NMAC BTWN AN LGT ON A NOISE ABATEMENT DEP CLB; WHO STOPPED CLBING AND DSNDED IN RESPONSE TO A TCASII RA; AND AN UNKNOWN ACFT THAT JUST POPPED UP ON ATC RADAR. THE SEE AND AVOID IS STILL APPLICABLE SINCE ANOTHER ACFT COULD POP UP ILLEGALLY AT ANY TIME AND TURN ON ITS XPONDER.

Narrative

N DEP OUT OF COS WITH TURN TO 100 DEGS. JUST FINISHED CLEANUP; BUT STILL IN NOISE ABATEMENT (REDUCED PWR) CLB. TCASII TA OF ACFT ABOUT 1 O'CLOCK; 3 MI; DSNDING AND CONVERGING. SHORTLY FOLLOWED BY ATC RPTING 'JUST POPPED UP' TFC. WE VISUALLY ACQUIRED TFC JUST PRIOR TO TCASII RA OF 'DSND.' WE COMPLIED WITH RA (AND OUR GUT REACTION). WHITE BONANZA WITH BLUE STRIPE PASSED DIRECTLY ABOVE US IN OUR WINDSCREEN. NO INDICATION HE OR SHE EVER SAW US. WITHOUT TCASII WE PROBABLY WOULD NOT HAVE SEEN TFC AND ALMOST CERTAINLY WOULD HAVE HAD A MIDAIR. GLAD WE WERE NOT FLYING A CARGO FLT WITHOUT TCASII. COMMENTS: HOW DOES AN ACFT SUDDENLY 'POP UP' ON CTLR'S SCREEN -- SO CLOSE TO THE ARPT; AND IN A DSCNT? I KNOW CTLRS ARE NOT REQUIRED TO SEPARATE FROM UNKNOWN VFR TFC; BUT A RADAR VECTOR OUT OF HARM'S WAY WOULD SURE BE APPRECIATED; ESPECIALLY SINCE TCASII IS STRICTLY A VERT TOOK AT THIS POINT. COS HAS NUMEROUS VFR TRAINING FLTS JUST E OF THE ARPT. WITH THE EVER INCREASING AMOUNT OF AIRLINE SVC IN AND OUT OF COS; SOMETHING NEEDS TO BE DONE ABOUT SEPARATING THE 2 (IE; DEFINED PRACTICE AREAS; SIDS OR STARS). I DOUBT THIS IS AN ISOLATED INCIDENT AND EVEN WITH TCASII; SEE AND AVOID IS A LAST DITCH -- UNRELIABLE METHOD OF KEEPING ACFT APART.