Narrative:

Were being vectored for ILS runway 9R; on the base leg of a 180 degree heading; at assigned altitude of 5;000 ft; 210 KTS. Both had TCAS displayed on our mfds; and were in IMC. Pointed out traffic on the TCAS. It was established on the runway 10 final; at our altitude. I replied that he was established on the final for the parallel runway; but thought it strange that he was at our altitude. He asked if we should query ATC. I hesitated because the frequency was very busy; and I was afraid we would block ATC's transmission to turn us onto the final; thus worsening the potential hazard. As we got closer to the final; it was clear that ATC was going to overshoot us through the final; creating a collision hazard. We made the turn to intercept; but finally noticing the altitude error; the controller cleared us for an immediate (emergency) descent to 4;000 ft. I disengaged the autopilot and manually flew this maneuver at maximum bank angle. The TCAS gave us a TA; but not an RA. We were then broken off the approach; and given vectors for a second. Landed uneventfully. After reviewing the tapes and radar data; he admitted that it was not only an operational error; but an illegal operation that resulted in a loss of separation. The fact that both the first officer and I both had our mfd displaying TCAS gave us situational awareness that allowed us to be aware of the potential hazard; and respond to the hazard in a timely manner. Better awareness of ATC policies and procedures for conducting parallel approaches would be helpful. I think I would have been a little less hesitant to question our altitude assignment had I known that it was indeed against policy for ATC to do that.

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Original NASA ASRS Text

Title: Air carrier landing ORD being vectored at 5000 FT for Runway 9R experienced conflict with traffic on parallel Runway 10; ATC admitted operational error.

Narrative: Were being vectored for ILS Runway 9R; on the base leg of a 180 degree heading; at assigned altitude of 5;000 FT; 210 KTS. Both had TCAS displayed on our MFDs; and were in IMC. Pointed out traffic on the TCAS. It was established on the Runway 10 final; at our altitude. I replied that he was established on the final for the parallel runway; but thought it strange that he was at our altitude. He asked if we should query ATC. I hesitated because the frequency was very busy; and I was afraid we would block ATC's transmission to turn us onto the final; thus worsening the potential hazard. As we got closer to the final; it was clear that ATC was going to overshoot us through the final; creating a collision hazard. We made the turn to intercept; but finally noticing the altitude error; the Controller cleared us for an immediate (emergency) descent to 4;000 FT. I disengaged the autopilot and manually flew this maneuver at maximum bank angle. The TCAS gave us a TA; but not an RA. We were then broken off the approach; and given vectors for a second. Landed uneventfully. After reviewing the tapes and radar data; he admitted that it was not only an operational error; but an illegal operation that resulted in a loss of separation. The fact that both the First Officer and I both had our MFD displaying TCAS gave us situational awareness that allowed us to be aware of the potential hazard; and respond to the hazard in a timely manner. Better awareness of ATC policies and procedures for conducting parallel approaches would be helpful. I think I would have been a little less hesitant to question our altitude assignment had I known that it was indeed against policy for ATC to do that.

Data retrieved from NASA's ASRS site as of April 2012 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.