Narrative:

During approach to the pbi approach and level at 4000 ft, on an assigned heading of 230 degrees, we observed a contact on TCASII at about our 12 O'clock position. It was first observed as an open diamond with -15 below it. As we got closer, the diamond changed to a blue DOT, then to a yellow DOT, accompanied by a 'traffic' warning from TCASII. We acquired the airplane visually at our left 11:30 O'clock position and level with us. It was not moving in the windscreen and it was soon apparent that we were on a collision course. At this point, there was a cloud backgnd, and I couldn't see a horizon to determine if in fact we were level with this aircraft or not. So I disconnected the autoplt and started a climb, hoping to see the other aircraft descend in the windscreen. It did, confirming that we were at the same altitude. We climbed 500 ft above our assigned altitude, and passed directly over the other aircraft. I estimate the other aircraft heading to be about 300 degrees. Our miss distance was about 300-400 ft vertically. If I had not taken evasive action, there is no doubt in my mind that we would have collided. We immediately reported our altitude deviation to ZMA who said that the only traffic nearby was reported to be 2000 ft below us. After landing at pbi, I called ZMA, who had started an investigation. The type of airplane we encountered appeared to be a light twin, silver and blue in color, with winglets. It was most likely a piper navajo or similar aircraft. When I started my evasive maneuver, the other airplane was rapidly getting larger, and the analysis I described above was done in just a few short seconds.

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Original NASA ASRS Text

Title: DC9 FLC INITIATE TCASII MANEUVER DUE TO UNRPTED TFC WHILE UNDER ZMA CTL.

Narrative: DURING APCH TO THE PBI APCH AND LEVEL AT 4000 FT, ON AN ASSIGNED HDG OF 230 DEGS, WE OBSERVED A CONTACT ON TCASII AT ABOUT OUR 12 O'CLOCK POS. IT WAS FIRST OBSERVED AS AN OPEN DIAMOND WITH -15 BELOW IT. AS WE GOT CLOSER, THE DIAMOND CHANGED TO A BLUE DOT, THEN TO A YELLOW DOT, ACCOMPANIED BY A 'TFC' WARNING FROM TCASII. WE ACQUIRED THE AIRPLANE VISUALLY AT OUR L 11:30 O'CLOCK POS AND LEVEL WITH US. IT WAS NOT MOVING IN THE WINDSCREEN AND IT WAS SOON APPARENT THAT WE WERE ON A COLLISION COURSE. AT THIS POINT, THERE WAS A CLOUD BACKGND, AND I COULDN'T SEE A HORIZON TO DETERMINE IF IN FACT WE WERE LEVEL WITH THIS ACFT OR NOT. SO I DISCONNECTED THE AUTOPLT AND STARTED A CLB, HOPING TO SEE THE OTHER ACFT DSND IN THE WINDSCREEN. IT DID, CONFIRMING THAT WE WERE AT THE SAME ALT. WE CLBED 500 FT ABOVE OUR ASSIGNED ALT, AND PASSED DIRECTLY OVER THE OTHER ACFT. I ESTIMATE THE OTHER ACFT HDG TO BE ABOUT 300 DEGS. OUR MISS DISTANCE WAS ABOUT 300-400 FT VERTICALLY. IF I HAD NOT TAKEN EVASIVE ACTION, THERE IS NO DOUBT IN MY MIND THAT WE WOULD HAVE COLLIDED. WE IMMEDIATELY RPTED OUR ALTDEV TO ZMA WHO SAID THAT THE ONLY TFC NEARBY WAS RPTED TO BE 2000 FT BELOW US. AFTER LNDG AT PBI, I CALLED ZMA, WHO HAD STARTED AN INVESTIGATION. THE TYPE OF AIRPLANE WE ENCOUNTERED APPEARED TO BE A LIGHT TWIN, SILVER AND BLUE IN COLOR, WITH WINGLETS. IT WAS MOST LIKELY A PIPER NAVAJO OR SIMILAR ACFT. WHEN I STARTED MY EVASIVE MANEUVER, THE OTHER AIRPLANE WAS RAPIDLY GETTING LARGER, AND THE ANALYSIS I DESCRIBED ABOVE WAS DONE IN JUST A FEW SHORT SECONDS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.