Date: 201701
Local time of day: 1201-1800
Location: D01.TRACON, CO
Light: Dusk
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01
Second aircraft: Medium Large Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Distraction; Situational Awareness; Communication Breakdown
Second reporter function: Approach
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: I was relieving a controller at the Final position and to my memory I believe I had plugged in to watch for a satisfactory amount of time in order to become comfortable with what was happening. During the briefing; Aircraft X on a visual approach to 16R was eventually 600 ft below Aircraft Y who was also on a visual approach; but to 16L. Aircraft Y went through his localizer but was already on tower frequency. When I assumed the position and noticed Aircraft Y on the wrong localizer; I gave my brain a chance to assess the situation and I think I noticed the Aircraft Y making a correction back to 16L. I didn't say anything to Aircraft X because of the correction that Aircraft Y was making back to the 16L localizer and after Aircraft Y was no longer a factor for Aircraft X; I shipped Aircraft X to the tower.I think calling traffic 'again' would be good but I'm not sure that Aircraft X would have been able to see him. Both aircraft were issued traffic prior to this happening and both reported the other in sight. The timing of this event was unfortunate because it happened during a relief briefing.


Second narrative: I was working FR2 which is the East Finals. In this case I was working finals for 16L and 17R. Traffic was moderate to busy; my 16L final was about 20 miles long and my 17R final was about 15 miles long. Compression was notable. Aircraft Y was given an intercept heading of 200 to join the 16L final and cleared for a visual approach approximately 20 miles out. The pilot read back the clearance and turned on the 200 heading. The 16 runways are not widely spaced; and thus we need to insure the aircraft are established on their heading to join before losing standard separation with the 16R final. Aircraft Y and the parallel traffic both turned in time for legal intercepts.A couple miles outside of the outer marker I advised Aircraft Y that there would be traffic on the parallel final and switched them to the tower. Aircraft Y reported the traffic in sight and took the change. The controller next to me was working the other final position for 16R only. His final was about the same length as mine; and was also busy with downwind and base leg traffic. He advised Aircraft X who was established on 16R of my Aircraft Y traffic and changed Aircraft X to the tower.Aircraft Y failed to intercept the localizer and continued the 200 heading all the way through the 16R localizer as well. Targets merged with Aircraft Y on top of Aircraft X by 4-500 feet. Neither I nor the other final controller saw this happen as we were focused on our sequence points 20 miles from the airport. Conflict alert never sounded; and it is my belief that the Local controller did not see the incident either. I was made aware of the incident a week later by my supervisor who was alerted by the region. The 16s operation is problematic and at times unsafe. Because of the proximity of the runways we have special intercept rules that we don't have on other runways. 16R also cannot allow reduced separation because of the high-speed location. Because of DEN's elevation we also see a substantial amount of TCAS go-arounds. We've been briefed that this is because the aircraft are still high enough that their TCAS is in en-route mode; and causes the go-around. In a VFR land south operation Denver is always landing on the 16s as primaries and using 17R as cargo or an offload only. An operation to the 17s; or a combination of 17s and one of the 16s would all be widely spaced and safer.It is my opinion that to cut down on taxi times; and to make the tower's workload easier we are consistently running a less than safe operation. I have seen hundreds of errors on the 16s and dozens of go-arounds.


Synopsis: Denver TRACON Final controllers reported a loss of separation when the leading aircraft went through the other aircraft's final.