Date: 201509
Local time of day: 1801-2400
Location: SEA.Airport, WA
Flight conditions: VMC
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B SEA
ATC / advisory: TRACON S46; Tower SEA
Second aircraft: Commercial Fixed Wing
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Human factors: Situational Awareness
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Conflict Airborne Conflict
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Executed Go Around / Missed Approach; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Procedure; Human Factors; Airspace Structure
Primary problem: Ambiguous

Narrative: We were on approach to Runway 16R at Seattle.  We received an RA while on approach. This occurred approximately 3;000 feet above the ground. It was a 'Descend; Descend' RA. The aircraft causing our RA was at our seven o'clock high. We had visual contact the entire time. It was night time but clear and unlimited visibility. We were in a situation where we had to descend in order to comply with the RA. The only choice in that situation was to do a go-around followed by a safe approach. The go-around became problematic because we could not climb without violating the RA. We had to communicate to the Tower that we needed a right hand turn and then a climb. This request seem confusing to the Tower personnel. They eventually allowed us to do a right turn to a heading up 190 at which time the RA stopped and we executed a normal go-around.Here are the data points as I remember them. Approach Control asked us if we had the field and our traffic to follow in sight. We answered in the affirmative. We were given a 130 heading to intercept the Localizer. The autopilot intercepted the Localizer.  We intercepted glideslope at 4;000 feet MSL. The autopilot was flying the airplane with the autothrottles engaged. We were at flaps five and 190 knots.  At approximately 3;400 feet MSL; we received a traffic TA. The symbology indicated an aircraft at our seven o'clock.  I looked at our seven o'clock and could see an aircraft on final for 16L. Very quickly we received a 'Descend; Descend' RA. I disconnected the autopilot and autothrottle; and complied with the RA. I felt the only logical decision at that point was to do a go-around. I instructed the First Officer to tell Approach we needed to go around. Approach instructed us to contact Tower. We contacted the Tower and told them we were responding to a RA; descending; and needed to go around. He misunderstood and cleared us to land. We told him we needed to go around and we also needed a right hand turn. He instructed us to climb straight ahead runway heading to 2;000 feet. We repeated that we needed a right hand turn and then a climb. He seemed confused but finally complied and gave us a turn into a heading of 190. Shortly after the heading change; the RA went away and we executed a normal go-around.I did not configure the airplane past flaps five. We were gear up the entire time. I saw no point in adding more drag to the airplane when we were certainly going to do a go-around. The 'Descend; Descend' RA was steady from 3;000 feet AGL to 1;100 feet AGL. The 'Descend; Descend' RA was forcing us to go below glideslope. The ground was clearly visible and not a threat. We did not receive any GPWS warnings. I am not sure if the 'Descend; Descend' RA stopped because of the horizontal separation provided by the 190 heading or if the 'Descend; Descend' RA was simply inhibited at approximately 1;100 feet as it is designed.I called SEA Approach about the event and they said that for visual approaches there is no required interval. I suspect that the aircraft that approached us from our seven o'clock was aggressively decelerating and descending. We were very stable so he must have been doing something very aggressive to upset my TCAS so much. I felt our response was safe and in compliance with our manuals/procedures.


Second narrative: When we contacted Tower; I stated we were responding to an RA; descending; and we would be going around. Tower responded with 'Cleared to land; Runway 16R.' I stated again that we were responding to an RA and would be going around. The Tower then stated to 'climb to 2;000 feet; maintain runway heading.' This would have been contrary the RA. The Captain; while continuing to descend on the RA guidance; and now two dots below the glideslope; told Tower we needed a turn. At this point; we were approaching 1;000 feet AGL; and still had RA guidance to descend. Tower finally gave us a 190 heading; diverging from the traffic; and a climb to 2;000 feet. The go-around was flown normally; followed by vectors for a visual approach and uneventful landing.


Synopsis: A B737-700 flight crew reported receiving an RA on a visual approach to SEA from an aircraft at their 7 o'clock position. TCAS commanded a descent to about 1;000 feet AGL and two dots below the glideslope; at that point the flight crew executed a missed approach.