Date: 201808
Local time of day: 1801-2400
Location: S46.TRACON, WA
Flight conditions: Marginal
Weather elements / visibility: Rain; Thunderstorm
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class B SEA
ATC / advisory: TRACON S46
Second aircraft: Medium Large Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Communication Breakdown; Time Pressure; Workload
Anomaly: ATC Issue All Types; Airspace Violation All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Contributing factors: Weather; Staffing; Procedure; Airspace Structure; Human Factors
Primary problem: Procedure

Narrative: Before this situation occurred it was noted that there were too many airplanes in the airspace with the amount of deviations occurring and NO TMC (Traffic Manager Coordinator) staffed to help with the traffic volume. The whole situation started with Aircraft Z from the west that was on an unusual vector to avoid weather. Aircraft Z was the first aircraft I had that would eventually encounter the weather to the north. The final was full and Aircraft Z needed to be turned to a downwind heading. The pilot refused the instruction; fly heading 340; because of weather and was consequently taken across the final behind another airplane using divergence for resequencing. This had a cascading effect on the sequence as multiple aircraft were subject to this pilot's inaction. I had to repeat the instructions three times. Fly heading 130; fly heading 160; he finally started turning and I had him roll out on a 070 heading to pass behind. I just needed the aircraft to turn.Resequencing that aircraft caused other aircraft to extend on the east downwind. All of the aircraft were contained in final's airspace. There were multiple aircraft left at higher altitudes such as 5000 ft because of how far out I was turning to final and I didn't want to impact other sectors with aircraft at lower altitudes and potentially going outside the Class B airspace. I was busy communicating with my airplanes in the airspace due to the prior situation and I was continuing to formulate my plan to work around the weather deviations and reel the final back in. I then noticed another aircraft; Aircraft Y; from another sector out of place and in my airspace. Aircraft Y was southwest bound level at 5000 ft; heading right for Aircraft X and I immediately issued a traffic alert. The Aircraft X pilot said they had the aircraft on TCAS but not in sight. I wasn't sure what the other controller was going to do so I instructed the pilot to turn left immediately heading 230 to avoid a potential collision. This was a clear airspace violation and a potentially dangerous situation due to the split focus between low sectors in bad weather and the inaction of the other controller once the CA (Collision Avoidance) alarms went off and they realized the grave state of affairs. Aircraft X and the other aircraft were sequenced appropriately following this event and all landed safely without further incident. We had at one time at least 16 Seattle Arrivals deviating for weather with a 46 arrival rate which is not the ILS rate into this airport. All the sectors were combined up. This has been a continuing trend at this facility and these kinds of events have been noted in the past. I'm just glad I had a backup plan. Due to the weather deviations and the lack of traffic oversight the arrival rate should have been set appropriately. The lack of awareness in the room as to what the weather was doing; how it was moving and interacting with final and other sectors is unacceptable; especially after the CIC (Controller in Charge) was made aware by another controller before I sat down and I gave them a heads up too. Having positions combined up during significant weather events is a bad idea and inherently dangerous given the current event stated above. The thought process of every controller in the building should be at minimum these two things: Scanning for potential conflicts (They may not be in your airspace 'yet') and say something if it looks wrong or out of place. The second thing is too stay out of the final controller's way and out of final's airspace; period.




Synopsis: TRACON Controller working combined sectors with weather deviations observed an aircraft entering their airspace without a handoff conflicting their arrivals.