Narrative:

Aircraft X was [enroute] to rdm; from the north. He requested an RNAV Z 23 approach. Aircraft Y was from the east into rdm. He requested direct to a fix; hutki; that's about 10 or 12 miles out from the airport.the weather was pretty good; high layer (110) of scattered clouds; but 10+ visibility.the timing was such that someone had to be first and someone had to be second. The RNAV Z approach has a segment that points downwind before doing a curved segment-to-final; and I figured that would be a good spot to have the aircraft X turn on his lights and let the aircraft Y get him in sight; then do a visual to follow to the airport.I cleared the aircraft Y to 'slow to approach speed for sequencing' (thinking it'd be around 180-200); but in hindsight I don't think he slowed that much (he was a long way out and probably didn't think I meant that much).I turned aircraft Y further away from the airport (towards the north) to get some more spacing; then turned him in towards the airport and had the aircraft X turn on his lights. Aircraft Y reported aircraft X in sight and I cleared him for a visual approach to follow. Aircraft X had also reported the aircraft Y in sight.as aircraft Y proceeded down towards the runway; I became concerned with his speed and how aircraft X's turn to final was going; aircraft X was off course not flying a 'normal' base leg; but instead was on the approach. It was going to be very tight and as aircraft X started turning in; while aircraft Y was still maybe 4-5 miles out; I asked aircraft Y if he would be able to do some s-turns on final to remain behind the aircraft X. He said 'affirmative'.as they got closer I just was not comfortable with the amount of space that aircraft Y would have behind aircraft X. We recently had issues at rdm with a go-around; and while that was in a different situation (leading aircraft on a visual approach did go-around; came off airport; and turned 180 degrees to fly downwind for another shot at it; on this approach; the leading aircraft; aircraft X; would fly basically straight ahead if he missed) I was more fearful that the aircraft Y was just going to overrun aircraft X and be in a position of a destabilized approach; on maybe a 4 to 7 mile final; below the mia (minimum IFR altitude) and maneuvering around on a dark night (granted; good weather; but still definitely well after sunset and I didn't know if there was a moon.(as it turns out; the moon is waxing crescent; and on wednesday night was only 7% visible; so it was likely very dim or almost invisible; during bright moon; with lots of snow on the ground; the terrain is very visible on that side of the redmond airport at night.)I told aircraft Y that I just didn't think there was going to be enough room as the aircraft X turned in front of him; so I told aircraft Y to execute a left 360. I chose left because it's away from higher terrain; the mia on the north and eastern side of his position is 075; and to the south it is 070. He was at about 070 at the time.aircraft Y executed the left 360; but 'he kept descending'. By 180 degrees through his turn he was at 062 and continuing to go down; and I realized that he wasn't exactly on a visual approach and he wasn't on an instrument approach and really wasn't getting any separation from the terrain. I asked him if he had good ground contact (he said he did) and I think I told him that the minimum IFR altitude was 070 in his vicinity.in hindsight; I think this could easily be called a separation error with mia. He was technically maneuvering on a visual-to-follow approach; but by giving him a 360 (and he never did break 3 miles separation from aircraft X because I turned him in time) I don't think that would apply here.I think the single biggest thing would be to not assume that the pilot would level during a 360. In looking back; I'm sure that the pilot had good ground contact the entire time and never got closer than 1000 feet above terrain during the 360. I apologized to him but said that the closure rate was such that I didn't think he would have enough room for a safe landing and would have wound up going around anyway and he said 'no problem' and didn't sound bent out of shape or anything.I'm confident that spinning him was a good move. The closure rate sucked; the aircraft X has a 180 knot maximum speed on that segment of the approach and so he didn't get inbound quickly enough to get good spacing in front of aircraft Y.technically; I could have just let aircraft Y go; and it would be on the pilot to maintain separation on the visual-to-follow; if he couldn't; then he would have to either s-turn or go around. The problem with the s-turn option is that the carriers have put a lot of emphasis lately upon pilots getting nicely stabilized well outside of the airport and flying a nice; stabilized final; screwing around with s-turns and trying to slow while keeping energy state at a good point is pretty tough work during VMC on a sunny day. At night; it's asking for trouble.so spinning him was good.but; next time; once I spin a guy; I will issue a climb to the highest mia that is around; or ask him if a contact approach will work and verify that he can maintain his own terrain/obstruction clearance.I'll also probably only do a 180 degree turn to begin with; so I can see how the altitude will work out with whenever he gets turned back into the airport and issued a visual approach.

