A DC8 CREW; CRUISING OVER NORTHEASTERN CANADA; ATTEMPTING AN INS UPDATE FLUSH; UNINTENTIONALLY COMMANDED A MANUAL TRACK CHANGE OF TRUE N; CAUSING THE ACFT TO TURN IN THAT UNWANTED DIRECTION.

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DURING A FLT FROM EBBR TO JFK; ABOUT 100 MI ENE OF TAFFY INTXN; MY FO ASKED IF WE COULD REVIEW HOW TO FLUSH INS UPDATES. WE WERE NAVIGATING AND COUPLED ON #1 INS. NEITHER INS HAD ANY UPDATES INSERTED AT THAT TIME. I DEMONSTRATED HOW TO FLUSH THE UPDATES USING BOTH INS #1 AND #2. I ACCIDENTALLY LEFT OUT THE FIRST STEP OF THE PROC; WHICH IS TO PUSH THE HOLD BUTTON. BY PERFORMING THE NEXT 3 STEPS OF THE PROC -- SELECTING DESIRED TRACK; PRESSING 'ZERO;' AND PRESSING INSERT; I UNKNOWINGLY COMMANDED A MANUAL TRACK CHANGE OF TRUE N. BOTH MY FO AND MYSELF WERE ENGAGED IN A PRACTICE POS UPDATE AND FLUSH OF THE #2 INS. THE AUTOPLT USED SUCH A SLOW AND SHALLOW TURN THAT WE HAD TURNED ABOUT 70 DEGS WHEN MY FO NOTICED THE CHANGE. WE HAD JUST STARTED TURNING BACK WHEN MONTREAL CTR ASKED ABOUT THE TURN. SUPPLEMENTAL INFO FROM ACN 534679: THE STEPS ARE: 1) SELECT DESIRED TRACK STATUS; 2) PRESS HOLD; 3) PRESS ZERO; 4) PRESS INSERT. HE INADVERTENTLY LEFT OUT STEP #2 TO PRESS HOLD. LEAVING OUT THAT STEP COMMANDS THE INS TO FLY A SELECTED COURSE IN REF TO TRUE N. WE DID NOT SELECT A COURSE TO TRACK; THEREFORE; THE INS BEGAN A R TURN TO TRUE N SINCE THE DISPLAY READS 000.0 WITH NO INPUT FROM US. MONTREAL CTR SAID HE NOTICED WE WERE TURNING N. WE TOLD HIM WE HAD AN INS SCREW-UP AND WERE CORRECTING IT. ONLY LATER; AFTER READING THE INS MANUAL; WERE WE ABLE TO DETERMINE WHAT HAD HAPPENED.