A LIGHT PLANE PILOT FAILED TO CANCEL HIS IFR FLIGHT PLAN.

Narrative

I DEPARTED DIRECT TO GNB; UNDER VFR FLT FOLLOWING. ENRTE I RECEIVED VFR FLT FOLLOWING ADVISORIES FROM GRAY APCH; ZAB; AMARILLO APCH; AND ZDV. WHILE I WAS IN DETERIORATING VMC WEST OF FREMONT (16V); ZDV PROVIDED VFR COURSE GUIDANCE AROUND A TSTM AND REPORTED GNB IN VMC. DUE TO CONTINUING DETERIORATION OF VMC; I REQUESTED INFLT IFR; WHICH WAS PROVIDED BY ZDV WITH A DIRECT COURSE TO GNB AT AN IFR FLT LEVEL. AT APPROX THE DOBEE INTXN; I REPORTED TO ZDV THAT I WAS IN VMC WITH THE GNB BEACON IN SIGHT AND REQUESTED VFR AND GO TO ADVISORY FREQ. ZDV RESPONDED WITH A TRANSMISSION REGARDING CANCELLATION OF IFR CLRNC AND GOING TO ADVISORY FREQ; WHICH I READ BACK. I INADVERTENTLY INTERPRETED THE TRANSMISSION AS AUTO TO CANCEL IFR; SO I DID NOT INITIATE FURTHER COM WITH ZDV. I PROCEEDED TO GNB IN VMC UNDER PRESUMED VFR; AND LANDED WITHOUT INCIDENT. MY FAILURE TO CLOSE MY IFR FLT PLAN WAS NOT DELIBERATE. UPON MY RETURN HOME I WAS SURPRISED TO FIND A MESSAGE THAT THE FAA AT ZDV WAS FILING A PLT DEV REPORT FOR FAILURE TO CLOSE MY FLT PLAN. THE FILING AGENT HAS ADVISED ME THAT ZDV PROVIDED INSTRUCTIONS FOR CANCELLING IFR; AND DID NOT AUTHORIZE CANCELLATION OF IFR. I UNDERSTAND NOW THAT THIS IS CORRECT; AND THAT THERE WAS A MISCOM. I REALIZE NOT THAT INSTEAD OF REQUESTING 'VFR;' I SHOULD HAVE REQUESTED 'CANCELLATION OF IFR;' AND ALSO THAT I SHOULD HAVE LISTENED FOR ZDV TO REPORT 'IFR CANCELLED;' IF I WAS TO ASSUME IFR WAS CANCELLED. I ALSO SHOULD HAVE ESTABLISHED A BETTER MENTAL TRANSITION TO IFR IN SPITE OF THE THREE PLUS HOURS OF VFR FLT THAT PRECEDED INITIATION IFR.