Narrative:

The incident flight was from phl to lns and return. We had slowed to 190 KIAS. Turbulence was continuous light chop. As we crossed 10 mi northwest of mxe VORTAC and were outbound on the 295 degree radial, we were advised by phl departure to change to mdt approach. After checking in with mdt, we were informed of traffic at '1 O'clock 5500 ft, descending.' we intensified our scan and checked our 1 O'clock. Negative contact. Approximately 1 min later we were informed traffic now as 12-1 O'clock 4500 ft descending 2 mi sebound. Since west were in a northwest direction we were closing in pretty fast on our traffic. Mdt approach cleared us to 3000 ft and asked us to expedite our descent. Still no contact. As were passing through 3600 ft I saw a cessna (152 or 172 type) pass underneath us. Had we been 100-150 ft lower when passing, a collision would have been unavoidable. My first officer was the PF, but I immediately grabbed the yoke and stopped the descent since I saw traffic first. Time frame from the time I saw traffic until he was gone was 2 seconds tops. I asked mdt approach if the traffic passing under us was the traffic we were supposed to look for since our traffic reportedly was higher than us. He confirmed that indeed that was our traffic and he seemed surprised when I said that the traffic passed under instead of over us. He also verified the cessna's altitude of being 3500 ft. I told the controller that I had no choice but to file a report on this near hit. Remainder of flight proceeded without incident and arrival and landing in lns was uneventful. I don't know where the system failed but it was either the cessna's mode C who was giving the controller false readings or he was descending at a rate that was excessive and the controllers equipment couldn't keep up. Incident occurred in class east airspace. Had we had a TCASII onboard we might have been more enlightened of the situation. If separation requirements between IFR and VFR traffic in VMC was stricter this incident would never have happened. The blame was no ones, really. I am just glad that we didn't have a collision this time.

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Original NASA ASRS Text

Title: NMAC. CONFLICT WITH VFR ACFT ON DSCNT. UNDER ATC CTL AND COM.

Narrative: THE INCIDENT FLT WAS FROM PHL TO LNS AND RETURN. WE HAD SLOWED TO 190 KIAS. TURB WAS CONTINUOUS LIGHT CHOP. AS WE CROSSED 10 MI NW OF MXE VORTAC AND WERE OUTBOUND ON THE 295 DEG RADIAL, WE WERE ADVISED BY PHL DEP TO CHANGE TO MDT APCH. AFTER CHKING IN WITH MDT, WE WERE INFORMED OF TFC AT '1 O'CLOCK 5500 FT, DSNDING.' WE INTENSIFIED OUR SCAN AND CHKED OUR 1 O'CLOCK. NEGATIVE CONTACT. APPROX 1 MIN LATER WE WERE INFORMED TFC NOW AS 12-1 O'CLOCK 4500 FT DSNDING 2 MI SEBOUND. SINCE W WERE IN A NW DIRECTION WE WERE CLOSING IN PRETTY FAST ON OUR TFC. MDT APCH CLRED US TO 3000 FT AND ASKED US TO EXPEDITE OUR DSCNT. STILL NO CONTACT. AS WERE PASSING THROUGH 3600 FT I SAW A CESSNA (152 OR 172 TYPE) PASS UNDERNEATH US. HAD WE BEEN 100-150 FT LOWER WHEN PASSING, A COLLISION WOULD HAVE BEEN UNAVOIDABLE. MY FO WAS THE PF, BUT I IMMEDIATELY GRABBED THE YOKE AND STOPPED THE DSCNT SINCE I SAW TFC FIRST. TIME FRAME FROM THE TIME I SAW TFC UNTIL HE WAS GONE WAS 2 SECONDS TOPS. I ASKED MDT APCH IF THE TFC PASSING UNDER US WAS THE TFC WE WERE SUPPOSED TO LOOK FOR SINCE OUR TFC REPORTEDLY WAS HIGHER THAN US. HE CONFIRMED THAT INDEED THAT WAS OUR TFC AND HE SEEMED SURPRISED WHEN I SAID THAT THE TFC PASSED UNDER INSTEAD OF OVER US. HE ALSO VERIFIED THE CESSNA'S ALT OF BEING 3500 FT. I TOLD THE CTLR THAT I HAD NO CHOICE BUT TO FILE A RPT ON THIS NEAR HIT. REMAINDER OF FLT PROCEEDED WITHOUT INCIDENT AND ARR AND LNDG IN LNS WAS UNEVENTFUL. I DON'T KNOW WHERE THE SYS FAILED BUT IT WAS EITHER THE CESSNA'S MODE C WHO WAS GIVING THE CTLR FALSE READINGS OR HE WAS DSNDING AT A RATE THAT WAS EXCESSIVE AND THE CTLRS EQUIP COULDN'T KEEP UP. INCIDENT OCCURRED IN CLASS E AIRSPACE. HAD WE HAD A TCASII ONBOARD WE MIGHT HAVE BEEN MORE ENLIGHTENED OF THE SIT. IF SEPARATION REQUIREMENTS BTWN IFR AND VFR TFC IN VMC WAS STRICTER THIS INCIDENT WOULD NEVER HAVE HAPPENED. THE BLAME WAS NO ONES, REALLY. I AM JUST GLAD THAT WE DIDN'T HAVE A COLLISION THIS TIME.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.