MLG HAS NMAC WITH ACROBATIC BIPLANE.

Narrative

DUE TO TSTMS ON THE FLL APCH WE WERE VECTORED TO INTERCEPT THE HOMEY ARR INTO PBI AND TO EXPECT RADAR VECTORS WHEN PAST THE WX. AS PART OF MY NORMAL BRIEFING PROCS WHEN IN FLORIDA AND PRIOR TO OUR DSCNT BELOW 10000 FT; THE FO AND I DISCUSSED THE NEED TO BE ESPECIALLY ALERT FOR LIGHT ACFT BELOW THE SCATTERED CLOUD DECK AT ABOUT 8000 FT. ALL EXTERNAL ACFT LIGHTS WERE TURNED ON. AFTER PASSING TURNR INTXN; PBI APCH TURNED US S AND HANDED US OFF TO MIA APCH CTL. WE WERE HDG 180 DEGS AT 6000 FT WHEN APCH ADVISED US OF A TARGET IN OUR 11-12 O'CLOCK POS AT 5000 FT ABOUT 7-8 MI. WE HAD THIS TARGET INTERMITTENTLY ON TCASII AT THIS TIME. APCH THEN CALLED OUT THE TFC AT ABOUT 5 MI AND SAID IT APPEARED TO BE MANEUVERING AS ITS HDG AND ALT WERE CONSTANTLY CHANGING. I SAW THE TFC; A SMALL RED BI-WING AIRPLANE; LOW; AT ABOUT 3-4 MI AND IT WAS HDG IN AN EASTERLY DIRECTION AND APPEARED TO BE OF NO THREAT TO US. THE FO ADVISED APCH THAT WE HAD THE TFC VISUALLY. THE TCASII AT THIS POINT SHOWED A TA (YELLOW) 600 FT BELOW US AT 11-12 O'CLOCK. IT QUICKLY TURNED TO AN RA (RED) AND GAVE THE AURAL ALERT TO 'MONITOR VERT SPD' AND SHOWED A RED ARC BELOW LEVEL FLT (DON'T DSND). AT THIS POINT; WITH NO WARNING (I.E.; NO CLRING TURNS); THE TFC ABRUPTLY PULLED STRAIGHT UP TO THE VERT AND WAS COMING RIGHT AT US. I DISCONNECTED THE AUTOPLT AND STARTED A R CLBING TURN ALMOST SIMULTANEOUSLY AS THE TCASII AURAL CHANGED TO 'CLB; CLB.' I WAS ONLY ABLE TO TURN ABOUT 10 DEGS AND CLB 150 FT BEFORE WE PASSED THE TFC AND THE TCASII ALERT CEASED. THE FO ADVISED ME THE TCASII SHOWED -02 DURING THIS MANEUVER. I ESTIMATED THE HORIZ SEPARATION WAS LESS THAN 500 FT AS I COULD NOT SEE THE OTHER PLANE ONCE I STARTED EVASIVE ACTION. A 'NEAR MISS' WAS RPTED AND I CONTACTED MIA APCH CTL WHEN ON THE GND BY TELEPHONE WITH THE PERTINENT INFO. I ASKED IF THAT AIRPLANE WAS IN AN AREA WHERE HE WAS ALLOWED TO BE PERFORMING ACROBATICS AND WAS ADVISED HE WAS. BECAUSE OF THE TSTMS; WE WERE NOT ON A RTE NORMALLY USED BY COMMERCIAL ACFT SO IT APPEARS HE WAS NOT EXPECTING TO SEE US. IN ANY CASE; HE DID NOT PERFORM ANY CLRING TURNS PRIOR TO COMMENCING AN ACROBATIC MANEUVER. AT THIS TIME; MIA APCH HAS NOT BEEN ABLE TO IDENT WHO THE OTHER ACFT WAS. MIA APCH CTL DID AN EXCELLENT JOB OF ISSUING ACFT ADVISORIES AND THE TCASII PERFORMED COMMENDABLY. I GUESS THE NEXT TIME I SEE A BI-WING ACFT I'LL ASSUME IT'S DOING ACROBATICS AND GIVE IT A LITTLE WIDER BERTH.