Narrative:

Climbing out of dfw on the TRYTN2; we were initially cleared to 17;000 feet; then re-cleared to 16;000 feet due to VFR NORDO traffic crossing our route ahead. Climbing through about 15;000 feet; we got 'traffic; traffic' from the TCAS. I immediately leveled off with 'vertical speed zero'; trying to minimize the conflict while we looked for it. I was a little confused because we were still about 2500 feet below where I thought the traffic was. Very shortly thereafter; we got 'descend; descend' and immediately commenced the TCAS procedure although I still couldn't understand why we were getting it so early. By now; the ATC controller had re-entered the scenario; and ordered a 20 degree right turn for traffic separation. Turns out; the VFR NORDO had commenced a descent which had gone unnoticed by ATC. Both our aircraft; and the VFR NORDO; were operating in accordance with their respective IFR/VFR rules. However; in this case; the rules were insufficient to provide standard separation without close scrutiny and input from ATC. The controller had other traffic to deal with; and the momentary diversion of his attention just happened to coincide with the VFR aircraft's descent initiation.I think a change in ATC protocol regarding mixing VFR NORDO traffic with any traffic under ATC control is warranted. Specifically; I think ATC should immediately stop allowing IFR traffic to simply under fly NORDO traffic. Worked traffic should be vectored behind; or at least 3 miles ahead of; crossing NORDO traffic for the simple reason that you just can't know when the NORDO will change course; either horizontally or vertically. Furthermore; most turboprop aircraft could easily attain a 3500 fpm descent with very little notice. It seems to me; that this could easily overwhelm the TCAS system and make it unable to plot a solution and get the pilot to comply in time to avoid a collision. I think pilots should be given more latitude to initiate separation action before the actual RA is displayed. The RA waits until there is little time remaining to save the aircraft. A pilot could see the situation developing but without ATC clearance; (the controller is busy with another plane) would almost certainly face disciplinary action for flight not in accordance with his clearance. It's like the system is only interested in 'close calls'; so converting them to 'long calls' isn't a priority.

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Original NASA ASRS Text

Title: A320 flight crew reported a TCAS RA event with a VFR aircraft not in communication with ATC.

Narrative: Climbing out of DFW on the TRYTN2; we were initially cleared to 17;000 feet; then re-cleared to 16;000 feet due to VFR NORDO traffic crossing our route ahead. Climbing through about 15;000 feet; we got 'TRAFFIC; TRAFFIC' from the TCAS. I immediately leveled off with 'Vertical Speed Zero'; trying to minimize the conflict while we looked for it. I was a little confused because we were still about 2500 feet below where I thought the traffic was. Very shortly thereafter; we got 'DESCEND; DESCEND' and immediately commenced the TCAS procedure although I still couldn't understand why we were getting it so early. By now; the ATC controller had re-entered the scenario; and ordered a 20 degree right turn for traffic separation. Turns out; the VFR NORDO had commenced a descent which had gone unnoticed by ATC. Both our aircraft; and the VFR NORDO; were operating in accordance with their respective IFR/VFR rules. However; in this case; the rules were insufficient to provide standard separation without close scrutiny and input from ATC. The controller had other traffic to deal with; and the momentary diversion of his attention just happened to coincide with the VFR aircraft's descent initiation.I think a change in ATC protocol regarding mixing VFR NORDO traffic with any traffic under ATC control is warranted. Specifically; I think ATC should immediately stop allowing IFR traffic to simply under fly NORDO traffic. Worked traffic should be vectored behind; or at least 3 miles ahead of; crossing NORDO traffic for the simple reason that you just can't know when the NORDO will change course; either horizontally or vertically. Furthermore; most turboprop aircraft could easily attain a 3500 fpm descent with very little notice. It seems to me; that this could easily overwhelm the TCAS system and make it unable to plot a solution and get the pilot to comply in time to avoid a collision. I think pilots should be given more latitude to initiate separation action before the actual RA is displayed. The RA waits until there is little time remaining to save the aircraft. A pilot could see the situation developing but without ATC clearance; (the controller is busy with another plane) would almost certainly face disciplinary action for flight not in accordance with his clearance. It's like the system is only interested in 'close calls'; so converting them to 'long calls' isn't a priority.

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.