Narrative:

ATC assigned heading of 170 degrees, altitude 12000 ft, awaiting clearance to requested cruise altitude. Center called traffic at our 10- 11 O'clock position at 12500 ft. We were unable to obtain a visual on the aircraft at that time, however, the aircraft appeared on our TCASII indicating 500 ft above our altitude. Another aircraft appeared on TCASII in our 5 O'clock position, also 500 ft above our altitude. TCASII issued a TA, followed by an RA, 'monitor vertical speed.' the 2 aircraft seemed to be converging on us 500 ft above us. At that time, ATC called the traffic at our 11 O'clock position at 12500 ft, but did not call the traffic at our 5 O'clock position. TCASII issued an RA 'descend, descend, descend.' we followed the command and advised ATC we were descending as a result of the RA. It was at that point we made a visual on the 11 O'clock traffic at approximately 1 to 1 1/2 mi. We notified ATC we had that traffic, however, we never saw the other aircraft. It just disappeared off the screen. We advised climbing back up to the assigned altitude of 12000 ft. Conflict over. After the event, I question whether the aircraft at our 5 O'clock position was really a true target or just a false target.

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Original NASA ASRS Text

Title: FLC OF B737-400 HAS TCASII INDICATION OF 2 ACFT AT 500 FT ABOVE THEM. ATC HAS ISSUED ONE TFC ALERT. TCASII GOES TO RA AND FLC DSNDS SIGHTING THE ACFT IN THE PROCESS. NEVER SIGHTED THE SECOND ACFT.

Narrative: ATC ASSIGNED HDG OF 170 DEGS, ALT 12000 FT, AWAITING CLRNC TO REQUESTED CRUISE ALT. CTR CALLED TFC AT OUR 10- 11 O'CLOCK POS AT 12500 FT. WE WERE UNABLE TO OBTAIN A VISUAL ON THE ACFT AT THAT TIME, HOWEVER, THE ACFT APPEARED ON OUR TCASII INDICATING 500 FT ABOVE OUR ALT. ANOTHER ACFT APPEARED ON TCASII IN OUR 5 O'CLOCK POS, ALSO 500 FT ABOVE OUR ALT. TCASII ISSUED A TA, FOLLOWED BY AN RA, 'MONITOR VERT SPD.' THE 2 ACFT SEEMED TO BE CONVERGING ON US 500 FT ABOVE US. AT THAT TIME, ATC CALLED THE TFC AT OUR 11 O'CLOCK POS AT 12500 FT, BUT DID NOT CALL THE TFC AT OUR 5 O'CLOCK POS. TCASII ISSUED AN RA 'DSND, DSND, DSND.' WE FOLLOWED THE COMMAND AND ADVISED ATC WE WERE DSNDING AS A RESULT OF THE RA. IT WAS AT THAT POINT WE MADE A VISUAL ON THE 11 O'CLOCK TFC AT APPROX 1 TO 1 1/2 MI. WE NOTIFIED ATC WE HAD THAT TFC, HOWEVER, WE NEVER SAW THE OTHER ACFT. IT JUST DISAPPEARED OFF THE SCREEN. WE ADVISED CLBING BACK UP TO THE ASSIGNED ALT OF 12000 FT. CONFLICT OVER. AFTER THE EVENT, I QUESTION WHETHER THE ACFT AT OUR 5 O'CLOCK POS WAS REALLY A TRUE TARGET OR JUST A FALSE TARGET.

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.