TCASII RA.

Narrative

THERE WAS RAIN IN THE AREA. CEILING WAS ABOUT 2000 FT WITH VISIBILITY GOOD BELOW THE CEILING. NEW ORLEANS APCH CTL WAS VECTORING US FOR A RWY 19 APCH. WE WERE AT 1500 FT ON A 280 DEG HDG; W OF THE BRIDGE AND E OF THE APCH COURSE. ANOTHER B-737 WAS AHEAD OF US FOR RWY 19. MY FO SAW A COMMUTER COMING FROM OUR R ON AN INTERCEPTING COURSE. HE WAS EITHER AT OUR ALT OR DSNDING TO OUR ALT. AS I RECEIVED AN RA TO DSND; I DSNDED TO 1000 FT AND THE COMMUTER PASSED OVERHEAD; MISSING US BY ABOUT 600 FT. WE INFORMED APCH CTL OF OUR EVASIVE MANEUVER AND COMPLETED THE APCH WITHOUT FURTHER INCIDENT. THE APCH CTL SUPVR TOLD ME THAT HE THOUGHT THE COMMUTER MISIDENTED THE B-737 IN FRONT OF US AS THE ACFT HE WAS TO FOLLOW. APCH CTL HAD INTENDED HIM TO FOLLOW US. THE APCH CTL SUPVR WAS CONCERNED THAT THE APCH CTLR MAY HAVE GIVEN INCOMPLETE INSTRUCTIONS TO THE COMMUTER. TCASII AND ALERT ACTION ON OUR PART PREVENTED A COLLISION. SUPPLEMENTAL INFO FROM ACN 278239: WE WERE BEING VECTORED N OF THE ARPT AROUND A TSTM WHICH WAS ALONG THE APCH COURSE OF THE LOC BC RWY 19. OUR POS WAS JUST UNDER THE LOWEST LAYER OF CLOUDS N OF THE TSTM. THE CTLR ASKED US IF WE HAD OUR TFC TO FOLLOW AT 2 O'CLOCK AND SOME MI (I DO NOT RECALL THE DISTANCE AT THIS TIME). WE BOTH SAW TFC AT OUR 2 O'CLOCK POS AND RPTED 'TFC IN SIGHT.' WE WERE THEN CLRED TO FOLLOW THAT ACFT TO THE RWY AND CLRED FOR THE VISUAL APCH TO RWY 19. AS WE GOT CLOSER WE WERE INSTRUCTED OUR TFC WAS SQUARING THE TURN TO FINAL. AT THAT TIME; WE NOTICED AN ACFT OFF THE L 9 O'CLOCK POS WHICH WAS ABOUT 500 FT BELOW AND 1/2 MI AWAY. WE TOLD THE CTLR WE WERE FOLLOWING THE ACFT ON FINAL APCH. HE TOLD US WE WERE SUPPOSED TO BE FOLLOWING THE ACFT AT OUR 9 O'CLOCK; WHO WAS NOT BEHIND US AT 1 MI. WE HAD A NEAR MISS WITH A BOEING 737 TYPE WHILE FOLLOWING THE WRONG ACFT TO THE RWY. THIS WAS NOT AN INTENTIONAL EVENT -- WE THOUGHT WE WERE FOLLOWING THE RIGHT ACFT!!