DC10 CREW HAD GROSS NAV ERROR IN RJTG AIRSPACE.

Narrative

ON DSCNT; WE DISCOVERED A PROB WITH THE MECHANICAL DIGITAL READOUT OF OUR HDG SELECTOR. DURING CRUISE; WHILE THE AUTOPLT WAS RECEIVING NAV GUIDANCE FROM THE INS; THE CAPT AND FE DID SOME TROUBLESHOOTING OF THE PROB SO THAT AN AML ENTRY COULD BE MADE. DURING THIS PROCESS; APPARENTLY THE HDG SELECTOR KNOB WAS PUSHED IN; CAUSING THE ACFT TO GO INTO HDG HOLD. AS THE PF; I DID NOT RECOGNIZE THIS. OVER TIME; DUE TO CHANGING WINDS AND VARIATION; WE DRIFTED L OF COURSE. THE FE ACTUALLY NOTICED THIS NOTING THAT THE CDI BAR WAS NOT CTRED. AT THAT TIME I REALIZED WE WERE IN HDG HOLD. WE IMMEDIATELY CORRECTED BACK TO THE AIRWAY. I ESTIMATE THAT WE WERE APPROX 20 MI OFF COURSE. IT IS VERY EASY TO BECOME COMPLACENT TOWARD NAV WHEN YOU OPERATE WITH A GOOD AUTOPLT AND INS. WHEN YOU COUPLE IT UP; IT FLIES THE RTE VERY PRECISELY. IN THE FUTURE; I WILL ENSURE THAT THE ACFT IS STILL IN THE PLANNED MODE AFTER ANYONE TOUCHES ANY TYPE AUTOPLT CTL. I WILL ALSO TRY TO BE MORE VIGILANT AS THE PF TO THE FLT MODE ANNUNCIATORS DURING CRUISE. I TAKE A LOT OF PAINS TO STAY ON TOP OF NAV; USING CHARTS AND NAVAIDS; BUT OUT OVER WATER THIS ERROR SLIPPED BY ME.