Narrative:

We were being vectored by the denver TRACON for the ILS 34L approach. The ATIS advertised triple simultaneous ILS approaches to 34L; 35L; and 35R. We were in instrument meteorological conditions and expected an approach to near minimums. We began to accrete ice; so I began to look up our ice accumulation approach speeds. We were issued a turn to final and the first officer mentioned a concern he had with an aircraft on our TCAS display. Just as I was about to query ATC we were given a turn to final and clearance to intercept the 34L localizer. At that moment we received a traffic advisory (TA). The first officer turned off the autopilot and tightened the turn. We then received a resolution advisory (RA) instructing us to descend. I took the controls and complied with the RA. The TCAS then gave a climb command. I added emergency power and complied. After obtaining the necessary FPM climb we; once again; got a 'descend; descend' command from the TCAS. At this point I pulled the aircraft in a very steep bank in the direction that ATC gave us and complied with the last descend command given by the TCAS. At this point I passed controls back to the first officer and I saw we were still on the west side of the 34L localizer. The first officer complied with the remaining radar vectors and climbed back to 10;000 ft. At this point the aircraft was shuddering noticeably due to moderate mixed icing conditions; but we were able to maintain airspeed of 210 KTS (assigned) and climb with adequate safety margins. I asked the controller for a phone number so the situation could be further discussed on the ground. I was told an rj flew through the 35L localizer and was not on his assigned altitude. This was consistent with what we witnessed on the TCAS. I called the TRACON after arrival and was told that the parallel traffic was 2;000 ft below his assigned altitude and was established on the inbound course for 35L. I was told that this was the cause of the traffic alerts given. This did not explain the lack of lateral separation that was encountered as it would not be expected to receive an RA if we were on the west side of our approach course and the other aircraft was established inbound on the proper course. He also said that separation was lost. Vertical separation was 0 and lateral separation was no closer than 2 miles. These runways are well within this range and evasive maneuvers were necessary to avoid collision. So in our view; lateral separation must have also been lost if the other aircraft turned westbound and transgressed into our course at our altitude. I am also unsure as to why we were getting conflicting resolution advisories. I do not know the situation in the other cockpit but we attempted to comply immediately with the RA instructions and ended up getting very concerning instructions. The other aircraft appeared to be heading westbound before any TCAS advisories were given (the TRACON supervisor told me that they were complying with an ATC instruction). I don't know the timing of these events on the east side of the airport; but it seemed that we were put into a very dangerous situation due to either multiple errors or approach design tolerances. Proper compliance with TCAS instructions; approach clearances; and altitudes. I am unsure as to what role ATC had in vectoring the aircraft into our course; but undoubtedly some responsibility as discussed on the phone with the TRACON supervisor.

Related reports:

Google
 

Original NASA ASRS Text

Title: A Light Transport flight crew experiences a TCAS RA with a CRJ during simultaneous instrument approaches to Runways 34L and 35L at DEN. TCAS issued a descend; then a climb; then another descend instruction to avoid the CRJ which was apparently at the wrong altitude.

Narrative: We were being vectored by the Denver TRACON for the ILS 34L approach. The ATIS advertised triple simultaneous ILS approaches to 34L; 35L; and 35R. We were in instrument meteorological conditions and expected an approach to near minimums. We began to accrete ice; so I began to look up our ice accumulation approach speeds. We were issued a turn to final and the First Officer mentioned a concern he had with an aircraft on our TCAS display. Just as I was about to query ATC we were given a turn to final and clearance to intercept the 34L localizer. At that moment we received a traffic advisory (TA). The First Officer turned off the autopilot and tightened the turn. We then received a resolution advisory (RA) instructing us to descend. I took the controls and complied with the RA. The TCAS then gave a climb command. I added emergency power and complied. After obtaining the necessary FPM climb we; once again; got a 'Descend; Descend' command from the TCAS. At this point I pulled the aircraft in a very steep bank in the direction that ATC gave us and complied with the last descend command given by the TCAS. At this point I passed controls back to the First Officer and I saw we were still on the west side of the 34L localizer. The First Officer complied with the remaining radar vectors and climbed back to 10;000 FT. At this point the aircraft was shuddering noticeably due to moderate mixed icing conditions; but we were able to maintain airspeed of 210 KTS (assigned) and climb with adequate safety margins. I asked the Controller for a phone number so the situation could be further discussed on the ground. I was told an RJ flew through the 35L localizer and was not on his assigned altitude. This was consistent with what we witnessed on the TCAS. I called the TRACON after arrival and was told that the parallel traffic was 2;000 FT below his assigned altitude and was established on the inbound course for 35L. I was told that this was the cause of the traffic alerts given. This did not explain the lack of lateral separation that was encountered as it would not be expected to receive an RA if we were on the west side of our approach course and the other aircraft was established inbound on the proper course. He also said that separation was lost. Vertical separation was 0 and lateral separation was no closer than 2 miles. These runways are well within this range and evasive maneuvers were necessary to avoid collision. So in our view; lateral separation must have also been lost if the other aircraft turned westbound and transgressed into our course at our altitude. I am also unsure as to why we were getting conflicting resolution advisories. I do not know the situation in the other cockpit but we attempted to comply immediately with the RA instructions and ended up getting very concerning instructions. The other aircraft appeared to be heading westbound before any TCAS advisories were given (The TRACON Supervisor told me that they were complying with an ATC instruction). I don't know the timing of these events on the east side of the airport; but it seemed that we were put into a very dangerous situation due to either multiple errors or approach design tolerances. Proper compliance with TCAS instructions; approach clearances; and altitudes. I am unsure as to what role ATC had in vectoring the aircraft into our course; but undoubtedly some responsibility as discussed on the phone with the TRACON Supervisor.

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.