Date: 201604
Local time of day: 1201-1800
Location: S46.TRACON, WA
Flight conditions: VMC
Light: Daylight
Aircraft: Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class B SEA
ATC / advisory: TRACON S46
Second aircraft: Large Transport
Reporter organization: Government
Reporter function: Departure
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Time Pressure; Situational Awareness; Distraction; Human-Machine Interface
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; 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; Air Traffic Control Provided Assistance
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: Working Departure East (A) sector. Gorgeous VMC. The SOP allows SEA ATCT to launch successive departures utilizing pilot provided visual separation. The ATCT must modify the secondary scratchpad in the STARS data-tag to 'VV' to advise the TRACON that pilot provided visual separation is being used. However; in this case the first aircraft is Aircraft X and that type of aircraft is notorious for climbing out much slower than most other aircraft. According to the FALCON replay; the trailing Aircraft Y is 1.95NM behind Aircraft X with approximately a 30 knot overtake. In light of the situation that had already occurred; I stopped Aircraft Y at 3000' on the climbout. I do not recall if I restricted his speed; but I believe that I did (230kts). Immediately after observing this situation developing; I flipped on our stop departure lights to terminate automatic releases for SEA. I simultaneously made the supervisor aware of the new situation. A final aircraft departed behind Aircraft Y. Because Aircraft Z was airborne; I decided to turn Aircraft Y out to the northwest in an attempt to give myself some room to work. I also restricted Aircraft Z to 4000' on initial contact to ensure vertical separation between Aircraft Z and Aircraft X; in case that became an overtake situation as well. All aircraft subsequently continued their departures without further incident.This is a common problem that we experience at SEA with Aircraft X [type] departures. I do not know why this specific aircraft type climbs out so differently; but it is common knowledge at the TRACON. I would propose that this situation could be corrected by changing the LOA with SEA ATCT to exclude Aircraft X [type] departures from the provision allowing the use of pilot provided visual separation between successive departures. If SEA ATCT had utilized the standard 3-5NM in-trail spacing that would have been required during IMC; this event would have been completely avoided.




Synopsis: S46 controller reported of a loss of separation due to a particular type of aircraft that climbs out slower than most other aircraft. Controller would like to see LOA changed between Tower and TRACON to exclude this type of aircraft from use of visual separation during climb out.