NMAC BTWN A PIPER TOMAHAWK AND A C560XL IN THE ARR AREA OF ILM. THE TOMAHAWK WAS NOT DETECTED BY ATC RADAR OR TCASII ON BOARD THE C560XL.

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WHILE ON DSCNT INTO ILM; PASSING THROUGH 3000 FT MSL; BEING VECTORED FOR VISUAL APCH RWY 24; HAD A NEAR MISS WITH A PIPER TOMAHAWK CONVERGING FROM THE 1 O'CLOCK POS. EVASIVE ACTION WAS NECESSARY TO AVOID IMPACT. WE WERE IN A DSCNT AND MISSED THE OTHER ACFT FROM ABOVE. IT WAS CLOSE ENOUGH THAT I WAS ABLE TO SEE THE PLT'S FACE. TCASII WAS IN RA/TA MODE; HE DID NOT APPEAR ON THE SCREEN NOR WAS AN RA TRIGGERED. APCH WAS ADVISED AND THEY DID NOT HAVE THE OTHER AIRPLANE ON RADAR EITHER. WX WAS VFR; DAYTIME; WITH FLT VISIBILITY OF 20+ MI. OUR APPROX HEADING WAS 120 DEGS; OTHER ACFT ESTIMATED HEADING WAS 310 DEGS. THIS IS MY CLOSEST POINT OF POSSIBLE CONTACT WITH ANOTHER ACFT INFLT TO DATE. PREVENTABLE IN THE FUTURE? YES. HOW? MANDATORY USE OF A MODE C OR BETTER XPONDER. WHILE NOT ALL ACFT HAVE TCASII; OURS DID; IT ONLY TAKES 1 ACFT TO TAKE CORRECTIVE ACTION. HOWEVER; IF 1 ACFT HAS NO XPONDER; TCASII IS USELESS; AS IS THE MAJORITY OF THE AIR TFC CTLRS RADAR. I AM AN FO FOR A FRACTIONAL CPR OWNERSHIP COMPANY. I HAVE OVER 4000 HRS OF FLT EXPERIENCE; FROM SMALL CESSNAS TO BOEING 727'S. THIS IS A SIT I FEEL WILL BECOME MORE AND MORE COMMONPLACE WITH THE RISE IN FRACTIONAL ACFT OWNERSHIP. WE FLY INTO MANY UNCTLED FIELDS WHERE THIS TYPE OF SIT IS MOST LIKELY TO OCCUR.