SOP; HOWEVER; NEITHER PLT WAS ABLE TO IDENT AND RECTIFY THE SIT AND THEY CALLED ME UP TO THE COCKPIT. THANKFULLY; WE WERE OPERATING IN THE NATS AT AN OFF PEAK TIME AND THERE WAS NO TFC CONFLICT. CONTRIBUTING FACTORS TO THIS EVENT WERE; 1) LACK OF THIS PARTICULAR AUTOPLT SYS. IT GIVES THE FLT CREW LITTLE NOTICE THAT IT HAS REVERTED TO CTL WHEEL STEERING FROM COMMAND. ALL IT TAKES IS A FEW LBS OF PRESSURE ON THE YOKE AND IT REVERTS TO CTL WHEEL STEERING. WE THINK THAT THE RELIEF PLT BUMPED THE YOKE WHILE TAKING HIS SEAT AND THUS THE AUTOPLT REVERTED TO CTL WHEEL STEERING. THERE IS NO AURAL WARNING WHEN THIS HAPPENS. IN FACT; THE ONLY WAY TO TELL THAT THE AUTOPLT IS OUT OF COMMAND (OTHER THAN THE AUTOPLT SWITCH POS) IS THE AUTOPLT ANNUNCIATOR PANEL GREEN NAV FLAG DISAPPEARS; NO RED/AMBER FLAGS/LIGHTS.