Date: 201508
Local time of day: 1801-2400
Location: S46.TRACON, WA
Light: Night
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B SEA
ATC / advisory: TRACON S46
Second aircraft: Medium Large Transport
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Human-Machine Interface; Confusion
Second reporter function: Enroute; Trainee
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Contributing factors: Company Policy; Human Factors; Procedure
Primary problem: Procedure

Narrative: Seattle North flow; all the aircraft coming into the airspace on the HAWKS 4 Arrival had to be vectored off the arrival to be sequenced with the other arrival legs. I took radar from ZSE Sector 2 on Aircraft X and when he checked in; the first thing I said was fly heading 340; reduce speed to 210 and descend to 8000ft. I did not hear it at the time; but the [company] that checked in was Aircraft Y descending via the HAWKS 4 Arrival. I did not have the hand-off on Aircraft Y nor was he flashing to me from ZSE Sector 2. I gave multiple control instructions to Aircraft Y thinking it was Aircraft X until I questioned the aircraft that the heading I gave him did not appear he was flying. That is when Aircraft Y said he was heading 320 descending out of FL240 to 8000. At that time I realized what was happening and told Aircraft Y to disregard my control instructions and return to his previous frequency and I called ZSE Sector 2 and told them I needed to talk to Aircraft X who was joining the localizer at Seattle as the HAWKS 4 instructs them to do. Because Aircraft X was not sequenced; final did not have the hand-off on them and had cleared Aircraft Z for a visual approach. I told Final what was happening and he vectored Aircraft Z to the northeast and at that point; Aircraft X checked on and I vectored him to the northwest. The loss was right around 2.9 miles instead of the needed 3.This is a similar sounding call sign and an attention to detail with listening to call sign problem. I should have heard the wrong call sign when they checked in and the airline needs to ensure that they don't schedule similar sounding call signs to arrive at the airport at the same time.


Second narrative: I am training on R02 and had a busying sequencing session. I had Aircraft X on frequency; giving vectors and altitude changes to work him into my sequence. He was number 2 in my sequence of 6+ aircraft. After minutes or so of having him on my frequency; Aircraft Y checked on; 40 +/- miles in trail. I gave Aircraft Y descend via the HAWKZ 4 arrival. And he took it. Around the same time; Aircraft X was near S46 airspace and he also was given descend via with a speed restriction. When I was finished with Aircraft X and S46 had the hand off; I shipped him to 125.6 with published speeds and Aircraft Y took the frequency change. I was busy and missed the read back. Aircraft Y then proceeded to check on with S46 and take control instructions for Aircraft X. The S46 controller eventually noticed the situation and had Aircraft Y return to my frequency. When he checked back on; we then realized what had happened; reissued a clearance to Aircraft Y and tried to ship Aircraft X to 125.6. It was brought to my attention that this hear back read back error resulted in a loss of 2.8 miles in Seattle approaches airspace. I would recommend a serious look at the similar sounding call signs; this is a frequent problem and it can result in serious situations.


Synopsis: S46 and ZSE Controllers reported confusion with similar call signs that led to a loss of separation.