EC135 HELI OPERATED WHEN AN EXHAUST HEAT SHIELD WAS CRACKED.

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DURING PREFLT FOR ANNUAL PART 135 CHK WITH COMPANY CHK PLT AND FAA OBSERVING; NOTICED CRACK ON EXHAUST HEAT SHIELD WHERE IT ATTACHED TO EXHAUST. ACFT WAS DOWNED AND DISCREPANCY WAS WRITTEN UP. AFTER TALKING TO MECH; HE INDICATED THAT THE PART WAS ON ORDER AND IT HAD BEEN CRACKED FOR SEVERAL DAYS. THE PART WAS NOT DEFERRABLE ON MEL. I HAD FLOWN THE ACFT 3 DAYS EARLIER AND HAD NOT SEEN THE CRACKED PART. MECH HAD CALLED COMPANY ABOUT PROB BUT WAS NOT TOLD IT WAS NON DEFERRABLE. THIS WAS MECH'S FIRST ASSIGNMENT FOR PART 135 OPERATOR. I ESTIMATE I FLEW THE ACFT 3-4 TIMES BEFORE FINDING IT HAD A DOWNING DISCREPANCY. SUPPLEMENTAL INFO FROM ACN 545082: ALTHOUGH THIS WAS NOT A SAFETY OF FLT ISSUE; SINCE THE MECH HAD NOT WRITTEN THE INFO IN THE LOGBOOK OTHER PLTS LIKE MYSELF OVERLOOKED THE PART ON OUR PREFLT DUE TO ITS INCONSPICUOUS LOCATION. AS A RESULT OF THIS; I HAD FLOWN THE ACFT THE DAY BEFORE IN THIS CONDITION. THE MECH WAS NOT AWARE OF MAINT PROCS FOR DEFERRAL OF ITEMS NOT ON MEL LOCATED IN THE COMPANY GENERAL OPS MANUAL.