Narrative:

While being radar vectored to join the localizer for runway 35L at den; we were instructed to maintain 9;000 ft until established. We were given successive speed reductions from ATC until we later abandoned the approach. The surface winds were out of the north northeast at approximately 020 degrees at 25 KTS with low overcast ceilings and visibility less than one mile in blowing snow. A CAT IIIB autoland approach had been briefed and all checklists had been completed per SOP. We were subsequently cleared for the ILS approach to runway 35L. The aircraft joined and captured the localizer in level flight at 8;000 ft and; once the glideslope became active; the aircraft began its descent on the glideslope in instrument meteorological conditions. The aircraft continued to track the localizer and glideslope with very little lateral or vertical deviation. While passing through approximately 8;000 ft; ATC issued us another speed reduction from the 'final monitor'. At the same instant; as we were slowing; a descending RA annunciated on both mfds. Prior to the RA; we never had any indication that there was traffic on the parallel runway in any form; neither by means of an ATC advisory or a TA icon on our instrumentation. I do not remember hearing the aircraft order a descent; but instead remember seeing a red traffic icon and a descending RA command replace the vsi. It quickly became apparent that the crosswind from the right had caused the traffic on the parallel runway to overshoot his final approach course. At this point; the intruder aircraft was below us; which caused confusion in the cockpit. The captain; who was the pilot flying; leveled the aircraft to prevent further descent and the TCAS system seemed to agree prompting a 'monitor vertical descent'. The TCAS system then commanded an aggressive descent. We; however; did not comply with the orders because we both noticed that the intruder aircraft was approximately -200 ft below our altitude. Soon after starting a climb; the TCAS system ordered an aggressive climb; which we complied with to 9;000 ft. During our climb and realizing that separation was an issue; the tower issued go-around instructions to us and we climbed to 9;000 ft while continuing to track the localizer inbound while performing a go-around per SOP. We then received radar vectors to the parallel runway; 35R and landed uneventfully. I believe that ATC severely misjudged the lateral and vertical spacing during parallel runway operations. From my viewpoint in the cockpit; ATC attempted to stagger the spacing on the parallel localizers by sequentially reducing our assigned speed. Unfortunately; the intruder aircraft was not slowed properly and blundered through his parallel localizer course causing our RA. I am not sure why our TCAS system commanded a descending RA with the intruder aircraft below our position and; most likely; continuing to descend on his glide path. I am; however; grateful that we were able to maintain situational awareness and successfully maneuver away from the conflicting traffic thanks to our training and crew coordination.

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Original NASA ASRS Text

Title: An A320 on a DEN Runway 35L ILS received a severe DOWN RED arrow TCAS alerted which the crew interpreted as a descend command and also an aural command; MONITOR VERTICAL SPEED. The crew did not descend because the target was also displayed but they incorrectly interpreted the TCAS command.

Narrative: While being radar vectored to join the localizer for Runway 35L at DEN; we were instructed to maintain 9;000 FT until established. We were given successive speed reductions from ATC until we later abandoned the approach. The surface winds were out of the north northeast at approximately 020 degrees at 25 KTS with low overcast ceilings and visibility less than one mile in blowing snow. A CAT IIIB autoland approach had been briefed and all checklists had been completed per SOP. We were subsequently cleared for the ILS approach to Runway 35L. The aircraft joined and captured the localizer in level flight at 8;000 FT and; once the glideslope became active; the aircraft began its descent on the glideslope in instrument meteorological conditions. The aircraft continued to track the localizer and glideslope with very little lateral or vertical deviation. While passing through approximately 8;000 FT; ATC issued us another speed reduction from the 'Final Monitor'. At the same instant; as we were slowing; a descending RA annunciated on both MFDs. Prior to the RA; we never had any indication that there was traffic on the parallel runway in any form; neither by means of an ATC advisory or a TA icon on our instrumentation. I do not remember hearing the aircraft order a descent; but instead remember seeing a red traffic icon and a descending RA command replace the VSI. It quickly became apparent that the crosswind from the right had caused the traffic on the parallel runway to overshoot his final approach course. At this point; the intruder aircraft was below us; which caused confusion in the cockpit. The Captain; who was the pilot flying; leveled the aircraft to prevent further descent and the TCAS system seemed to agree prompting a 'MONITOR VERTICAL DESCENT'. The TCAS system then commanded an aggressive descent. We; however; did not comply with the orders because we both noticed that the intruder aircraft was approximately -200 FT below our altitude. Soon after starting a climb; the TCAS system ordered an aggressive climb; which we complied with to 9;000 FT. During our climb and realizing that separation was an issue; the Tower issued go-around instructions to us and we climbed to 9;000 FT while continuing to track the localizer inbound while performing a go-around per SOP. We then received radar vectors to the parallel Runway; 35R and landed uneventfully. I believe that ATC severely misjudged the lateral and vertical spacing during parallel runway operations. From my viewpoint in the cockpit; ATC attempted to stagger the spacing on the parallel localizers by sequentially reducing our assigned speed. Unfortunately; the intruder aircraft was not slowed properly and blundered through his parallel localizer course causing our RA. I am not sure why our TCAS system commanded a descending RA with the intruder aircraft below our position and; most likely; continuing to descend on his glide path. I am; however; grateful that we were able to maintain situational awareness and successfully maneuver away from the conflicting traffic thanks to our training and crew coordination.

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.