Date: 202106
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Rain
Light: Daylight
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Landing; Final Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01
Second aircraft: Medium Large Transport
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Confusion; Workload
Second reporter function: Approach
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Aircraft RA; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Executed Go Around / Missed Approach; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Airspace Structure; Procedure
Primary problem: Procedure

Narrative: I was the Line Captain in the First Officer's seat. After numerous delaying vectors which cut into our planned landing fuel; we were finally cleared for the ILS Runway 35L. Passing through 9;000 feet we noticed an aircraft approaching our aircraft from the east at 8;000 feet. As we descended on the glideslope; passing 8400 feet; the TCAS generated an RA on the aircraft; which was still at 8;000 feet. We came within 400 feet. of merging with this aircraft! We circled back for another approach; cleaned up to conserve fuel; and advised Approach that we were now at a minimum fuel state. We circled back for another ILS to Runway 35L; which we completed normally. I called the TRACON Supervisor on my cell phone after parking. He said the other aircraft was supposed to intercept the final for Runway 35R but neglected to do so; which resulted in a rather severe loss of separation. According to the TRACON Supervisor; the other pilot blamed his automation for not capturing the 35R localizer.


Second narrative: Working Final Radar. Because of weather; it was necessary for me to use the ILS approach for my runway; 35L. The parallel final for Runway 35R was able to use visual approaches. I had cleared Aircraft X for the ILS to Runway 35L at Denver International. I switched communications to Tower. A few seconds later; the pilot of Aircraft X called on my frequency that he was responding to a TCAS RA and climbing. Aircraft Y was cleared for a visual approach to Runway 35R at 8000 ft. but was crossing his localizer. Aircraft Y ended up turning virtually underneath Aircraft X before heading back northeast to Runway 35R. Aircraft Y was at 8000 ft.; Aircraft X was indicating 8400 ft. and climbing. I cancelled the approach clearance for Aircraft X; turned west; and climbed to 9000 ft. Aircraft X was re-sequenced and landed safely on Runway 35L.North landing configuration is the preferred operation. This morning; the wind was out of the south at 8 to 12 kt. Although pilots were complaining about tailwinds; we maintained the north operation. In addition; convective weather was south of the airport; sitting on the final approaches. This affected [the] pilots' ability to see the airport. The only other weather in the airspace was a small area east of the airport that had no impact. In a north configuration; the runways; 35L/35R; are direct parallel runways. If we had landed south; as the wind and weather favored; we would have used a widely spaced configuration and landed on runways 16R and 17R. We also could have easily used visual approaches and the aircraft would have been in clear weather all the way to the airport and had a headwind. Landing north unnecessarily was a major contributing factor in this event. Also; the parallel final for Runway 35R was training and the Developmental forgot to turn Aircraft Y in a timely manner resulting in the overshoot. This could have also been avoided in a south operation because of much more spacing between the runways.


Synopsis: Air carrier Line Captain and TRACON Controller reported an NMAC between two air carriers on final approach due to the other aircraft flying through the final approach course.