FLC OF LGT RESPONDED TO TCASII RA. OBSERVED TARGET OF INTRUDER ACFT PASS 800 FT BELOW AFTER 500 FT CLB TO AVOID. LATE TA.

Narrative

WAS LEVEL AT 12000 MSL INBOUND TO DTW; BTWN LAYERS WITH GOOD VISIBILITY. ACFT WAS TCASII EQUIPPED AND IT WAS ON AND FUNCTIONING. AT APPROX THE ABOVE NAMED FIX; WE RECEIVED A TCASII TA. WE SWITCHED TO 10 NM SCALE AND NOTICED ACFT LIGHTS AT OUR 2 O'CLOCK POS. DUE TO NIGHT LIGHTING CONDITIONS IT WAS HARD TO DETERMINE HIS EXACT ALT AND DIRECTION OF FLT. WE THEN RECEIVED A TCASII RA 'CLB; CLB NOW' COMMAND AND I PLACED THE VERT SPD INDICATOR IN THE GREEN ARC THAT WAS INDICATED. THE OTHER CREW MEMBERS NOTED THAT HIS ALT WAS NOW 11700 MSL. WE REACHED 12500 WHEN THE VERT SPD INDICATOR GREEN ARC INDICATED A DSCNT AND I RETURNED THE ACFT TO ITS ASSIGNED ALT. WHEN I INITIATED THE RA ESCAPE MANEUVER; MY FO (ANOTHER CAPT WHO IS A LINE CHK AIRMAN AND WAS GIVING ME MY INITIAL OPERATING EXPERIENCE AS A CAPT NEW TO THIS ACFT) ADVISED ATC OF WHAT WE WERE DOING. THE CTLR THEN ADVISED US OF THE INTRUDING TFC -- THE FIRST AND ONLY ADVISORY WE RECEIVED. WHEN ASKED AGAIN ABOUT HOW LONG THE TFC HAD BEEN ON HIS SCOPE; HE ANSWERED 'I DON'T KNOW.' THE TCASII ALERTS AND COMMANDS WERE BOTH TIMELY AND ACCURATE. THE ESCAPE MANEUVER DIRECTED WAS FOLLOWED IN A SMOOTH MANNER SUCH THAT THE COCKPIT CREW WERE THE ONLY PEOPLE ON THE ACFT AWARE OF THE PROBLEM. HAD WE NOT HAD THE SYS ON BOARD; OUR VERT SEPARATION WOULD HAVE BEEN LESS THAN 300 FT AND; QUITE POSSIBLY; SOMEONE ELSE WOULD NOW BE WRITING A MUCH DIFFERENT RPT FOR THE FRONT PAGE OF A NEWSPAPER. SIGN ME A TCASII BELIEVER.