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.

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Original NASA ASRS Text

Title: 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.

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.

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.