Narrative:

I was cleared by pittsburgh approach for a left base visual approach to agc when approximately 10 miles directly south of agc. I flew a course of approximately 020 to position myself for the left base and began a descent from 4;000 feet to tpa of 2;250 feet. I contacted agc tower and advised of the visual approach.when approximately 6 miles out; I saw an aircraft out of the corner of my eye at about my 2 to 3 o'clock position. The aircraft was at my altitude and flying a westerly/south westerly course. I would estimate that the aircraft was within 300 feet horizontally and at my same altitude.I immediately dove the airplane to create vertical separation. I would estimate that we descended 100 to 200 feet quickly.the aircraft passed over the top of us and took no evasive action.this incident took place in an odd location in the airspace system: aircraft on an IFR flight plan; in visual conditions; just having been handed off from approach to tower; but still outside (I believe) of agc class D airspace. At this point; it seems that no one was particularly watching: approach had handed off; tower had me outside of the class D; no one in the ATC world had responsibility for separating me (IFR) from VFR traffic. I was focused on the approach and expecting; perhaps; traffic advisories as I was on an IFR flight plan.lesson: pay particular attention while on approach at the seam of controlled airspace where planes may be flying to avoid having to talk to class D controllers. Lesson 2: get ads-B into my airplane.

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Original NASA ASRS Text

Title: PA32 pilot reported being on an IFR flight plan and experiencing a NMAC with another aircraft 6 NM south of AGC during descent. The other aircraft did not deviate while the reporter made an evasive descent. The reporter believed that the Tower should have advised him of the traffic.

Narrative: I was cleared by Pittsburgh Approach for a left base visual approach to AGC when approximately 10 miles directly south of AGC. I flew a course of approximately 020 to position myself for the left base and began a descent from 4;000 feet to TPA of 2;250 feet. I contacted AGC Tower and advised of the visual approach.When approximately 6 miles out; I saw an aircraft out of the corner of my eye at about my 2 to 3 o'clock position. The aircraft was at my altitude and flying a westerly/south westerly course. I would estimate that the aircraft was within 300 feet horizontally and at my same altitude.I immediately dove the airplane to create vertical separation. I would estimate that we descended 100 to 200 feet quickly.The aircraft passed over the top of us and took no evasive action.This incident took place in an odd location in the airspace system: aircraft on an IFR flight plan; in visual conditions; just having been handed off from Approach to Tower; but still outside (I believe) of AGC Class D airspace. At this point; it seems that no one was particularly watching: Approach had handed off; Tower had me outside of the Class D; no one in the ATC world had responsibility for separating me (IFR) from VFR traffic. I was focused on the approach and expecting; perhaps; traffic advisories as I was on an IFR flight plan.Lesson: pay particular attention while on approach at the seam of controlled airspace where planes may be flying to avoid having to talk to Class D controllers. Lesson 2: get ADS-B into my airplane.

Data retrieved from NASA's ASRS site 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.