Narrative:

Den WX was IFR, using simultaneous VOR/DME 17L and localizer 18 approachs. As we were tuned on final, TCASII gave TA for traffic slightly lower than our altitude, behind and to the right. About this time, we were switched to tower. TCASII gave another alert. We checked on with tower, and inquired whether the traffic behind and below was on approach to the parallel runway. The response was for us to turn 90 degree left, maintain 8000 ft. We turned and climbed. Approximately 30 seconds later, another jet reported inbound -- just where we would have been if we had not turned. After we got on the ground, we called approach TRACON and asked what the reason was for our turn, and what our conflict was. They said that our traffic was indeed for runway 18, an light transport jet, whom they had not yet talked to. They said their procedure is to turn the aircraft with whom they have contact off the approach, so we were turned. We could not see the traffic visually, but were getting alerts on TCASII. I'm still not sure if we actually had another airplane on the same approach (17L) or if, indeed, the light transport on 18 was giving the alert. If it was 17L traffic, there was an error. If it was 18 traffic, why didn't they simply tell us it was 18 traffic. If we question a TCASII alert while IFR, they need to tell us about the traffic, not turn us off the approach and burden the system with another aircraft in the pattern while they are already busy. You can imagine our thoughts when we felt we had almost had a midair collision!

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Original NASA ASRS Text

Title: AN MLG HAD TO BE REMOVED FROM THE ILS BECAUSE OF A TCASII ALERT IN IMC.

Narrative: DEN WX WAS IFR, USING SIMULTANEOUS VOR/DME 17L AND LOC 18 APCHS. AS WE WERE TUNED ON FINAL, TCASII GAVE TA FOR TFC SLIGHTLY LOWER THAN OUR ALT, BEHIND AND TO THE R. ABOUT THIS TIME, WE WERE SWITCHED TO TWR. TCASII GAVE ANOTHER ALERT. WE CHKED ON WITH TWR, AND INQUIRED WHETHER THE TFC BEHIND AND BELOW WAS ON APCH TO THE PARALLEL RWY. THE RESPONSE WAS FOR US TO TURN 90 DEG L, MAINTAIN 8000 FT. WE TURNED AND CLBED. APPROX 30 SECONDS LATER, ANOTHER JET RPTED INBOUND -- JUST WHERE WE WOULD HAVE BEEN IF WE HAD NOT TURNED. AFTER WE GOT ON THE GND, WE CALLED APCH TRACON AND ASKED WHAT THE REASON WAS FOR OUR TURN, AND WHAT OUR CONFLICT WAS. THEY SAID THAT OUR TFC WAS INDEED FOR RWY 18, AN LTT JET, WHOM THEY HAD NOT YET TALKED TO. THEY SAID THEIR PROC IS TO TURN THE ACFT WITH WHOM THEY HAVE CONTACT OFF THE APCH, SO WE WERE TURNED. WE COULD NOT SEE THE TFC VISUALLY, BUT WERE GETTING ALERTS ON TCASII. I'M STILL NOT SURE IF WE ACTUALLY HAD ANOTHER AIRPLANE ON THE SAME APCH (17L) OR IF, INDEED, THE LTT ON 18 WAS GIVING THE ALERT. IF IT WAS 17L TFC, THERE WAS AN ERROR. IF IT WAS 18 TFC, WHY DIDN'T THEY SIMPLY TELL US IT WAS 18 TFC. IF WE QUESTION A TCASII ALERT WHILE IFR, THEY NEED TO TELL US ABOUT THE TFC, NOT TURN US OFF THE APCH AND BURDEN THE SYS WITH ANOTHER ACFT IN THE PATTERN WHILE THEY ARE ALREADY BUSY. YOU CAN IMAGINE OUR THOUGHTS WHEN WE FELT WE HAD ALMOST HAD A MIDAIR COLLISION!

Data retrieved from NASA's ASRS site as of July 2007 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.