CLOSE PROX ACR-MLG GA-SMA IN MDT TRSA.

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I WAS FLYING AT 4000 FT MSL RECEIVING VECTORS FROM HARRISBURG APCH FOR AN ILS TO RWY 31 AT HARRISBURG INTL ARPT. WE WERE IN AND OUT OF LOW CUMULUS CLOUDS; WITH DENSE HAZE. WE WERE ON A NW TO N VECTOR 8-9 MI SE OF ARPT AND INSTRUCTED TO DSND TO 3500 FT. ON AUTOPLT; I SELECTED 1000 FPM VERT VELOCITY. AT APPROX 3700 FT; APCH CALLED WITH 'POP UP' TFC LESS THAN A MI AT 3500 FT. RETURN TO 4000 FT. THE URGENCY IN HIS VOICE WAS OBVIOUS. I SELECTED A CLB ON THE AUTOPLT IMMEDIATELY BUT INSTANTLY REALIZED THAT IT WOULD NOT RESPOND ENOUGH. I GRABBED THE YOKE AND PULLED DISENGAGING THE AUTOPLT. AS WE BEGAN THE CLB; A LIGHT ACFT PASSED DIRECTLY UNDERNEATH US FROM R TO L. COPLT ESTIMATES 100-200 FT. HAD I NOT OVERRODE THE AUTOPLT; WE WOULD HAVE COLLIDED. I CALLED APCH CTL ON THE GND AND THANKED THEM FOR THEIR QUICK ACTION. TO MY KNOWLEDGE; THE LIGHT ACFT WAS NOT TALKING TO ANYONE; WHICH I THINK HE SHOULD HAVE BEEN; ESPECIALLY CONSIDERING THE MARGINAL WX. HAVING RECENTLY STARTED FLYING INTO HARRISBURG AND ALLENTOWN; I CONSIDER THAT AREA EXTREMELY HAZARDOUS DUE TO THE LIGHT ACFT TFC AND THE SEEMINGLY UNPROFESSIONAL MANNER IN WHICH THEY COM ON THE RADIOS. JUST PRIOR TO OUR NEAR MISS; I HAD REQUESTED IMMEDIATE VECTORS FROM APCH FOR CLRNC AROUND ANOTHER LIGHT ACFT WHO APPARENTLY HAD AN IMPROPER ALTIMETER SETTING AND WAS TO PASS BENEATH US AT 4500 FT WITH US AT 5000 FT. HIS ALT READOUT INDICATED 5000 FT. HIS RADIO TERMINOLOGY INCLUDED NUMEROUS CB PHRASEOLOGY. IT WOULD BE INTERESTING FOR YOU TO REVIEW THE TAPES OF BOTH THE INITIAL APCH CTL AND THE FINAL CTLR WHEN WE HAD OUR NEAR MISS.