Narrative:

As we were descending to 9;000 ft. We received an RA to monitor vertical speed. The first officer confirmed the RA and I disconnected the autopilot and followed the commands to increase the descent rate. We had a target at 12 o'clock position at +200 ft just off the nose. It seemed improbable but we followed the TCAS RA commands as trained. I told the first officer to advise ATC that we were following the RA commands and we were not able to stop the descent at 9;000. He did so and asked for lower. ATC said they had no other traffic near us and to stop the descent at 9;000 ft. I said no we needed lower and we went through 9;000 ft. First officer advised ATC and we were cleared to 8;000 ft. I continued down and by now we were aware that the target was not going away. It was still in the same position on our displays. I leveled at 8;000 ft and ATC stated to go no lower because of departing traffic from mia at 7;000 ft. Mia was in a westerly operation. They again stated that they had no traffic near us. The first officer and I agreed to select TA on the TCAS because we felt assured that this was a TCAS failure. The target stayed in the same position on our displays for the rest of the descent and approach to landing. An emergency was not declared because we were cleared to 8;000 ft and when level at 8;000 ft we had figured out that this had to be a TCAS failure. I was at the 9;000 ft crossing just about to take the communication and declare an emergency but we were cleared to lower. The aircraft had the lower TCAS antenna replaced by maintenance. Also first officer stayed one second in front of me and kept up an open communication with ATC. If I thought of it he seems to have been on the way to doing it.

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Original NASA ASRS Text

Title: B737 Captain experienced a TCAS RA during descent into MIA and took evasive action. Relative position of intruder aircraft did not change while ATC advised is no traffic in the area; leading the crew to assume a TCAS anomaly.

Narrative: As we were descending to 9;000 FT. We received an RA to monitor vertical speed. The First Officer confirmed the RA and I disconnected the autopilot and followed the commands to increase the descent rate. We had a target at 12 o'clock position at +200 FT just off the nose. It seemed improbable but we followed the TCAS RA commands as trained. I told the First Officer to advise ATC that we were following the RA commands and we were not able to stop the descent at 9;000. He did so and asked for lower. ATC said they had no other traffic near us and to stop the descent at 9;000 FT. I said no we needed lower and we went through 9;000 FT. First Officer advised ATC and we were cleared to 8;000 FT. I continued down and by now we were aware that the target was not going away. It was still in the same position on our displays. I leveled at 8;000 FT and ATC stated to go no lower because of departing traffic from MIA at 7;000 FT. MIA was in a westerly operation. They again stated that they had no traffic near us. The First Officer and I agreed to select TA on the TCAS because we felt assured that this was a TCAS failure. The target stayed in the same position on our displays for the rest of the descent and approach to landing. An emergency was not declared because we were cleared to 8;000 FT and when level at 8;000 FT we had figured out that this had to be a TCAS failure. I was at the 9;000 FT crossing just about to take the communication and declare an emergency but we were cleared to lower. The aircraft had the lower TCAS antenna replaced by maintenance. Also First Officer stayed one second in front of me and kept up an open communication with ATC. If I thought of it he seems to have been on the way to doing it.

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.