Date: 201404
Local time of day: 1201-1800
Location: DEN.Airport, CO
Flight conditions: IMC
Weather elements / visibility: Windshear; Snow
Light: Daylight
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01
Aircraft component: Traffic Collision Avoidance System (TCAS)
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Human factors: Situational Awareness; Confusion; Communication Breakdown; Distraction
Second reporter function: Pilot Flying; Captain
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Contributing factors: Environment - Non Weather Related; Human Factors; Procedure; Weather
Primary problem: Ambiguous

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.


Second narrative: I was the pilot flying on a flight to DEN. We had prepared and briefed for the Cat IIIB ILS 35L to an autoland. The current weather was varying between 2;400 to 4;500 RVR; blowing snow; low ceilings; and winds out of the northeast at approximately 25 KTS. We were given a heading and assigned 9;000 FT until established cleared for the ILS 35L. We then joined the localizer just outside of CHOLA in level flight at 9;000 FT and assigned speed of 170 KTS. I remained at 9;000 FT until we intercepted the glideslope at CELBI and commenced our normal descent on the glideslope. ATC assigned us another speed reduction of 160 KTS. Our proceeding traffic on [Runway] 35L was showing about 6 miles in front of us; it was a heavy. I just figured we must be gaining on them. I was looking at the TCAS for traffic on the parallels and didn't see any other traffic displayed; nor did ATC advise us of any other traffic. While descending through approximately 8;000 FT; 2;500 AGL; we get a RA commanding a steep descent; approximately full scale down. As I initiated the descent complying with the RA; I saw on the TCAS it was showing our intruder in red and 500 FT below us and to our right. It appeared to me that we were descending into them. I felt we were getting conflicting information which caused some confusion; so I elected to level off immediately to better assess the situation; at this point it's showing them only 200 FT lower than us. The RA at this point is commanding 'MONITOR VERTICAL SPEED' followed by a 'CLIMB' command a short time later. I then complied immediately to the 'CLIMB' command; at the same time Denver Final Monitor told us to slow to minimum approach speed and my First Officer responded unable we are complying with an RA and going around. The Controller then said go-around and track the localizer inbound and climb to 9;000 FT. While we were complying with the RA on the climb the RA announced clear of conflict; we leveled at 9;000 FT and were then radar vectored to the ILS 35R and landed uneventfully. ATC never advised us of the traffic that was being vectored from the east onto the ILS 35R that in my opinion was to be staggered in front of us on the parallel Runway 35R; and was the reasoning for our speed reductions. I believe the other aircraft grossly flew through the 35R localizer causing the RA. ATC should have advised us of the traffic that was attempting to join the Localizer on 35R and given proper separation on parallel approaches. The aircraft joining the 35R localizer shouldn't have overflown the localizer.


Synopsis: 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.