Date: 201712
Local time of day: 0601-1200
Location: S46.TRACON, WA
Flight conditions: Marginal
Aircraft: Light Transport; High Wing; 2 Turboprop Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B SEA
ATC / advisory: TRACON S46
Second aircraft: EMB ERJ 170/175 ER/LR
Reporter organization: Government
Reporter function: Departure
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Workload
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance
Contributing factors: Procedure; Human Factors; Airspace Structure; Company Policy
Primary problem: Procedure

Narrative: I was working departure sectors combined. Aircraft X (turboprop) was a SID departure off sea direct climbing to 15000 ft on course. Aircraft Y was a departure climbing to 15000 ft on a SID departure. Normally an Embraer behind (truboprop like Aircraft X) isn't much of a problem in this scenario because the E175 turns on the SID to diverge from the departure gate before speed ever becomes a problem. I was busy dealing with a pilot deviation and trying to establish standard separation between two other aircraft.After I got that squared away at I looked up towards SEA to check my other aircraft and I saw Aircraft X; in front was at about 10500 ft at 180 knots and Aircraft Y behind him was at about 9600 ft at 250 knots. I saw that there was barely 3 miles so I stopped Aircraft Y at 11000 ft. The way he was climbing; it was too late to stop him at 10000 ft. I figured Aircraft X would be out of 11000 ft any second so it would be fine. I also issued Aircraft Y a vector 110 heading to establish divergence and lateral. I then issued traffic; but Aircraft Y was in the clouds. Normally I would have recognized this earlier but I was tunnel-visioned on another airport where the pilot deviated and turned into another airplane and had a loss. This sort of situation; unfortunately; happens a lot. Usually we are able to stop the second aircraft and vector off the SID to establish separation. Our letter of agreement with tower gives them the opportunity to launch like departure routes with only 3 miles separation; even if the aircraft characteristics are different. The only caveat we have with characteristics is B757 behind Airbus which requires 5 miles in trail. We used to get 5 miles in trail with all departures and this never happened. But now it happens constantly; where we are forced to increase our complexity significantly by speed restricting departures; vectoring off SIDs; and delaying aircraft to gain longitude separation. It increases coordination between us and the center; and increases chances for hear back read back errors. We are required to give the center 5 miles in trail and we have to make that somehow. Same or like type aircraft need more than 3 miles in trail initial separation; especially with different types. These particular aircraft came off about 8 miles in trail and the overtake was still a problem. I see 'almost losses' happen every shift because of these procedures. We are working so hard to make 5 miles in trail out of 3 miles in trail. When you have a string of aircraft all going the same way this becomes extremely taxing and you are likely to miss something. Unfortunately; because of my focus on the loss that was happening elsewhere and the focus that was needed to correct it; I didn't recognize the overtake in time. This is why the procedure is bad. Additionally; we have an even worse situation when the Tower uses pilot provided visual between aircraft that will be on the same course for 10 or more miles; and you have 3 airplanes within 3 miles of each other at the same altitude; on the same heading; speed overtakes; and 'visual separation'. Our letter of agreement creates dangerous situations because it focuses only on getting departures off the ground quickly; and doesn't factor in how safe it is once they are in the air.




Synopsis: S46 TRACON Controller reported that SEA Tower routinely launches successive departures with minimum separation regardless of aircraft performance differences.