Date: 201904
Local time of day: 1801-2400
Location: SEA.Airport, WA
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B SEA
ATC / advisory: TRACON S46
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Communication Breakdown; Confusion
Anomaly: ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: SEA was landing RWY 16 and ATIS told us to expect an RNAV-Y to RWY 16R. Ceilings were approximately 3500' Broken. ATC vectored us downwind over Puget Sound. We were expecting to get clearance to a fix on the RNAV arrival. Instead; the Controller cleared us to 'intercept the localizer'. We reminded him that SEA was advertising and were expecting the RNAV-Y 16R. He said 'OK; intercept the final approach course.' At this point we were outside of CELAK. There are three courses on the approach outside of CELAK. The segment between GRIFY and CELAK; the segment between Paine VOR and AGANE; and the segment between AGANE and CELAK. It wasn't clear which of these segments was intended. My First Officer asked me if he wanted me to extend the course from CELAK but ATC didn't make clear which segment we had been cleared to intercept. All my previous experience with RNAV approaches involve either direct to a fix or intercepting a final approach segment. The vague instructions left us uncertain how to proceed. We decided to see if the autopilot would automatically intercept the 164 course and when it was apparent it didn't; we selected direct CELAK and the FMA displayed FINAL APPROACH. The AP at that point flew the rest of the approach IAW SOP. The selection of direct CELAK approximated the segment from AGANE to CELAK but resulted in a slight overshoot of the extended centerline of the runway. ATC noticed our encroachment onto the final for 16C and advised us to correct back to our final (which had never been specifically assigned). Here was no other traffic inbound at the time the posed any risk of collision. Despite having flown this approach dozens of times in the simulator; I've never gotten a clearance like this. In fact I've never gotten a clearance like that in the real world either. ATC needs to be specific and needs to not assign localizer captures to aircraft who have been told to prepare for an RNAV approach. Furthermore; upon realizing his error; he should have issued more specific instructions rather than just 'intercept the final'. As a flight crew we should have queried ATC and selected direct to CELAK before an overshoot could occur.




Synopsis: Air carrier Captain reported a track deviation occurred on the arrival into SEA following a confusing clearance from ATC.