Date: 201312
Local time of day: 0601-1200
Location: DEN.Airport, CO
Flight conditions: IMC
Weather elements / visibility: Snow
Light: Daylight
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01; Tower DEN
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Aircraft component: Traffic Collision Avoidance System (TCAS)
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Human factors: Confusion; Human-Machine Interface; Situational Awareness; Training / Qualification
Second reporter function: Pilot Flying; Captain
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Less Severe; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Executed Go Around / Missed Approach; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Procedure; Weather
Primary problem: Procedure

Narrative: Denver was operating parallel approaches to 35L and 35R. We were on a straight in approach to Runway 35L in IFR conditions with reported heavy snow and parallel operations being conducted to Runway 35R. Approximately 23 miles from the field we were instructed to contact Tower. The Captain was flying the aircraft with both autopilots engaged for the briefed Cat III approach to autoland. At an approximate altitude of 10;000 MSL the TCAS gave a commanded descending RA because of an aircraft on approach to the parallel Runway 35R. All procedures and radio calls were made appropriately with the Captain disengaging the autopilot and immediately initiating a vertical descent while remaining on the lateral profile and I made a radio call to Tower of TCAS descent. While the Captain was primarily engaged with the maneuver I focused on the intruder position and mentioned he was to the right and below. We both agreed that a descent into the intruder did not make sense; but the TCAS command was followed as we do not know what the intruder is being commanded or if they were receiving any TCAS commands. I would approximate 10 seconds later the TCAS gave an abrupt reversal command for a maximum climb which the Captain immediately performed and I gave another call to Tower of a TCAS climb and told the Captain they were close to the right and 200 FT below.Shortly after initiating the TCAS climb the event was over. I asked the Captain if he would like to do a go-around which he instructed yes since we would not be able to get back on a stabilized vertical profile. We were instructed by Tower to maintain heading and climb to 10;000 FT and eventually we were switched over to Approach Control. At all times we remained on the proper lateral profile for the approach to Runway 35L. At no time did I hear any response from Tower to the two TCAS callouts made.Our TCAS was obviously operating in the TA/RA mode. Subsequent discussion with the Captain about the event was the Company FOM mentioning of an allowed procedure to switch the TCAS mode to TA only when intentionally operating in close environments to other aircraft to prevent RA's. In contrast to a PRM approach where there is a monitoring Controller; in this case we have two aircraft more than 20 miles from the airport on different Tower frequencies; on different approaches; to different runways; approximately 5;000 FT apart. While I understand the intention of the procedure of switching the TCAS mode to TA only and its benefits while in VFR conditions it should be noted this is not a required procedure and I personally do not agree with the procedure when in IFR conditions operating outside Tower's normal area of responsibility; not knowing who is responsible for monitoring the positions of close aircraft on non-PRM approaches; on different frequencies; coupled with the fact that this procedure will never prevent an overshoot or blunder into the path of an adjacent aircraft.Further discussions with the Captain was that I am sure even with parallel runways approximately 5;000 FT apart that Approach Control was attempting to stagger (offset) the aircraft to adjacent runways but for some reason it did not work out that way for our approach. Our second approach had the adjacent aircraft between 2-3 miles ahead of us on their profile and no TCAS events occurred. If possible I would like to receive notification of how close our two aircraft came to each other.


Second narrative: I was pilot flying on ILS 35L at DEN between CHOLA and CELBI descending between 10;000 FT and 9;000 FT on glideslope when we got a 'traffic; traffic' advisory followed by a Resolution Advisory (RA) commanding descent. Intruder was to our right and approximately below us. It did not make sense; but I followed the RA as required. A few seconds later TCAS reversed the command calling for a maximum TCAS climb which I immediately followed. The First Officer made the 2 appropriate 'TCAS descent' and 'TCAS climb' calls on Tower frequency. During the reversal the First Officer called out to me 'traffic close on right 200 FT below.' Within seconds after establishing the TCAS climb the event was over. Soon (seconds) thereafter I initiated a go-around because there was no way we were going to be able to get back to glideslope in a stabilized condition. We leveled at 10;000 FT; informed Tower and accepted vectors for another ILS 35L Approach which was normal. All proper TCAS maneuvers and Tower calls were made thus avoiding what really looked to us like a near midair situation. Tower never acknowledged our calls during the event. After leveling at 10;000 FT following the go-around I asked Tower about the position of the intruder aircraft. Tower said that the other aircraft was established on the Localizer for ILS 35R. I do not believe this was true. We had been established on and identified on the Localizer for ILS 35L since at least CPPER. It should be noted that the altitudes and positions noted in this report were reconstructed by myself and the First Officer following the event and were therefore approximate notes.


Synopsis: An A320 crew on the DEN 35L ILS took evasive action in response to a TCAS DESCEND command with lower abeam traffic on the 35R ILS; but the command reversed to the correct CLIMB command after about 10 seconds. A go-around was initiated at that point.