NMAC BTWN A YAK54 ON FINAL AND A C152 JUST AHEAD ON A STRAIGHT-IN APCH NOT TALKING TO THE TEMPORARY TWR ESTABLISHED FOR AN AVIATION SPECIAL EVENT.

Narrative

THE NEAR MISS EVENT OCCURRED AT THE THREE FORKS ARPT; MT (9S5). THIS ARPT WAS HOSTING A SPECIAL EVENT. THIS ARPT NORMALLY HAS A UNICOM AND CTAF ON 122.8 BUT FOR THIS EVENT; A TEMPORARY TWR WAS SET UP AND MANNED BY ATC PERSONNEL FROM HELENA; MT. THE CTAF FOR THIS EVENT WAS 123.9 AND PLTS WERE NOTIFIED BY NOTAM. I HAD CHKED THIS NOTAM PRIOR TO THE FLT BY DUAT. THE ARPT WAS QUITE BUSY THAT MORNING AS I APCHED FROM THE E; HAVING DEPARTED FROM BOZEMAN; MT; A FEW MINS EARLIER. I WAS FLYING A RUSSIAN MADE YAKOVLEV YAK54 2 SEAT COMPETITION AEROBATIC AIRPLANE. I WAS IN COM WITH THE TWR FROM ABOUT 6 MI TO THE E UNTIL AFTER LNDG. PRIOR TO THE INCIDENT; I MADE 2 LNDG APCHS WHICH HAD TO BE ABORTED BECAUSE OF TFC ON THE RWY. I WAS INSTRUCTED BY THE TWR TO ENTER R-HAND TFC FOR RWY 20; WHICH I DID. THERE WERE SEVERAL OTHER AIRPLANES IN THE PATTERN AT THE SAME TIME AND THERE WERE ALSO 2 PARACHUTISTS DSNDING FOR A LNDG AT THE ARPT. I TURNED BASE LEG AND THEN FINAL. AS I ROLLED OUT ON FINAL; I SAW ANOTHER ACFT; WHICH I BELIEVED TO BE A C152; DIRECTLY IN FRONT OF ME ALSO ON FINAL APCH. THE OTHER ACFT WAS NOT MORE THAN 2 AIRPLANE LENGTHS IN FRONT OF ME; AT MY ALT; AND THE DISTANCE BTWN US WAS CLOSING FAST. I IMMEDIATELY MADE AN ABRUPT 90 DEG L TURN USING FULL DEFLECTION OF THE CTLS TO AVOID A CERTAIN COLLISION. FORTUNATELY; MY ACFT WAS VERY CAPABLE OF DOING THIS. I THEN FOLLOWED TWR'S INSTRUCTION TO RE-ENTER THE PATTERN AND LAND UNEVENTFULLY. LATER THAT DAY; I SPOKE WITH THE ATC PERSON ON DUTY AT THE TIME. HE APOLOGIZED FOR WHAT HAPPENED AND INDICATED THAT THE OTHER AIRPLANE WAS NOT TALKING TO THEM ON 123.9. APPARENTLY THE OTHER AIRPLANE WAS MAKING A LONG STRAIGHT-IN APCH TO THE ARPT. I BELIEVE THAT THE INCIDENT WAS CAUSED BY 2 FACTORS: 1) FAILURE OF THE OTHER INCIDENT PLT TO BE AWARE OF THE NOTAM ON THE TEMPORARY TWR; AND 2) FAILURE OF THE TWR PERSONNEL TO MONITOR ADEQUATELY THE 122.8 UNICOM FREQ TO ADVISE SUCH ACFT THAT THEY NEED TO BE ON THE TWR FREQ. A CONTRIBUTING FACTOR IS THAT THE OTHER PLT WAS APPARENTLY BLITHELY UNAWARE OF THE LEVEL OF ACTIVITY AT THE ARPT. ANOTHER CONTRIBUTING FACTOR WAS THAT THE LEVEL OF ACTIVITY (PARACHUTISTS; OTHER PLANES IN PATTERN; ETC) MADE IT DIFFICULT FOR ME TO ADEQUATE SCAN THE AREA FROM WHICH THE OTHER INCIDENT AIRPLANE ARRIVED. NOTE THAT THE UNICOM IS NORMALLY MANNED BY THE FBO PERSONNEL. WHEN THE ARPT IS ABNORMALLY BUSY; AS ON THIS DAY; THE FBO IS ABSOLUTELY UNABLE TO MONITOR THE UNICOM. APPROX 130 ACFT WERE PARTICIPATING IN THE SPECIAL EVENT ON THIS DAY. FBO PERSONNEL HAVE INDICATED TO ME THAT THEY MADE NO ATTEMPT TO MONITOR THE UNICOM DURING THE SPECIAL EVENT AS THEY DID NOT HAVE THE PERSONNEL TO DO THIS AND HANDLE THE UNUSUAL NUMBER OF FUEL AND SVC REQUESTS. I BELIEVE THAT WHENEVER A TEMPORARY TWR IS ESTABLISHED FOR SPECIAL EVENTS THAT THE TWR PERSONNEL MUST TAKE RESPONSIBILITY FOR MONITORING THE NORMAL CTAF TO ADVISE UNAWARE PLTS OF THE TWR OP. FAILURE TO DO SO MAY ACTUALLY RESULT IN A REDUCTION OF SAFETY DUE TO THE TWR OP BECAUSE NOT ALL PLTS ARE USING THE SAME CTAF RESULTING IN SUCH INCIDENTS AS THAT RPTED HERE.