Date: 201605
Local time of day: 1201-1800
Location: CMH.TRACON, OH
Light: Daylight
Aircraft: Medium Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C DAY
ATC / advisory: TRACON CMH
Second aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Second reporter function: Pilot Flying; Captain
Anomaly: Conflict NMAC
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Contributing factors: Airspace Structure; Human Factors; Procedure
Primary problem: Airspace Structure

Narrative: I was working Middletown radar sector; and it was combined at the time with Xenia sector and Urbana sector. Aircraft X was an arrival from the south and I was vectoring him northbound for a visual approach to DAY. As Aircraft X was northbound about 8 miles east of DAY I turned him to a 270 heading to intercept the localizer. After my transmission; someone keyed up without using a callsign and just said 'Hang on'. The read back sounded like multiple transmissions were made over top of each other so I repeated my instructions to Aircraft X and he read back the instructions but also advised me that he saw traffic. He just said something to the effect of 'We have traffic out here about 1;000 feet.' I saw traffic after Aircraft X turned to the localizer; it was a primary radar return with no transponder. I asked Aircraft X to verify the traffic was 1;000 feet below him. Aircraft X said traffic was closer than 1;000 feet. I asked Aircraft X to say the estimated altitude of the traffic. Aircraft X said the traffic was at about 2;500. At the area the traffic was at the time; he was under the edge of DAY's class C airspace shelf; which is a minimum altitude of 2;400 feet between 5 and 10 miles from the airport. Aircraft X was still descending at the time; and I saw his altitude at 3;200 when he reported the traffic. I cleared Aircraft X for the approach and then switched him to DAY tower; and then just after I switched him I tried to catch him just to ask him if he had to do any evasive maneuvers for the traffic. Aircraft X had already switched; so I called DAY tower to have the pilot call us so we could ask him about the traffic. DAY local controller said he would ask; and also have the pilot call us. A minute or two later DAY local controller called and told me that Aircraft X reported that he did not have to make any maneuvers to miss the traffic. Aircraft X called later and assured the supervisor he did not wish to report any NMAC. So since I didn't notice any maneuvers on the scope from Aircraft X; the traffic was reported as at least 500 feet below Aircraft X at the time; and he also advised DAY he had to make no maneuvers to miss traffic; I felt it was not a significantly unsafe situation.The next day I was made aware that DAY had filed a report describing the situation as a Significant Event; and according to the report that the pilot told DAY he DID have to turn off his auto pilot and turn to miss the aircraft. That information was never passed to me from either the pilot or the DAY local controller. At some point the story seems to have changed. However; according to the data available and after listening to the recordings again; I saw nothing that would point to a NMAC or maneuvers to miss traffic. The recordings between myself and the pilot; as well as myself and DAY tower; also seemed to indicate the VFR traffic was at least 500 feet below Aircraft X; and at an altitude would even put the VFR aircraft below the Class C shelf. The traffic appeared to have just departed the New Carlisle; about 6 miles north of Aircraft X located about 8 miles east of DAY. I was actually coordinating and vectoring a few other aircraft at the time; and I did not see the VFR target pop up.With all the information available; it seems as though everyone was legally where they should have been at the time. There is also a LOA (Letter of Agreement) for a flying club for that area that spells out altitudes and restrictions when flying in that area; although I do not know if the VFR is part of that flying club or not. I would say the only recommendation would be for me to have seen the traffic sooner and call the traffic to Aircraft X. It seems like Aircraft X was spooked by seeing traffic he wasn't expecting. Also if the traffic was climbing to an altitude that would be 500 feet below Aircraft X; as Aircraft X was descending; I can see how traffic could surprise the pilot and look as though it is going to continue climbing. There is also thepossibility the VFR pilot had violated Class C airspace and was higher than Aircraft X reported; and if that was the case then I would recommend education for the VFR pilot; although he was low altitude with no transponder and we lost radar on him as he flew further northeast. So we have no way of knowing who the VFR pilot was. Another recommendation would be possibly something published on VFR charts to advise pilots in the area that multiple airliners are expected in the area descending to 3;000 feet and to use caution; if something similar is not already published there.


Second narrative: While being vectored on the downwind to runway 24L for a visual approach in DAY at 3000 feet we were getting ready for our base turn and I was looking out the left side window keeping the runway in sight; the First Officer announced 'airplane!' I looked back towards the front of the aircraft and saw a light colored high wing fabric tail dragger filling most of the windscreen on the right side of the aircraft. He was in a nose high attitude and appeared to be climbing into our flight path. At this time I switch off the automation and began an immediate left turn towards the final for runway 24L in Dayton.At some point shortly after I started the turn ATC issued us a turn to base and a descent and were told to 'standby.' I was not concerned at the time with ATC instructions and wanted to create more space between myself and the second aircraft. After it was clear that the other aircraft was no longer a threat I complied with the ATC instructions which happened to be identical to the direction that I chose to fly to avoid the other aircraft; I asked the controller where in the world this other aircraft had come from. He seemed puzzled by the question and then said that he could now see a primary only target; my guess being that it was no longer masked by our target on his screen.We joined the final and landed at DAY without further issue. On the taxi in Dayton control tower provided me with a phone number if I wanted to talk to Columbus approach. I called the number before I drove home after the trip and spoke to the manager there; and I cannot at this time recall his name. He spoke at length about what had happened and both concluded that there was no errors per se that caused this to happen. He stated that they have had issues before with NORDO aircraft departing a little satellite airport and skirting out under the airspace before. I asked about a 30 mile 'mode c' ring that I remembered being around airports the size of Dayton and asked if that was still a thing; as it has been many years since I have looked at any VFR Sectional charts. He indicated that there was no such ring around Dayton. Hole in ATC System as a whole; coupled with a NORDO aircraft departing from a smaller airport; and our flight being extended on a downwind due to another arriving aircraft from the north.I do not want to attempt to place numbers on how close to the other aircraft we were; but I will say that it was uncomfortable at best and I was not willing to allow my aircraft to get any closer than we already were; so I took action to not allow the situation to degrade any further than what it had. I do not feel that there were any errors committed by ATC; our aircraft; or the second aircraft; Its simple a 'hole in the system' I know that the FAA/ATC at different smaller airports that are in close proximity to other high traffic area will develop Notices or flyers to be placed at the smaller field pointing out where the faster higher volume traffic is coming from and to draw attention to it.


Synopsis: An air carrier being vectored onto the final approach to DAY took evasive action to avoid a VFR aircraft not displaying a transponder or in communication with TRACON.