Narrative:

Aircraft X checked in with me at 10;000 feet and reported severe turbulence. Aircraft X due to lower performance was taken out of the typical arrival procedure and was being handled the way we typically handle lower performance bfi arrivals in that traffic flow configuration. I had gotten PIREPS on departure aircraft reporting moderate turbulence starting at 050. Aircraft X was given a more northerly turn to avoid conflictions with sea and bfi departures. The pilot said that he didn't feel comfortable with that turn because he wanted to get out of the cloud and felt that a northerly turn would put him in worse turbulence. I decided to keep him on his present heading and descend him as quickly as I could below 050 reference terrain so that when the turbulence got better I could then turn him north again 5NM east of sea. I stopped auto turns for sea departures heading towards where he would be. As the pilot descended below 050 he reported that he was still getting the turbulence and complained of back and head injury. I descended him lower to 030 to see if that would help. As the pilot got close to 5NM east of sea a departure (aircraft Y) aircraft requested higher; I looked north to see where another aircraft; a bfi departure; and conflicting traffic for this sea departure was and his altitude and climbed the sea departure. As I did that; aircraft X had gotten within 5NM of sea and the departure course. I caught it and turned him to a heading 340. The pilot read the turn back correctly. Several radar sweeps went by and the pilot then asked if I wanted him on heading 340. I replied 'yes turn heading 340 immediately'. By the time he was on a northerly heading he was abeam the south end of sea about 1.5-2NM east of the airport still a bit northwest bound. At that time aircraft Z was tagging up and I began to call traffic to aircraft X who got him in sight and was instructed to maintain the visual and wake turbulence advisory. However it was too late and a loss had occurred. I should not have responded to aircraft Y requesting higher; or at least just responded with 'on request' in order to issue the turn to aircraft X at the best possible time. After I issued the turn I did not observe the aircraft turning but he read the turn back so I believed when I didn't see the turn happening it was due to a radar glitch that we get that will show an AC continuing on their last heading then after several sweeps will be suddenly on course and the targets potion updated. Or the glitch will show an aircraft suddenly in a rapid turn and then several sweeps later be corrected. I believed that this is what was happening until the pilot came back after several sweeps and wanted to know if I wanted him heading 340.

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Original NASA ASRS Text

Title: S46 Controller reported of a loss of separation due to a distraction and the pilot of the aircraft not flying the correct heading.

Narrative: Aircraft X checked in with me at 10;000 feet and reported severe turbulence. Aircraft X due to lower performance was taken out of the typical arrival procedure and was being handled the way we typically handle lower performance BFI arrivals in that traffic flow configuration. I had gotten PIREPS on departure aircraft reporting moderate turbulence starting at 050. Aircraft X was given a more northerly turn to avoid conflictions with SEA and BFI departures. The pilot said that he didn't feel comfortable with that turn because he wanted to get out of the cloud and felt that a northerly turn would put him in worse turbulence. I decided to keep him on his present heading and descend him as quickly as I could below 050 reference terrain so that when the turbulence got better I could then turn him north again 5NM east of SEA. I stopped auto turns for SEA departures heading towards where he would be. As the pilot descended below 050 he reported that he was still getting the turbulence and complained of back and head injury. I descended him lower to 030 to see if that would help. As the pilot got close to 5NM east of SEA a departure (Aircraft Y) aircraft requested higher; I looked north to see where another aircraft; a BFI departure; and conflicting traffic for this SEA departure was and his altitude and climbed the SEA departure. As I did that; Aircraft X had gotten within 5NM of SEA and the departure course. I caught it and turned him to a HDG 340. The pilot read the turn back correctly. Several radar sweeps went by and the pilot then asked if I wanted him on HDG 340. I replied 'yes turn HDG 340 immediately'. By the time he was on a northerly heading he was abeam the south end of SEA about 1.5-2NM east of the airport still a bit northwest bound. At that time Aircraft Z was tagging up and I began to call traffic to Aircraft X who got him in sight and was instructed to maintain the visual and wake turbulence advisory. However it was too late and a loss had occurred. I should not have responded to Aircraft Y requesting higher; or at least just responded with 'on request' in order to issue the turn to Aircraft X at the best possible time. After I issued the turn I did not observe the aircraft turning but he read the turn back so I believed when I didn't see the turn happening it was due to a radar glitch that we get that will show an AC continuing on their last heading then after several sweeps will be suddenly on course and the targets potion updated. Or the glitch will show an aircraft suddenly in a rapid turn and then several sweeps later be corrected. I believed that this is what was happening until the pilot came back after several sweeps and wanted to know if I wanted him HDG 340.

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.