PA32 PLT DEPARTED ON AN IFR FLT PLAN WITHOUT WAITING FOR A DEP RELEASE.

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ON JAN/WED/02; I WENT TO LOUISVILLE FSS AT LOUISVILLE BOWMAN AND OBTAINED A WX BRIEFING. THE CONDITIONS ON MY RTE REQUIRED AN IFR FLT PLAN. AFTER LEAVING FSS; I RETURNED TO MY HANGAR TO PREFLT. BECAUSE THE TWR WAS NOT OPEN; I CONTACTED LOUISVILLE RADIO FOR A TA. THE OPERATOR OFFERED TO ACTIVATE MY FLT PLAN; TO WHICH I ACCEPTED. WHILE TAXIING TO RWY 6; HE CALLED BACK WITH MY CLRNC INFO. I DOCUMENTED MY INFO; AND HE VERIFIED MY READBACK. WHILE HOLDING FOR CLRNC; TFC CHOPPER (THE XA40 TFC HELI) CALLED LOUISVILLE RADIO FOR A TA. THE OPERATOR TOLD THE HELI THAT THE ONLY KNOWN TFC WAS A LANCE THAT WOULD BE DEPARTING WITHIN 2 MINS. BASED ON THAT XMISSION; I QUICKLY FINISHED MY RUN-UP SO I COULD BE IN THE AIR WITHIN THE 2 MINS. I KNEW THE HELI WAS APCHING AND I WANTED TO BE GONE BEFORE HE ARRIVED. ONCE AIRBORNE; WHILE STILL IN VMC AND OUTSIDE CLASS C AIRSPACE; I CONTACTED LOUISVILLE DEP ON THE FREQ I WAS GIVEN; AND RPTED OFF BOWMAN. THE CTLR ACKNOWLEDGED MY PRESENCE AND I BEGAN MY FLT. SEVERAL MINS INTO THE FLT I WAS ADVISED THAT I DEPARTED BOWMAN WITHOUT CLRNC; HOWEVER; IT WASN'T A BIG DEAL BECAUSE THERE WERE NO OTHER ACFT INVOLVED. SHORTLY BEFORE I LEFT THEIR AIRSPACE; I WAS ASKED TO CONTACT LOUISVILLE APCH CTL AFTER LNDG. THIS WAS PURELY A CASE OF MISUNDERSTANDING BTWN THE PLT (MYSELF) AND LOUISVILLE RADIO. I MISUNDERSTOOD A COM BTWN RADIO AND A HELI TO INFER THAT I WAS ABOUT TO DEPART. IF RADIO WOULD HAVE INFORMED THE HELI THAT I WAS STILL HOLDING FOR A CLRNC; I WOULD NOT HAVE DEPARTED. I AM USED TO EITHER DEPARTING WITH A TWR; OR IN UNCTLED AIRSPACE.