Narrative:

Arriving dfw at night; VMC; approximately 13 miles southwest of the airport on a northeast heading; descending through 4000 feet (all altitudes MSL); being vectored to a runway 36L intercept just outside of charr when we were cleared for a visual approach. First officer (first officer) was the pilot flying and I was the pilot monitoring. Shortly after; the aircraft ahead of us announced they were responding to a TCAS RA. This got my attention. I looked down at the navigation display and observed a TCAS target approximately 2.5 miles ahead at 3000 feet on what appeared to me to be a converging course. I alerted approach control to the presence of this aircraft and he instructed us to maintain 4000 feet. By this time we were already descending through 3500 feet and as the first officer adjusted the flight path we also received a TCAS RA to which the first officer responded. Some very long seconds later we received the 'clear of conflict' indications. At the point of the execution of this maneuver the TCAS indicated 200 feet vertical separation and no discernible horizontal separation. We were re-cleared to execute the visual approach to 36L and landed uneventfully. Neither the first officer nor I acquired the traffic visually but our jump seater witnessed the aircraft pass 'about' 200 feet below our left wing.at the completion of the parking checklist I phoned TRACON and spoke with the manager on duty (mod). He had witnessed the incident from TRACON and said that the aircraft involved was an aircraft that had departed an area outlying airport headed westbound and checked in at 2000 feet with a transponder with an inoperative mode C. According to the mod; their radar returns indicated 'all zeros;' which I assume means there was no data. About 20 minutes later the mod called me back and said that after review of their data they had acquired one brief altitude readout that showed his altitude at 2800 feet and climbing as he crossed the 35L localizer about 10 miles south of the airport. By this time they had spoken with the pilot and he swore that he maintained 2000 feet. I believe otherwise.the above narrative includes the facts as I understand them to be. Now for my assessment. This is the closest I have ever come to losing an aircraft in my 40 years of flying. Were it not for TCAS technology; our training and the inexplicable decision on the part of the small aircraft pilot leave a malfunctioning mode C transponder in the altitude reporting position I am convinced that the outcome would have been very dire indeed. Though I have tried to relay these events as dispassionately as possible; suffice it to say that all of us in the cockpit were shaken. Regardless; my first officer handled the situation in a very calm and professional manner and I must also commend his actions for the successful outcome.several things occur to me. First of all; the aircraft we almost hit was not at 2000 feet. As I understand it; the TCAS will not command an RA unless it has valid mode C data from the other aircraft. We have three independent altitude verifications; two from TCAS indications and one from ground-based radar. We also have the eyewitness account of the jump seat occupant. What was witnessed by the crew of the aircraft in front of us I cannot say.secondly; a heads-up would have been nice. Why did ATC not alert either crew to the presence of a VFR aircraft transitions the final approach course in such close proximity to the airport; even if they believed it to be 1000 feet below us? I don't know what the consequences would be for a controller if we were to collide with an aircraft for which he was not strictly required to provide separation; but the consequences for my passengers; my crew and myself would be the same were we to collide with any aircraft--IFR or VFR. Finally; why would an aircraft with a malfunctioning transponder be allowed cross 5 final approach paths; at night; in such close proximity to one of the busiest airports in the world at rush hour? This is a max workload environment for us and the chances of seeing a [small aircraft] at night against the backdrop of a major metropolitan environment are slim.it is not my intent to disparage the fine work of our ATC professionals. They do a terrific job. But; as the airspace grows more crowded; there does seem to be a growing disconnect between what is desirous for maximizing traffic flow and the capabilities of our aircraft and crews. I would like to see this incident investigated; in particular; but in the light of what I perceive to be a larger problem--too may airplanes and too little airspace.

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Original NASA ASRS Text

Title: Air carrier Captain reported a near-mid-air-collision with a small aircraft during a night VMC approach to DFW.

Narrative: Arriving DFW at night; VMC; approximately 13 miles SW of the airport on a NE heading; descending through 4000 feet (all altitudes MSL); being vectored to a runway 36L intercept just outside of CHARR when we were cleared for a visual approach. First Officer (FO) was the Pilot Flying and I was the Pilot Monitoring. Shortly after; the aircraft ahead of us announced they were responding to a TCAS RA. This got my attention. I looked down at the Navigation Display and observed a TCAS target approximately 2.5 miles ahead at 3000 feet on what appeared to me to be a converging course. I alerted approach control to the presence of this aircraft and he instructed us to maintain 4000 feet. By this time we were already descending through 3500 feet and as the FO adjusted the flight path we also received a TCAS RA to which the FO responded. Some very long seconds later we received the 'clear of conflict' indications. At the point of the execution of this maneuver the TCAS indicated 200 feet vertical separation and no discernible horizontal separation. We were re-cleared to execute the visual approach to 36L and landed uneventfully. Neither the FO nor I acquired the traffic visually but our jump seater witnessed the aircraft pass 'about' 200 feet below our left wing.At the completion of the Parking checklist I phoned TRACON and spoke with the Manager on Duty (MOD). He had witnessed the incident from TRACON and said that the aircraft involved was an aircraft that had departed an area outlying airport headed westbound and checked in at 2000 feet with a transponder with an inoperative Mode C. According to the MOD; their radar returns indicated 'all zeros;' which I assume means there was no data. About 20 minutes later the MOD called me back and said that after review of their data they had acquired one brief altitude readout that showed his altitude at 2800 feet and climbing as he crossed the 35L localizer about 10 miles south of the airport. By this time they had spoken with the pilot and he swore that he maintained 2000 feet. I believe otherwise.The above narrative includes the facts as I understand them to be. Now for my assessment. This is the closest I have ever come to losing an aircraft in my 40 years of flying. Were it not for TCAS technology; our training and the inexplicable decision on the part of the small aircraft pilot leave a malfunctioning Mode C transponder in the altitude reporting position I am convinced that the outcome would have been very dire indeed. Though I have tried to relay these events as dispassionately as possible; suffice it to say that all of us in the cockpit were shaken. Regardless; my FO handled the situation in a very calm and professional manner and I must also commend his actions for the successful outcome.Several things occur to me. First of all; the aircraft we almost hit was NOT at 2000 feet. As I understand it; the TCAS will not command an RA unless it has valid Mode C data from the other aircraft. We have three independent altitude verifications; two from TCAS indications and one from ground-based radar. We also have the eyewitness account of the jump seat occupant. What was witnessed by the crew of the aircraft in front of us I cannot say.Secondly; a heads-up would have been nice. Why did ATC not alert either crew to the presence of a VFR aircraft transitions the final approach course in such close proximity to the airport; even if they believed it to be 1000 feet below us? I don't know what the consequences would be for a controller if we were to collide with an aircraft for which he was not strictly required to provide separation; but the consequences for my passengers; my crew and myself would be the same were we to collide with any aircraft--IFR or VFR. Finally; why would an aircraft with a malfunctioning transponder be allowed cross 5 final approach paths; at night; in such close proximity to one of the busiest airports in the world at rush hour? This is a max workload environment for us and the chances of seeing a [small aircraft] at night against the backdrop of a major metropolitan environment are slim.It is not my intent to disparage the fine work of our ATC professionals. They do a terrific job. But; as the airspace grows more crowded; there does seem to be a growing disconnect between what is desirous for maximizing traffic flow and the capabilities of our aircraft and crews. I would like to see this incident investigated; in particular; but in the light of what I perceive to be a larger problem--too may airplanes and too little airspace.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.