AN SA227 FLC FAILED TO CHK CLRED RTE AGAINST THE FLT PLAN AND FLEW THE WRONG RTE DEPARTING CDB.

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WE OBTAINED OUR CLRNC FROM CDB RADIO TO DUT; WHICH READ 'CLRED AS FILED; MAINTAIN FL200; SQUAWK XXXX; CONTACT CTR ABEAM MORDI ON 121.4.' DISPATCH ALWAYS FILES OVER THE AIRWAYS AND CDB J115 DUT IS THE ONLY JET RTE TO DUT. WE DID NOT VERIFY OUR RTE; THERE WAS NO REASON TO BELIEVE WE WERE FILED ANY OTHER RTE. CONTACTING CTR 50 MI SW OF CDB (ZAN RCAG AT CDB WAS OTS) WE WERE ASKED TO VERIFY WE WERE ON THE AIRWAY. WE CONFIRMED; AND THEN WERE TOLD ZAN SHOWED US FILED OFF AIRWAY CDB.MORDI.HORTH.DUT. WE THEN REQUESTED J115 DUT; BUT WAS TOLD 'UNABLE;' SO WE PROCEEDED DIRECT MORDI. BEFORE REACHING MORDI WE WERE CLRED DIRECT DUT. PRIMARY FACTOR WAS THAT WE DID NOT VERIFY OUR RTE; SINCE THERE IS ONLY ONE JET RTE TO DUT FROM CDB AND DISPATCH VERY RARELY FILES US OFF AIRWAY WHEN JET RTES ARE AVAILABLE. J115 IS THE MOST DIRECT RTE TO DUT FROM CDB. IN OUR CLRNC 'ABEAM MORDI' MADE SENSE TO US RATHER THAN OVER MORDI WHICH WOULD HAVE RAISED A FLAG. OTHER FACTORS: WE WERE AT THE END OF OUR 9 HR DUTY DAY IN WHICH WE FLEW 6.6 HRS.