Narrative:

RNAV approaches; especially arcing ones; at den have been a hot issue lately. There is even a bulletin emphasizing that these should be flown with use of the autopilot. The 16R RNAV Z also has a turning arc; to reverse course just outside the 'FAF' for many of the approaches. On this approach; autopilot on; we could see oncoming traffic while on base; and rolling final. This converging traffic; we presumed; was for the parallel runway. Our convergence caused a TCAS RA with 300 feet vertical separation. We had a descending RA then climbing one; after which the traffic could be viewed as on a parallel path and below us. In the climb; we dropped gear and flaps so that we could slow and not abandon the approach. We were able to stabilize the approach around 1000 feet AGL; though due to the confusion of ATC handoff; configuration change; and new procedures; we did not turn the TCAS to TA only and ignored it once on a parallel path; where the TCAS event disappeared shortly thereafter. Landing was uneventful and the event was debriefed in the cockpit. Some of the issues that arose were as follows:visual approach to the left runway and RNAV Z approach for the right runway were converging on final at about the same place. In the visual only environment; the convergence would be better monitored; but an instrument approach and visual traffic; at night no less; presents an unexpected convergence where perhaps separation is anticipated. The TRACON gave a traffic advisory; but there was confusion due to expected separation of the 'RNAV'. Procedurally; TCAS can be selected to TA in a close proximity or parallel approach environment; but we were still converging to our respective finals. In accepting the RNAV approach; were we somehow subjecting ourselves to this close proximity environment? It wasn't noted that way on the jeppesen charts either. It seems TCAS is really still needed in this situation for any overshoot.should the TRACON turn visual approach traffic at the same turn point as the RNAV approach; or stagger it? Is it ok for TRACON to break separation with instrument approach aircraft with a traffic call? Perhaps the TCAS software is too slow and sensitive and unnecessarily responds. Better TRACON coordination is needed as the continuous radius final intercept on the RNAV presents at a much steeper angle than the typical 30-degree intercept and sets off TCAS. Requiring an autopilot to be on in this situation (by bulletin) may not lead to greater awareness; anticipation; or prevention of the situation. Guidance in flight operation manual and 737 aircraft manual do not seem to indicate any procedural latitude once the traffic is acquired and separation is ensured. I think this event will provide some valuable insight for other parties; particularly TRACON; and fleets.

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Original NASA ASRS Text

Title: B737 Captain reported the design of the DEN RNAV Z 16R STAR led to a TCAS RA alert for traffic on the parallel runway.

Narrative: RNAV approaches; especially arcing ones; at DEN have been a hot issue lately. There is even a bulletin emphasizing that these should be flown with use of the autopilot. The 16R RNAV Z also has a turning arc; to reverse course just outside the 'FAF' for many of the approaches. On this approach; autopilot on; we could see oncoming traffic while on base; and rolling final. This converging traffic; we presumed; was for the parallel runway. Our convergence caused a TCAS RA with 300 feet vertical separation. We had a descending RA then climbing one; after which the traffic could be viewed as on a parallel path and below us. In the climb; we dropped gear and flaps so that we could slow and not abandon the approach. We were able to stabilize the approach around 1000 feet AGL; though due to the confusion of ATC handoff; configuration change; and new procedures; we did not turn the TCAS to TA only and ignored it once on a parallel path; where the TCAS event disappeared shortly thereafter. Landing was uneventful and the event was debriefed in the cockpit. Some of the issues that arose were as follows:Visual approach to the left runway and RNAV Z approach for the right runway were converging on final at about the same place. In the visual only environment; the convergence would be better monitored; but an instrument approach and visual traffic; at night no less; presents an unexpected convergence where perhaps separation is anticipated. The TRACON gave a traffic advisory; but there was confusion due to expected separation of the 'RNAV'. Procedurally; TCAS can be selected to TA in a close proximity or parallel approach environment; but we were still converging to our respective finals. In accepting the RNAV approach; were we somehow subjecting ourselves to this close proximity environment? It wasn't noted that way on the Jeppesen charts either. It seems TCAS is really still needed in this situation for any overshoot.Should the TRACON turn visual approach traffic at the same turn point as the RNAV Approach; or stagger it? Is it ok for TRACON to break separation with instrument approach aircraft with a traffic call? Perhaps the TCAS software is too slow and sensitive and unnecessarily responds. Better TRACON coordination is needed as the continuous radius final intercept on the RNAV presents at a much steeper angle than the typical 30-degree intercept and sets off TCAS. Requiring an autopilot to be on in this situation (by bulletin) may not lead to greater awareness; anticipation; or prevention of the situation. Guidance in Flight Operation Manual and 737 Aircraft Manual do not seem to indicate any procedural latitude once the traffic is acquired and separation is ensured. I think this event will provide some valuable insight for other parties; particularly TRACON; and fleets.

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.