G4 CREW HAD A TRACK DEV WHEN THE FILED FLT PLAN WAS CHANGED.

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THE ACFT ARRIVED AT MMU FOR A QUICK TURN TO DROP OFF 1 PAX AND THEN CONTINUE ON TO ITS DEST. AFTER LNDG; I RAN THE 'AFTER LNDG' CHKLIST AND THEN CALLED CLRNC DELIVERY FOR THE OUTBOUND CLRNC. AFTER RECEIVING THE CLRNC; I LOADED THE FILED FLT PLAN FROM THE DATA XFER UNIT TO THE FMS; SET THE NEW XPONDER CODE; AND SET THE INITIAL CLRED ALT IN THE ALT ALERTER; MAKING A MENTAL NOTE THAT THE FILED FLT PLAN WAS DIFFERENT THAN THE ATC CLRED FLT PLAN AND THAT THE RTE WOULD NEED TO BE CHANGED IN THE FMS. THE FILED RTE WAS COATE; LZV; J106; BEWEL; J70; PMM AND THE CLRED RTE WAS MMU5; RV; COATE; J36; FNT; PMM. AT THIS TIME THE ACFT WAS ENTERING THE RAMP AREA SO MY ATTN WAS DIVERTED TO THE OUTSIDE OF THE ACFT AS WE PARKED ON THE CONGESTED RAMP. WE RAN THE 'SHUTDOWN' CHKLIST AND AS I LEFT THE COCKPIT TO ASSIST AND DEPLANE THE PAX; I ADVISED THE OTHER CREW MEMBER THAT THE PERFORMANCE STILL NEEDED TO BE LOADED IN THE FMS; FAILING TO MENTION THE DISCREPANCY IN THE FILED VERSUS CLRED RTE. I ASSISTED THE PAX IN DEPLANING AND ASSURED HIS SAFE DEP FROM THE RAMP AREA BEFORE BOARDING THE ACFT FOR THE FLT ON TO HOME BASE. ENG START; TAXI; TKOF; AND DEP WERE NORMAL. AFTER GETTING SEVERAL RADAR VECTOR HDGS AND STEP CLB ALTS; WE WERE THEN CLRED DIRECT COATE ON COURSE WITH CLRNC TO CLB TO FL430. SHORTLY AFTER PASSING COATE AND CLBING THROUGH THE UPPER 20'S; I CHKED THE FMS; SAW LZV (WILKES BARRE) AS THE NEXT FIX AND IMMEDIATELY BEGAN TO PROGRAM J36 INTO THE FMS (OUTBOUND COURSE THAT WE WERE ON WAS 287 DEGS AND THE J36 COURSE WAS 311 DEGS). AS WE BEGAN TO MAKE THE CORRECTION BACK ON COURSE; ATC REQUESTED A 40 DEG TURN TO THE R WITH WHICH WE COMPLIED; AND SHORTLY THEREAFTER ATC RECLRED US DIRECT TO LHY (LAKE HENRY) ON COURSE. CHAIN OF EVENTS: 1) THE PROB AROSE WHEN TOO MANY TASKS WERE TRYING TO BE COMPLETED AT THE SAME TIME. 2) CONTRIBUTING FACTOR WAS THE DESIRE TO KEEP GND TIME TO A MINIMUM AT MMU. 3) THE PROB WAS DISCOVERED WHEN I CONFIRMED THE COURSE AFTER PASSING A FIX. HUMAN PERFORMANCE CONSIDERATIONS: 1) THE PERCEPTION THAT THE REMAINING PAX ON BOARD THE ACFT WANTED MINIMUM GND TIME AT MMU. 2) NOT FOLLOWING SOP'S; COPYING CLRNC WHILE TAXIING; NOT CONFIRMING WITH THE OTHER CREW MEMBER; NOT COMPARING FILED FLT PLANS TO CLRED FLT PLANS.