AERONCA DEP FROM RVS TOOK OFF WITHOUT ATC CLRNC CAUSING GAR.

Narrative

AT RVS ARPT; AERONCA WAS ASKED BY GND CTL TO CONTACT TWR N SIDE OF RWY 13. UPON SWITCHING TO TWR FREQ 120.3 I HEARD 'ACFT Y CLRED ON COURSE.' I RESPONDED 'ACFT Y CLRED FOR TKOF;' AND PROCEEDED WITH THE TKOF ROLL. NO FURTHER RESPONSE CAME FROM THE TWR UNTIL AFTER I WAS AIRBORNE. AFTER LIFTOFF; I OBSERVED A SMALL CESSNA PASSING OVERHEAD ON THE 'GAR.' THE CESSNA WAS APPROX 150 FT ABOVE AND IN FRONT OF ME. I WAS APPROX 200 FT IN THE AIR WHEN THE TWR INFORMED ME THAT I HAD NOT BEEN CLRED FOR TKOF. I RESPONDED THAT I THOUGHT I HAD BEEN CLRED. THE TWR ACKNOWLEDGED THAT MY CALL SIGN HAD BEEN USED BUT THAT NO CLRNC WAS GIVEN. WHEN THE TWR SAID 'ACFT Y CLRED ON COURSE;' I PRESUMED THEY WERE IN A HURRY TO GET ME OFF AND I ACKNOWLEDGED AND RESPONDED BY COMMENCING THE TKOF. I BELIEVE THE TWR HAD INTENDED THE 'CLRNC ON COURSE' FOR ANOTHER ACFT AND INADVERTENTLY USED MY CALL SIGN WITH SUCH CLRNC; RESULTING IN CONFUSION. I ACKNOWLEDGED THE CLRNC AS A TKOF CLRNC AND GOT NO CORRECTIVE RESPONSE FROM THE TWR. I FLY BIG PLANES INTO BIG ARPTS AND IT IS NOT UNUSUAL FOR THE TWR TO INITIATE THE CALL AFTER BEING SWITCHED BY GND CTL TO TWR.