Narrative:

A tfr had just been lifted with multiple aircraft holding and inbound to the airport. Numerous runway changes were being issued to various aircraft; including ours; by approach control. After our runway change from 13R to 13L and being cleared for a visual approach by approach control and cleared to land on runway 13L by tower; 'ILS 13L OTS' as stated by the ATIS; we were beginning the process of configuring for landing. Initial flaps and landing gear had just been extended and we were about to configure to landing flaps around the FAF. At that time; with no previous 'traffic' warning; we got a completely unexpected RA with the voice prompt 'descend now' and the vsi presentation of a descent. The sic (PF) disconnected the ap and started a slow descent while we looked for traffic and tried to gain situational awareness. A few seconds after the onset of the RA and with no visual contact with traffic; the PIC commanded; 'follow the RA now!' this was done by the sic. However the conflicting traffic continued a descent matching our descent rate and the RA continued with no further voice prompt but with a visual display commanding a descent on the vsi. This continued until the sic became concerned about the RA possibly leading to a low altitude and a CFIT issue. About this time the PIC gained visual contact with another aircraft above and behind our position on the parallel visual to runway 13R. It was obvious that this was the traffic causing the event and it was on a normal visual to the parallel runway and was no threat. About that time the RA went away with no voice prompt. A stabilized approach was continued to landing. No report was made to ATC because no deviation from ATC clearance was experienced and no intrusion by the other aircraft was experienced and there was no failure with ATC. Whether the other aircraft experienced an RA or had visual contact with us is unknown. This is believed by the crew to be a nuisance RA. Regarding human factors: the unexpected TCAS RA; with no earlier warning and in an unusual area of flight operations for a TCAS event; and under high workloads; was initially thought by both pilots to be an anomaly or malfunction. However in the case of no other evidence of a malfunction; the RA was followed to the extent as not to jeopardize the aircraft further by entering into a GPWS or CFIT event. This presents a dual-minded pilot. That is; the innate desire not to descend in close proximity to the ground or below a normal glidepath and the desire to descend via the RA as is regularly instilled in the pilot during simulator and classroom instruction. The delay in this decision making process can exceed the time allotted to execute the RA action with success. In addition; the belief; due to frequency of the event; that the RA is not 'real' and is generated by another aircraft on the other parallel visual approach leads the pilots to complacency in the event of an RA in this situation; when in fact the RA may not be a nuisance RA and is in fact a potential collision event. This complacency can also lead to the pilots not reacting in time to successfully resolve a real collision possibility. The tendency of the pilot after discovering the RA event was a 'nuisance RA' is to not report the event to ATC because no party was really responsible for the event 'trigger;' and in this event; no deviation from ATC clearance was experienced by either aircraft.

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Original NASA ASRS Text

Title: An air carrier flight crew at 2;000 feet on a visual approach to DAL Runway 13L followed a TCAS RA command from an unannounced aircraft on approach to 13R.

Narrative: A TFR had just been lifted with multiple aircraft holding and inbound to the airport. Numerous runway changes were being issued to various aircraft; including ours; by Approach Control. After our runway change from 13R to 13L and being cleared for a visual approach by Approach Control and cleared to land on Runway 13L by Tower; 'ILS 13L OTS' as stated by the ATIS; we were beginning the process of configuring for landing. Initial flaps and landing gear had just been extended and we were about to configure to landing flaps around the FAF. At that time; with no previous 'TRAFFIC' warning; we got a completely unexpected RA with the voice prompt 'DESCEND NOW' and the VSI presentation of a descent. The SIC (PF) disconnected the AP and started a slow descent while we looked for traffic and tried to gain situational awareness. A few seconds after the onset of the RA and with no visual contact with traffic; the PIC commanded; 'follow the RA now!' This was done by the SIC. However the conflicting traffic continued a descent matching our descent rate and the RA continued with no further voice prompt but with a visual display commanding a descent on the VSI. This continued until the SIC became concerned about the RA possibly leading to a low altitude and a CFIT issue. About this time the PIC gained visual contact with another aircraft above and behind our position on the parallel visual to Runway 13R. It was obvious that this was the traffic causing the event and it was on a normal visual to the parallel runway and was no threat. About that time the RA went away with no voice prompt. A stabilized approach was continued to landing. No report was made to ATC because no deviation from ATC clearance was experienced and no intrusion by the other aircraft was experienced and there was no failure with ATC. Whether the other aircraft experienced an RA or had visual contact with us is unknown. This is believed by the crew to be a nuisance RA. Regarding human factors: The unexpected TCAS RA; with no earlier warning and in an unusual area of flight operations for a TCAS event; and under high workloads; was initially thought by both pilots to be an anomaly or malfunction. However in the case of no other evidence of a malfunction; the RA was followed to the extent as not to jeopardize the aircraft further by entering into a GPWS or CFIT event. This presents a dual-minded pilot. That is; the innate desire not to descend in close proximity to the ground or below a normal glidepath and the desire to descend via the RA as is regularly instilled in the pilot during simulator and classroom instruction. The delay in this decision making process can exceed the time allotted to execute the RA action with success. In addition; the belief; due to frequency of the event; that the RA is not 'real' and is generated by another aircraft on the other parallel visual approach leads the pilots to complacency in the event of an RA in this situation; when in fact the RA may not be a nuisance RA and is in fact a potential collision event. This complacency can also lead to the pilots not reacting in time to successfully resolve a real collision possibility. The tendency of the pilot after discovering the RA event was a 'nuisance RA' is to not report the event to ATC because no party was really responsible for the event 'trigger;' and in this event; no deviation from ATC clearance was experienced by either aircraft.

Data retrieved from NASA's ASRS site as of July 2013 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.