Narrative:

I was the pilot flying during a TCAS commanded RA event while on final to runway 17C at dfw in solid IMC conditions that required a descend below ILS glide slope indications in order to comply with the TCAS RA commands. It was a very busy morning at dfw. Dfw was landing on runway 17L; 17C; and 18R. All runways had heavy traffic. The base leg for 17C was at 20 DME. Once established on final and tracking the centerline of the localizer ATC told us to slow to final approach speed while we were still approximately 15 DME from dfw due to traffic saturation.we were fully configured on final approach speed when we entered solid IMC conditions approaching zingg due to a low overcast cloud bank. We were conducting a practice cat III HUD landing and I was already hand flying the aircraft. We were aware of the multiple parallel landing traffic on runways 17L and 18R.shortly after entering IMC conditions we received a TCAS RA commanding a descent in response to depicted traffic off the left side of our aircraft (presumingly landing traffic to 17L). The first officer reported our RA maneuvering response to ATC. ATC did not indicate any aircraft traffic separation concerns for the RA we were receiving. Complying with the minimum descent necessary to remain out of the red commanded area of the TCAS RA maneuver required an uncomfortable descend below the ILS glide slope for 17C. I considered several options during this very time compressed event to include [requesting priority handling] and initiating a go-around that would have been counter to the TCAS guidance and inhibited by the TCAS traffic above and in close proximity to our aircraft.[requesting priority handling] and establishing an offset or breakout right of course to establish lateral separation from the depicted TCAS traffic on our displays that would have compromised obstacle clearance even more that staying on the ILS centerline and would put us in conflict with aircraft landing 18R. At some point during this rapidly occurring event the HUD guidance also disappeared; but the flight director guidance on the pfd remained. Having flown into dfw for years I was aware that my main concern in this area was the radio towers either side of centerline and not terrain in the immediate vicinity of the aircraft. I did shallow out my TCAS maneuver approaching jiffy as a compromise between obstacle clearance and TCAS commands due to the radio tower in the vicinity of jiffy. We re-entered VMC conditions at approximately 1;200 feet AGL. We were never able to establish a visual contact with the potential TCAS target aircraft once we clear of all clouds. I do not know if this was a ghost target; an equipment malfunction; or an actual legitimate RA event. I have never felt so boxed in without a clear safe choice of action in all my years of flying. This all happened over a very short time span with a lot of conflicting information to process. My primary concern is a reoccurrence of this event due to either inadequate distance separation between runways at dfw for parallel operations; equipment malfunction; or the other TCAS event aircraft either not receiving or not complying with TCAS RA traffic resolution maneuvers as we never diverged from the other aircraft during the TCAS event. Having given this situation considerable thought I believe the best resolution to this situation would have been to declare an emergency and then find a fast way to communicate the situation to ATC and demand that they have the other aircraft execute and immediate go-around. This option obviously requires open communication channels that seldom exist during heavy landing operations at dfw.

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Original NASA ASRS Text

Title: B737NG Crew reported that while flying IMC on the DFW Runway 17R ILS inside of 15 DME; they descended below the glideslope as they took evasive action from a Runway 17L aircraft TCAS RA.

Narrative: I was the pilot flying during a TCAS commanded RA event while on final to Runway 17C at DFW in solid IMC conditions that required a descend below ILS glide slope indications in order to comply with the TCAS RA commands. It was a very busy morning at DFW. DFW was landing on Runway 17L; 17C; and 18R. All runways had heavy traffic. The base leg for 17C was at 20 DME. Once established on final and tracking the centerline of the localizer ATC told us to slow to final approach speed while we were still approximately 15 DME from DFW due to traffic saturation.We were fully configured on final approach speed when we entered solid IMC conditions approaching ZINGG due to a low overcast cloud bank. We were conducting a practice Cat III HUD landing and I was already hand flying the aircraft. We were aware of the multiple parallel landing traffic on Runways 17L and 18R.Shortly after entering IMC conditions we received a TCAS RA commanding a descent in response to depicted traffic off the left side of our aircraft (presumingly landing traffic to 17L). The First Officer reported our RA maneuvering response to ATC. ATC did not indicate any aircraft traffic separation concerns for the RA we were receiving. Complying with the minimum descent necessary to remain out of the red commanded area of the TCAS RA maneuver required an uncomfortable descend below the ILS glide slope for 17C. I considered several options during this very time compressed event to include [requesting priority handling] and initiating a go-around that would have been counter to the TCAS guidance and inhibited by the TCAS traffic above and in close proximity to our aircraft.[Requesting priority handling] and establishing an offset or breakout right of course to establish lateral separation from the depicted TCAS traffic on our displays that would have compromised obstacle clearance even more that staying on the ILS centerline and would put us in conflict with aircraft landing 18R. At some point during this rapidly occurring event the HUD guidance also disappeared; but the flight director guidance on the PFD remained. Having flown into DFW for years I was aware that my main concern in this area was the radio towers either side of centerline and not terrain in the immediate vicinity of the aircraft. I did shallow out my TCAS maneuver approaching JIFFY as a compromise between obstacle clearance and TCAS commands due to the radio tower in the vicinity of JIFFY. We re-entered VMC conditions at approximately 1;200 feet AGL. We were never able to establish a visual contact with the potential TCAS target aircraft once we clear of all clouds. I do not know if this was a ghost target; an equipment malfunction; or an actual legitimate RA event. I have never felt so boxed in without a clear safe choice of action in all my years of flying. This all happened over a very short time span with a lot of conflicting information to process. My primary concern is a reoccurrence of this event due to either inadequate distance separation between runways at DFW for parallel operations; equipment malfunction; or the other TCAS event aircraft either not receiving or not complying with TCAS RA traffic resolution maneuvers as we never diverged from the other aircraft during the TCAS event. Having given this situation considerable thought I believe the best resolution to this situation would have been to declare an emergency and then find a fast way to communicate the situation to ATC and demand that they have the other aircraft execute and immediate go-around. This option obviously requires open communication channels that seldom exist during heavy landing operations at DFW.

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.