A C172 PLT; APCHING HEF; DID A 360 DEG TURN TO AVOID A COLLISION; WHEN UNABLE TO CONTACT TWR.

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WE WERE RETURNING TO THE ARPT FROM THE W AFTER A TRAINING FLT. AFTER THE INITIAL CALL; THE CTL TWR DIRECTED US TO RPT A 2 MI R BASE FOR RWY 16L. WHEN APCHING THE ARPT AT APPROX 3 MI; WE SPOTTED TFC THAT WAS CONVERGING ON OUR COURSE. THE TFC WAS ESTABLISHED ON AN EXTENDED DOWNWIND FOR RWY 16R. OUR FLT PATH FOR THE BASE LEG FOR RWY 16L CROSSED THE EXTENDED DOWNWIND. THE CTL TWR FREQ WAS CONGESTED WITH MULTIPLE TARGETS INBOUND FOR LNDG. I WAS UNABLE TO CALL THE TWR (DUE TO FREQ CONGESTION) FOR A 360 DEG TURN BEFORE THE TFC CONFLICT BECAME TOO SERIOUS. I PROCEEDED WITH A 360 DEG TURN TO THE L TO AVOID THE COLLISION COURSE. AFTER APPROX 270 DEGS OF TURN; THE TWR ASKED WHO WAS DOING THE 360 DEGS. I RESPONDED WITH OUR CALL SIGN AND REASON FOR THE MANEUVER. THE TWR RESPONDED WITH 'ASK ME NEXT TIME BEFORE DOING ANY 360'S.' DUE TO THE FREQ CONGESTION; I WAS UNABLE TO CALL THE TWR BEFORE THE SIT BECAME A COLLISION. I TOOK THE ACTION TO AVOID A COLLISION; WHICH I FELT WAS GOING TO OCCUR WITH THE PRESENT HEADING. AFTER THE 360 DEGS; THE APCH AND LNDG WENT WITHOUT INCIDENT.