Date: 201905
Local time of day: 0001-0600
Location: PAE.Tower, WA
Flight conditions: VMC
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 D PAE
ATC / advisory: Tower PAE
Second aircraft: Skyhawk 172/Cutlass 172
Reporter organization: Government
Reporter function: Instructor; Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Confusion; Situational Awareness
Second reporter function: Trainee; Local
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft RA; Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: Aircraft X was inbound IFR on a visual approach to Runway 16R on a right base. Aircraft Y was on roughly a three mile final for Runway 16R. Aircraft X entered PAE Class Delta at high speed on a right base visual approach to Runway 16R. Aircraft X was not in communication with PAE until well within the Class Delta surface area and Aircraft X heading and altitude put him in conflict with Aircraft Y. Traffic was issued to both aircraft; Aircraft X reported traffic in sight. However; after reporting traffic in sight Aircraft X continued to fly directly toward Aircraft Y within 200 feet of Aircraft Y's altitude. The two aircraft appeared to merge on the radar screen; but out the window it was obvious that Aircraft X was taking evasive action to avoid Aircraft Y by descending below Aircraft Y's altitude and continuing eastbound. Around the time the targets merged on the radar screen LC1 instructed Aircraft Y to climb to avoid conflict. LC1 judged that Aircraft X was executing an RA and once Aircraft X was clear of traffic LC1 instructed Aircraft X to fly heading 360 and maintain 3;000 feet. Once Aircraft X was stabilized and climbing to the north LC1 instructed Aircraft X to contact Seattle Departure. I believe it was the late communication transfer of Aircraft X from S46 on the visual approach and the lack of coordination from S46 that led to this conflict. Aircraft X's approach to Runway 16R was very unexpected.It states in the PAE/S46 LOA that S46 must transfer communication of jet aircraft eight miles out. Had S46 adhered to this requirement the conflict would have been avoided. Also; had S46 coordinated that they were going to bring Aircraft X in on a tight right base visual approach to Runway 16R; PAE would have been able to prepare for it. The traffic that conflicted with Aircraft X on final for Runway 16R at PAE was clearly depicted on the radar screen at the time of the event.


Second narrative: I was working Local 1 and Local 2 at PAE. Runways 16L and 16R were in use. Aircraft Z was on final for Runway 16R. Aircraft Y was on an extended right downwind for Runway 16R to pass behind Aircraft Z and join final for the parallel runway. Aircraft Y had reported the traffic in sight and communicated they would comply with the instruction given. Aircraft X was on a visual approach to Runway 16R and appeared was turned right base by the Approach Controller and/or at the pilot's discretion. Generally; aircraft on visual approaches; including and especially commercial airliners; are vectored further north on this flow to allow IFR aircraft to be sequenced by Tower. Aircraft X had not communicated with Tower prior to joining a much earlier than expected right base; and I reached out to the pilot. Aircraft Z was instructed to change to Runway 16L and was cleared to land on Runway 16L to resolve the conflict with the Aircraft X arrival and track away from the inbound Embraer. Aircraft X had been advised of the Aircraft Y traffic that was tracking an extended right downwind for Runway 16R to the east/northeast to join final for Runway 16L. Aircraft X reported that traffic in sight.I expected Aircraft X to continue a right turn to Runway 16R without any conflict when Aircraft X maneuvered unexpectedly and turned north away from the runway and became a conflict with Aircraft Y tracking away to the east/northeast. It was a reasonable assumption that Aircraft X was responding to a TCAS RA at this time although the pilot did not state as such on frequency. A traffic alert was issued to the Cessna to climb based on how Aircraft X and Aircraft Y appeared out the window. Another [controller] coordinated with Approach to get a heading and altitude for Aircraft X. Aircraft X was advised to maintain terrain obstruction; fly heading 340; and maintain 3;000 (per Seattle Approach's relayed instruction). Aircraft Z landed. Aircraft Y tracked inbound to Runway 16L and was given a left 360 to descent to pattern altitude and rejoin final.1) Visual approaches should not be turned base inside of Class Delta airspace by Approach.2) Aircraft on visual approaches could either be on an extended right or left base outside of the airspace to allow the Tower Controller appropriate time to sequence.3) Conflicts should be reasonably resolved prior to switching; including and especially multiple inbound VFR aircraft.4) Pilots should not switch frequencies late; especially if they are on an early base turn inside Class Delta airspace.


Synopsis: PAE Tower controllers reported Approach Control gave them an aircraft on a base leg in conflict with straight in traffic which resulted in an NMAC.