TCASI RA DURING CLB. RPTR STOPPED CLB WHEN OBSERVING TCAS TFC. IT WAS A DC9 BEING VECTORED BY DEP CTL. OPERROR PUT ACFT TOO CLOSE AND TCASI PREVENTED THEM BEING CLOSER. RPTR CAPT WOULD LIKE TCASI TO PROVIDE MORE TFC READOUTS AND AT GREATER DISTANCES. RPTR SAYS TCASI USUALLY WILL ALERT ONLY ON IMMEDIATE THREATS WITHIN 2 MI.

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DEPARTED PDK ON VECTORS. CLRED TO CLB TO 9000 FT. PASSING 8300 FT GOT A 'TA' AND SAW TARGET AT 8700 FT ON MY L. I STOPPED CLB AT 8500 FT AND DC9 TYPE PASSED OVERHEAD. CTLR ALSO TOLD ME TO STOP CLB; BUT I ALREADY HAD. I DID NOT SEE TARGET UNTIL IT WAS CLOSE. THIS IS A FREQUENT PROB THAT IS CAUSED BY 'SELECTIVITY' PROGRAMMED INTO SET. I WASN'T TOLD OF THIS DURING TRAINING; BUT LEARNED IT THROUGH COMPLAINING TO TECHNICAL PEOPLE AT CESSNA. EQUIP IS BENDIX/KING/ALLIED SIGNAL TCASI. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT SAYS THAT 'SELECTIVITY' REFERS TO THE FAA CONCERNS OF OVER SATURATION OF THE TCAS SYS AND HAS POTENTIAL FOR DISRUPTING THEIR CAPABILITY. CONSEQUENTLY; THE SOFTWARE FOR TCASI WAS DEVELOPED TO ALERT ONLY IN CLOSE ENCOUNTERS AND NOT AS BROAD COVERAGE AS TCASII. RPTR SAYS THAT GENERALLY TCASI WON'T ALERT ON TFC MORE THAN 2 MI; BUT IF THERE IS VERY LIGHT TFC AND NOT MUCH INTERROGATIONS; IT IS POSSIBLE TO SEE TFC FARTHER AWAY. IF THE TFC IS PARALLELING THE TCASI ACFT; IT MAY DROP IT AS A THREAT; BUT IF THAT TFC TURNS AND CLOSES THE DISTANCE; THEN TCASI WILL BRING IT BACK. IN THIS CASE THE CTLR TURNED THE INTRUDER INTO THE RPTR ACFT AND; HAD THE RPTR NOT TAKEN EVASIVE ACTION BY LEVELING UNTIL CLR OF TFC; IT COULD HAVE BEEN A VERY NEAR MIDAIR.