CRJ ENCOUNTERS TCASII ALERT WITH INTRUDER PIPER CADET ON VISUAL APCH TO MLB.

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BEING VECTORED INTO THE MLG ARPT AREA; WE WERE ON BASE FOR RWY 27L AND CLRED FOR THE VISUAL APCH. ON TURNING FINAL; I USED THE LOC BACK COURSE FOR BACKUP AND THE GND LIGHTING SYS FOR THE DSCNT. I WAS HAND FLYING A NORMAL DSCNT AND HEARD A TFC ALERT ON THE TCASII. UPON REFING THE TCASII SCREEN; WE SAW THE TFC WAS 300 FT BELOW US AND DIRECTLY OFF THE NOSE. I LOOKED OUT AND THE AIRPLANE CAME INTO VIEW. IT WAS HARD TO SEE BECAUSE THE RIVER WAS BELOW US AND THE PLANE BLENDED IN. I ARRESTED MY DSCNT IMMEDIATELY AND THE PIPER CADET MAINTAINED HIS ALT AND WE PASSED OVER HIM AND HE FLEW UNDERNEATH US TO THE N. I DON'T KNOW IF HE EVER SAW US BECAUSE WE WERE AT HIS 5 O'CLOCK POS. HE MAY HAVE SEEN US AFTER HE TURNED ON FINAL FOR RWY 27R WHICH WAS WHERE HE WAS GOING. IT WAS PRETTY SCARY! IF I HAD CONTINUED IN MY DSCNT ON FINAL WE WOULD HAVE COLLIDED FOR SURE. AFTER LISTENING TO THE CONVERSATION BTWN THE CADET AND THE CTLR; WE FOUND OUT THIS: THE CTLR SAID HE INSTRUCTED THE CADET TO TURN A 360 DEG HDG (I BELIEVE TO THE S SIDE OF THE ARPT). THE CADET SAID HE COULD HEAR EVERY OTHER CTLR FREQ BUT HE WAS HAVING TROUBLE HEARING THE TWR FREQ. THE TWR AND HE DISCUSSED IT FURTHER; TWR TELLING HIM TO GET HIS RADIO FIXED. THE SIT RESULTED IN NO INCIDENT -- THANKFULLY. THANKS FOR THE NASA RPTING SYS.