HDG TRACK POS DEV. NAV ERROR. NON ADHERENCE TO ATC CLRNC INSTRUCTION.

Narrative

ON TRANSITION TO APCH AFTER DSCNT FROM CRUISE ALT; MSO APCH CTLR ISSUED '310 DEG HDG AFTER FGT VOR; MAINTAIN 11000 FT; 210 KTS.' I; PF; SET HDG BUG TO 310 DEGS IN ANTICIPATION OF INSTRUCTION ISSUED. AUTOPLT WAS FLYING ACFT TRACKING INBOUND TO PEPPR INTXN ON MEINZ 6 ARR WITH FGT VOR TO FOLLOW. I SPACED OUT AT PEPPR INTXN AND SELECTED HDG MODE ON AUTOPLT THINKING I HAD REACHED FGT VOR. PROCEEDED ON 310 DEG HDG FOR 2-3 MINS BEFORE CTLR CAUGHT MY EARLY TURN AND ISSUED NEW HDG (010 DEGS) FOLLOWED APPROX 1 MIN LATER BY CLB INSTRUCTIONS TO 12000 FT. THINK ERROR WOULD HAVE BEEN ELIMINATED BY BETTER SITUATIONAL AWARENESS ON MY PART (KNOWING WHICH INTXN/VOR I AM TRACKING TO INSTEAD OF SIMPLY THINKING 'OH; OK; 310 DEG HDG AFTER AUTOPLT PASSES NEXT FIX'). CONTRIBUTING FACTORS: 1) NIGHT FLT LATER THAN OUR USUAL FLT TIMES (MIND WAS ALREADY OFF DUTY). 2) CONCERN WITH OTHER FACTORS THAN PRIMARY DUTY OF ACFT CTL/NAV. WE HAD DECIDED TO DIVERT FROM ORIGINAL DEST TO MSP BECAUSE OF KEY RWYS/POOR WX. 3) OTHER COCKPIT CREWMEMBER WAS OUT OF FLYING LOOP. HE WAS BUSY MAKING GND TRANSPORTATION ARRANGEMENTS -- HANGAR; ETC; ON SEPARATE RADIO FREQ AND DID NOT CATCH MY NAV BLUNDER. CORRECTIVE ACTIONS: 1) MAINTAIN ALERTNESS DURING ALL PHASES OF FLT; ESPECIALLY THOSE CONCERNING DIFFERENT OR LESS THAN USUAL CIRCUMSTANCES; IE; UNUSUAL DUTY HRS; CHALLENGING WX AND ARPT CONDITIONS; ETC. 2) KEEP BOTH CREWMEMBERS ACTIVE IN FLYING. DURING ANY COURSE; HDG; ALT CHANGE BOTH PEOPLE SHOULD BE COGNIZANT OF SIT AND KNOW WHAT THE OTHER IS DOING.