Date: 201507
Local time of day: 0001-0600
Location: SEA.Airport, WA
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B SEA
ATC / advisory: TRACON S46
Second aircraft: Widebody; Low Wing; 2 Turbojet Eng
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Confusion; Situational Awareness; Workload; Distraction
Second reporter function: Pilot Flying; First Officer
Anomaly: ATC Issue All Types; Conflict NMAC
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Contributing factors: Airport; Human Factors; Procedure
Primary problem: Procedure

Narrative: I was the Captain/Pilot Monitoring on flight into SEA on the HAWKZ4 arrival to 34L. SEA Center told us to slow and gave us S vectors for metering into SEA. Told to Slow to 210 knots and given direct SONDR.  Then SEA approach told us to 'slow to 190; then down to 6;000 feet; don't worry about 6;000 foot restriction at SONDR.'  New SEA Appr frequency 133.65 very busy and checked in at 6;000 feet inside of SONDR and in the weather; IMC.  Approach immediately gave us 2 traffic calls for a Corporate jet 5 miles in front of us and a heavy jet. I responded with 'we are IMC at 6;000 feet; No Joy on traffic; request lower' (as best as I can remember the calls).  They said 'descend to 5;000 feet cleared ILS 34L'.  We were gear and Flaps 15 trying to go down and slow down further.  We broke out at about 5;300 feet.  I saw the Corporate aircraft and did not know where the heavy jet was.  First Officer (FO) called the runway in sight; I asked if he saw the heavy jet (I thought I heard the heavy jet call us in sight?); he said no; only the runway.   I called SEA appr and said 'Aircraft X has the field in sight' at about 12-11 DME. I zoomed in on the MAP; saw a TCAS target was off our right wing; less than a mile. Started looking out the FO's side windows and saw the heavy jet same altitude; less than a mile in a right hand turn to final; but it had some heading crossing angle; and I was concerned he may be overshooting.  By this time 10 DMEish; we were on LOC CAP/ GS CAP and FO was hand flying the aircraft.  I said; 'You have to watch that guy (heavy jet) close; he may be over shooting and I don't think he sees us!'  I called approach and said something like 'Does that heavy jet have us in sight? is he going to 34L or 34R' (at the same time I see him bank up; belly up to us at least 30 to 40 deg of bank; at the same time we are getting an RA).  Approach says 'he's going to 34R; contact tower' at about 10 DME we get and RA 'Climb' and RA symbology; my FO asks me if he should go around? I said 'No; stay visual with him; level off; and stay ABOVE him! pull up if he gets any closer'.  We both watch him snap turn away from us to the right and it appeared like they were heading to 34L; our runway.  We may have gotten a second RA or a continuation of the first to climb RA; we stayed level as we watched him descend and turn away from us; back to 34R's final.  I changed to tower frequency and called in 'Visual the field; the corporate and the heavy jet at 6.9 DME for the visual 34L'  'Aircraft X you are cleared to land 34L'  I think we got within 300-500 slant range of him and initially we were Co-Altitude or he was 50-100 lower than us.  I know he overshot 34R and maybe was trying to line up on 34L until I made that call on Appr frequency; as we were both getting RA's at 9-10 DME.  My FO was pretty rattled; landed on speed; 1;500 feet down and got on brakes pretty aggressively; I took Taxi Echo; Tango; Juliet; cross 34C and held for 5 -10 min short of 34R.  We had hot brakes when we pulled into the Gate.  Maintenance checked brakes and made a write up.  I believe the cause of this event was ATC vectoring large amounts of aircraft into minimum spacing and a heavy jet lost Situational Awareness and either overshot or was flying to the wrong runway and almost hit us.




Synopsis: A B737 flight crew on approach to SEA Runway 34L took evasive action from a heavy jet assigned Runway 34R as that aircraft's crew appeared confused about their assigned runway and drifted toward 34L several times while below 6;000 feet.