Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 2
Situational Awareness
ATC Issue All Types; Conflict NMAC
Air Traffic Control Separated Traffic; Air Traffic Control Issued Advisory / Alert
Manuals; Procedure; Human Factors; Environment - Non Weather Related
Aircraft Y was on final for Runway 10R (our south runway); talking to Local Control 2. I had Aircraft X with a westbound transition request to follow the Columbia River; opposite direction to our Runway 10L/R arrivals. Aircraft X's request is a routine request that usually occurs at 800 feet. On this day; the Pearson Airport; which is three miles west of PDX; right under our final; was unusually busy. I told Aircraft X that he either needed to divert to the north of Pearson for all that traffic; or climb above it. He elected to climb to 1100 feet for the westbound transition. When Aircraft X was approximately two miles west of PDX; I gave him the Aircraft Y traffic; who was now inside a 4 mile final for Runway 10R. He said he was looking. I immediately went back to him and told him to turn right 30 degrees for traffic (my concern in a more severe northerly turn; is that it would put him right on all this Pearson traffic). Aircraft X immediately came back to me and told me that he had Aircraft Y in sight. I told him to remain north of that traffic and to maintain visual separation. On the other side at local issued the Aircraft X traffic to Aircraft Y and told him that he was maintaining visual separation. Aircraft Y said he was responding to an RA and that Aircraft X was 'way too close.'This event is being brought to our attention because the region quality control staff (not the pilot) decided to file an NMAC. While this situation was not ideal; traffic advisories and visual separation had been successful. I followed the parameters of the 7110.65. I'd recommend that the region contact the pilot of Aircraft X and review the 'safe distance' to another aircraft while applying visual separation.
PDX Tower Controller reported instructing a VFR pilot to maintain visual separation from an arriving aircraft on approach. The pilot acknowledged the instruction; but the resulting conflict was logged as an NMAC.