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Original NASA ASRS Text

Title: ZSE Controller reports two aircraft joining final with the second aircraft being a little faster; but on a visual approach. The Controller has the second aircraft do a 360 to gain more spacing but does not assign an altitude to maintain. Controller realizes this and asks pilot if he has ground contact and can maintain clearance. Pilot says yes and the aircraft continues into the airport

Narrative: Aircraft X was [enroute] to RDM; from the north. He requested an RNAV Z 23 approach. Aircraft Y was from the east into RDM. He requested direct to a fix; HUTKI; that's about 10 or 12 miles out from the airport.The weather was pretty good; high layer (110) of scattered clouds; but 10+ visibility.The timing was such that someone had to be first and someone had to be second. The RNAV Z approach has a segment that points downwind before doing a curved segment-to-final; and I figured that would be a good spot to have the Aircraft X turn on his lights and let the Aircraft Y get him in sight; then do a visual to follow to the airport.I cleared the Aircraft Y to 'slow to approach speed for sequencing' (thinking it'd be around 180-200); but in hindsight I don't think he slowed that much (he was a long way out and probably didn't think I meant that much).I turned Aircraft Y further away from the airport (towards the north) to get some more spacing; then turned him in towards the airport and had the Aircraft X turn on his lights. Aircraft Y reported Aircraft X in sight and I cleared him for a visual approach to follow. Aircraft X had also reported the Aircraft Y in sight.As Aircraft Y proceeded down towards the runway; I became concerned with his speed and how Aircraft X's turn to final was going; Aircraft X was off course not flying a 'normal' base leg; but instead was on the approach. It was going to be very tight and as Aircraft X started turning in; while Aircraft Y was still maybe 4-5 miles out; I asked Aircraft Y if he would be able to do some S-turns on final to remain behind the Aircraft X. He said 'affirmative'.As they got closer I just was not comfortable with the amount of space that Aircraft Y would have behind Aircraft X. We recently had issues at RDM with a go-around; and while that was in a different situation (leading aircraft on a visual approach did go-around; came off airport; and turned 180 degrees to fly downwind for another shot at it; on this approach; the leading aircraft; Aircraft X; would fly basically straight ahead if he missed) I was more fearful that the Aircraft Y was just going to overrun Aircraft X and be in a position of a destabilized approach; on maybe a 4 to 7 mile final; below the MIA (Minimum IFR Altitude) and maneuvering around on a dark night (granted; good weather; but still definitely well after sunset and I didn't know if there was a moon.(As it turns out; the moon is waxing crescent; and on Wednesday night was only 7% visible; so it was likely very dim or almost invisible; during bright moon; with lots of snow on the ground; the terrain is very visible on that side of the Redmond airport at night.)I told Aircraft Y that I just didn't think there was going to be enough room as the Aircraft X turned in front of him; so I told Aircraft Y to execute a left 360. I chose left because it's away from higher terrain; the MIA on the north and eastern side of his position is 075; and to the south it is 070. He was at about 070 at the time.Aircraft Y executed the left 360; but 'he kept descending'. By 180 degrees through his turn he was at 062 and continuing to go down; and I realized that he wasn't exactly on a visual approach and he wasn't on an instrument approach and really wasn't getting any separation from the terrain. I asked him if he had good ground contact (he said he did) and I think I told him that the minimum IFR altitude was 070 in his vicinity.In hindsight; I think this could easily be called a separation error with MIA. He was technically maneuvering on a visual-to-follow approach; but by giving him a 360 (and he never did break 3 miles separation from Aircraft X because I turned him in time) I don't think that would apply here.I think the single biggest thing would be to not assume that the pilot would level during a 360. In looking back; I'm sure that the pilot had good ground contact the entire time and never got closer than 1000 feet above terrain during the 360. I apologized to him but said that the closure rate was such that I didn't think he would have enough room for a safe landing and would have wound up going around anyway and he said 'no problem' and didn't sound bent out of shape or anything.I'm confident that spinning him was a good move. The closure rate sucked; the Aircraft X has a 180 knot MAX speed on that segment of the approach and so he didn't get inbound quickly enough to get good spacing in front of Aircraft Y.Technically; I could have just let Aircraft Y go; and it would be on the pilot to maintain separation on the visual-to-follow; if he couldn't; then he would have to either S-turn or go around. The problem with the S-turn option is that the carriers have put a lot of emphasis lately upon pilots getting nicely stabilized well outside of the airport and flying a nice; stabilized final; screwing around with S-turns and trying to slow while keeping energy state at a good point is pretty tough work during VMC on a sunny day. At night; it's asking for trouble.So spinning him was good.But; next time; once I spin a guy; I will issue a climb to the highest MIA that is around; or ask him if a contact approach will work and verify that he can maintain his own terrain/obstruction clearance.I'll also probably only do a 180 degree turn to begin with; so I can see how the altitude will work out with whenever he gets turned back into the airport and issued a visual approach.

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.