BE35 PLT DEVIATES FROM CLRED TRACK IN CLE AIRSPACE AFTER INCORRECT INPUT TO NAV COMPUTER.

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DEPARTED BKL ON IFR CLRNC. CLRED TO FAILS INTXN; THEN DIRECT HFD. GIVEN VECTOR NBOUND 010 DEGS. LATER CLRED DIRECT FAILS INTXN. INPUT DIRECT FAILS INTO MY COMPUTER. BEARING GIVEN WAS SBOUND; SO I ASSUMED I WAS ALREADY N OF FAILS. TURNED TO SBOUND HDG; APPROX 180 DEGS. SOMETIME DURING THIS TIME THERE WAS A CTLR (FREQ) CHANGE. CTLR CALLED; 'ARE YOU HDG DIRECT FAILS?' 'TURN TO 360 DEG HDG.' PROCEEDED TO INTXN; FINISHED TRIP. PROBABLE CAUSE: FIGURED OUT THAT FLT PLAN WAS NOT ACTIVATED IN THE COMPUTER; SO 'DIRECT TO' INSTRUCTION DEFAULTED TO THE ARPT; ITS LAST DEST. ALSO; I MANAGED TO RATIONALIZE THE REST OF THE INFO (VOR/DME; ETC) DURING THIS BRIEF PERIOD. I HAVE USED THIS SAME EQUIP FOR MANY YRS; AND UNDOUBTEDLY HAVE COME TO DEPEND TOO HEAVILY ON ITS DIRECTION. CORRECTIVE ACTION: MUST CHK INSTRUMENTATION FEEDBACK WHEN INPUTTING DATA TO MAKE SURE IT IS ENTERED CORRECTLY. BE SURE THAT BACKUP INFO AGREES WITH PRIMARY NAV EQUIP. BE ALWAYS AWARE THAT TENDENCY IS TO LOOK AT INFO PRESENTED AND FORCE IT TO FIT OUR BELIEFS.