Narrative:

Atlanta center cleared us to descend from FL330 to FL230 and cleared us direct to the DCU VOR. The controller informed us that there would be crossing traffic at FL220 and we should expect lower once clear. We leveled at FL230 and visually sighted the traffic below us at our 1 o'clock position; passing from right to left about 5 NM away. In conjunction with the sighting; we had a TA on the TCAS showing the traffic 1;000 ft below us in a solid amber circle. As the traffic passed below us; I noticed a red square pop up on the TCAS display showing an intruder 500 ft above us; descending; and nearly superimposed on the solid amber circle. About a second later; the intruded showed 400 ft above us and still descending. At this instant; I heard the 'descend; descend' voice message and the TCAS commanding a 2;000-2;500 ft/minute descent rate. I got on the tcs switch and immediately began the descent. While I was descending; I informed atlanta center of the descending RA and turned on the fasten seat belt sign. Once established in the 2;000-2;500 ft/minute descent; I glanced at the TCAS display and noticed the red intruder symbol (RA) fully superimposed on the amber TA symbol for the crossing traffic we spotted earlier. At this instant; the symbols were at about our 10 o'clock position and still inside the 2 NM ring on my TCAS display. Shortly thereafter; we got the 'clear of conflict' message. I also noticed the intruder symbol was gone and the amber TA symbol was outside of the 2 NM ring on the TCAS display. I noted we lost 400-500 ft in the RA maneuver and began to return to our assigned altitude. We informed atlanta center of this and as we climbed through FL228; the controller cleared us to 11;000 ft. The center controller also asked us if we spotted the intruder visually and responded that we had not. She then assigned us a new frequency which we switched to. As we were climbing back to FL230; our flight attendant called us to make sure everything was ok. I informed him that we had to maneuver to avoid traffic and we were clear of the threat. I also asked our flight attendant if everything was ok in the cabin and he said it was. We then proceeded to destination and landed uneventfully. It took just over a minute for the incident I described above to occur. There was no time to second guess anything; just follow the TCAS guidance and inform ATC. Because the intruder which caused the RA appeared rapidly when the traffic at FL220 passed below us and in close proximity; plus we never saw another aircraft (within 2 NM and 500 ft above I believe we would have seen something filling the windscreen); I wonder if the transponder on this aircraft was putting out a weak; spurious signal which was picked-up by our TCAS. As a final note; when I returned to base; I had a voicemail from dispatch asking me to contact atlanta center. I gave them a description of the incident.

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Original NASA ASRS Text

Title: EMB145 flight crew experiences a TCAS RA at FL230; with the intruder 500 FT above and descending and superimposed on another target 1;000 FT below. The RA is complied with but once clear the target has vanished and ATC advises that they are not aware of any other target.

Narrative: Atlanta Center cleared us to descend from FL330 to FL230 and cleared us direct to the DCU VOR. The Controller informed us that there would be crossing traffic at FL220 and we should expect lower once clear. We leveled at FL230 and visually sighted the traffic below us at our 1 o'clock position; passing from right to left about 5 NM away. In conjunction with the sighting; we had a TA on the TCAS showing the traffic 1;000 FT below us in a solid amber circle. As the traffic passed below us; I noticed a red square pop up on the TCAS display showing an intruder 500 FT above us; descending; and nearly superimposed on the solid amber circle. About a second later; the intruded showed 400 FT above us and still descending. At this instant; I heard the 'DESCEND; DESCEND' voice message and the TCAS commanding a 2;000-2;500 FT/minute descent rate. I got on the TCS switch and immediately began the descent. While I was descending; I informed Atlanta Center of the descending RA and turned on the Fasten Seat belt sign. Once established in the 2;000-2;500 FT/minute descent; I glanced at the TCAS display and noticed the red intruder symbol (RA) fully superimposed on the amber TA symbol for the crossing traffic we spotted earlier. At this instant; the symbols were at about our 10 o'clock position and still inside the 2 NM ring on my TCAS display. Shortly thereafter; we got the 'CLEAR OF CONFLICT' message. I also noticed the intruder symbol was gone and the amber TA symbol was outside of the 2 NM ring on the TCAS display. I noted we lost 400-500 FT in the RA maneuver and began to return to our assigned altitude. We informed Atlanta Center of this and as we climbed through FL228; the Controller cleared us to 11;000 FT. The Center Controller also asked us if we spotted the intruder visually and responded that we had not. She then assigned us a new frequency which we switched to. As we were climbing back to FL230; our Flight Attendant called us to make sure everything was OK. I informed him that we had to maneuver to avoid traffic and we were clear of the threat. I also asked our Flight Attendant if everything was OK in the cabin and he said it was. We then proceeded to destination and landed uneventfully. It took just over a minute for the incident I described above to occur. There was no time to second guess anything; just follow the TCAS guidance and inform ATC. Because the intruder which caused the RA appeared rapidly when the traffic at FL220 passed below us and in close proximity; plus we never saw another aircraft (within 2 NM and 500 FT above I believe we would have seen something filling the windscreen); I wonder if the transponder on this aircraft was putting out a weak; spurious signal which was picked-up by our TCAS. As a final note; when I returned to base; I had a voicemail from Dispatch asking me to contact Atlanta Center. I gave them a description of the incident.

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.