Date: 201912
Local time of day: 1801-2400
Location: ZZZ.Tower, US
Flight conditions: IMC
Weather elements / visibility: Icing
Light: Night
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying
Second reporter function: Pilot Flying
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft RA; Automation Aircraft TA
When detected: In-flight
Result: Flight Crew Executed Go Around / Missed Approach; Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Procedure
Primary problem: Procedure

Narrative: We were operating Aircraft X; on approach to the ILS XXR to ZZZ. We were approximately 2-3 miles outside ZZZZZ; established on the ILS XXR; cleared for the approach and fully configured for landing. IMC conditions existed with moderate mixed icing in the clouds. Everything was normal until we received the following TA in exactly this cadence: 'traffic; traffic! climb; climb! increase climb; increase climb!' I commanded Go-Around; and the FO (First Officer) immediately disconnected the autopilot; selected TOGA and initiated a go-around maneuver; while I assisted with gear and flap clean up. While we were engaged with this we entered VMC conditions and I observed out of my left peripheral view aircraft position lights to our left and slightly below us; closing on us with no relative movement. I recall pulling back on the yoke with the FO and shouting 'climb; climb; climb!' As soon as we had done this we received a 'clear of conflict' message. This all took place in just a few seconds. I announced our go-around to ZZZ Tower; and we were told to track the 16R localizer course and climb to 9;000. When asked the reason for the go-around we told them we were responding to an RA. We were then handed off to ZZZ Approach; who advised us that there was another aircraft also on a go-around climbing to 11;000 feet. I'm not sure if this was the aircraft that caused our loss of separation; but I suspect so. We were then vectored for a subsequent approach to the ILS XXL which was accomplished uneventfully.Without hearing radio transmissions from the other Tower frequency I can't say for sure why the other aircraft was suddenly encroaching on our flight path; but I can say that the TCAS went from nothing to a red target to our immediate left with RA commands almost instantly. I don't know how close we came to the other aircraft; but in the brief time that we were VMC it seemed to me that we were very close.This is only conjecture; but I believe that an aircraft on the parallel approach to XXL executed a go-around; but failed to fly the localizer course and began turning into us; thus creating the loss of separation and RA event. For our part; we followed SOPs; as well as ATC directions. FO did an exceptional job of flying the aircraft first and getting us out of the path of the other aircraft. He also stated that when we received the RA; his VSI was completely red; with no safe 'green band' to even fly into.


Second narrative: I was recently asked to submit a report regarding a TCAS RA during the approach into ZZZ. This event occurred during the approach (ILS XXR) after we were cleared for the approach and fully configured. We were in IMC and icing conditions during night when the event occurred. Everything was proceeding normally and we were fully configured on a stabilized approach. I was the Pilot Flying and monitoring the instruments. There was no traffic displayed on TCAS and seemingly out of nowhere we received a TA and I saw the target appear on the MFD just to our left. Someone had been issued a go around just prior but I can't recall which runway that was from (XXL makes sense). Almost as soon as the target appeared with the TA; we received a climb RA. Despite the startle; I promptly complied with the RA and began the climb. Very quickly after the climb was initiated; we received the 'increase climb' command at which time I also complied. There was no RA vertical guidance given other than the aural climb instruction. Very shortly after this; we received the 'clear of conflict' aural message and continued cleaning up the aircraft and completed the go-around. The PM notified ATC that we were complying with an RA and that we were going around. We were vectored back around for the ILS XXR which was uneventful the second time. During the second approach; Tower inquired us about the other aircraft. Seemed like they (ATC) weren't entirely sure themselves who had transgressed into our final approach course and caused the RA. At the same time as our RA I seem to recall there was another aircraft executing a go around from a different runway.I believe another aircraft was executing a missed approach and transgressed into our final approach course. There was some confusion on the part of ZZZ Tower regarding who had gone around and where they were going.Seems to me that ATC should be slightly more vigilant in traffic separation and approach monitoring on final to ensure there are no transgressions. Overall seemed to be some confusion in the Tower with what had happened when we informed them of our RA and their subsequent query during our second approach about the traffic that had conflicted with us.


Synopsis: B737 flight crew reported that an unknown aircraft created a traffic conflict on final; and resulted in an RA and a go-around.