Narrative:

Myself (acting as CFI) and a student were on an IFR flight plan initiating from ZZZ inbound on the RNAV 31L approach at hio with instructions to circle to land runway 31R. I believe we were in the descent from 4;000 to 2;000 on course between nnike and intll in VMC when the portland approach controller called out VFR traffic to our left; approximately same altitude and traveling same direction. I got the traffic in sight; and informed the controller. The aircraft appeared to be a high wing cessna and they seemed to be slightly diverging; and we continued the approach as normal. A few seconds later I noticed the aircraft begin to make a left turn as if he were heading southbound away from our track; and at this time the approach controller advised he thought the traffic was turning away; no factor; and to contact hillsboro tower; I then noticed that the aircraft was actually making what appeared to be a left 270 degree turn which would have put it on a 90 degree intercept straight into our left side slightly higher and confirmed this on our adsb screen; since I could not see the aircraft at this point due to it being slightly higher and our wing blocking view; I immediately took controls from my student and decided to take evasive action by pulling power to idle and initiating a rapid descent to approx 1200 feet MSL (tpa at hio) until I felt we were clear of the conflict. Hillsboro tower did not call out the traffic in my call to them and I then asked if they were in contact with that aircraft that they were 'way too close' (less than 500 ft horizontal separation in my approximation) and they said 'roger' and asked us to call when we were on the ground. We; at this point; we'll below glide path for the approach; continued a normal visual approach and landing to runway 31R and after flight contacted hio tower and gave them my information and they said they were going to file a report and would be in further contact with me if need be.

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Original NASA ASRS Text

Title: Instructor pilot reported an NMAC during approach to HIO airport.

Narrative: Myself (acting as CFI) and a student were on an IFR flight plan initiating from ZZZ inbound on the RNAV 31L Approach at HIO with instructions to circle to land Runway 31R. I believe we were in the descent from 4;000 to 2;000 on course between NNIKE and INTLL in VMC when the Portland Approach Controller called out VFR traffic to our left; approximately same altitude and traveling same direction. I got the traffic in sight; and informed the Controller. The aircraft appeared to be a high wing Cessna and they seemed to be slightly diverging; and we continued the approach as normal. A few seconds later I noticed the aircraft begin to make a left turn as if he were heading southbound away from our track; and at this time the Approach Controller advised he thought the traffic was turning away; no factor; and to contact Hillsboro Tower; I then noticed that the aircraft was actually making what appeared to be a left 270 degree turn which would have put it on a 90 degree intercept straight into our left side slightly higher and confirmed this on our ADSB screen; since I could not see the aircraft at this point due to it being slightly higher and our wing blocking view; I immediately took controls from my student and decided to take evasive action by pulling power to idle and initiating a rapid descent to approx 1200 feet MSL (TPA at HIO) until I felt we were clear of the conflict. Hillsboro Tower did not call out the traffic in my call to them and I then asked if they were in contact with that aircraft that they were 'Way too close' (less than 500 ft horizontal separation in my approximation) and they said 'roger' and asked us to call when we were on the ground. We; at this point; we'll below glide path for the approach; continued a normal visual approach and landing to Runway 31R and after flight contacted HIO Tower and gave them my information and they said they were going to file a report and would be in further contact with me if need be.

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.