Narrative:

Air carrier X, yyc to geg, on approach into geg, VMC, cleared to descend to 5000 ft. Passing 7000 ft, approach called traffic at 5500 ft, 12 O'clock holding over 'phort' NDB. TCASII confirmed and visual contact was made. TCASII gave TA to sighted traffic, and ATC told us to expedite to 5000 ft. At 5200 ft, a second TCASII target appeared, -600 ft, 12 O'clock, with the TCASII symbols for the sighted traffic and the unannounced (by ATC) other aircraft Y almost superimposed. TCASII gave an RA, 'climb,' so we stopped our descent, looking for the new target. It was not seen, but TCASII indicated -100 ft on our position. The entire episode, from level at 5000 ft to 'clear of conflict,' took 15 seconds. We were reluctant to climb because the sighted aircraft, which had passed out of view down our left side, was at 5200 ft and the unrpted (by ATC), unsighted was considered we didn't want to climb back into a conflict. We called airport in sight and were cleared for visual to runway 21. We were not aware of the unrpted traffic, and had no visual on it, so we began a shallow descent towards the GS. We were uncertain of the existence of the unrpted target, with its symbol very close to the known traffic symbol. At the point of closure, we had 2 red symbols, but had seen the target reported by ATC, and felt it was possible a phantom image might have occurred. Phone conversation between the captain and ATC confirmed 2 targets, with ATC trying to put our large transport at 5000 ft on approach between 2 VFR aircraft, one at 4500 ft transiting the area, and the 2ND at 5500 ft.

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Original NASA ASRS Text

Title: ACR X DURING DSCNT TO 5000 FT RECEIVED TCASII TA ACFT AT 5500 FT ADDITIONAL TCASII RA WITH ACFT Y AT 4500 FT. ACR X HAD NMAC WITH ACFT Y. SEE AND AVOID CONCEPT.

Narrative: ACR X, YYC TO GEG, ON APCH INTO GEG, VMC, CLRED TO DSND TO 5000 FT. PASSING 7000 FT, APCH CALLED TFC AT 5500 FT, 12 O'CLOCK HOLDING OVER 'PHORT' NDB. TCASII CONFIRMED AND VISUAL CONTACT WAS MADE. TCASII GAVE TA TO SIGHTED TFC, AND ATC TOLD US TO EXPEDITE TO 5000 FT. AT 5200 FT, A SECOND TCASII TARGET APPEARED, -600 FT, 12 O'CLOCK, WITH THE TCASII SYMBOLS FOR THE SIGHTED TFC AND THE UNANNOUNCED (BY ATC) OTHER ACFT Y ALMOST SUPERIMPOSED. TCASII GAVE AN RA, 'CLB,' SO WE STOPPED OUR DSCNT, LOOKING FOR THE NEW TARGET. IT WAS NOT SEEN, BUT TCASII INDICATED -100 FT ON OUR POS. THE ENTIRE EPISODE, FROM LEVEL AT 5000 FT TO 'CLR OF CONFLICT,' TOOK 15 SECONDS. WE WERE RELUCTANT TO CLB BECAUSE THE SIGHTED ACFT, WHICH HAD PASSED OUT OF VIEW DOWN OUR L SIDE, WAS AT 5200 FT AND THE UNRPTED (BY ATC), UNSIGHTED WAS CONSIDERED WE DIDN'T WANT TO CLB BACK INTO A CONFLICT. WE CALLED ARPT IN SIGHT AND WERE CLRED FOR VISUAL TO RWY 21. WE WERE NOT AWARE OF THE UNRPTED TFC, AND HAD NO VISUAL ON IT, SO WE BEGAN A SHALLOW DSCNT TOWARDS THE GS. WE WERE UNCERTAIN OF THE EXISTENCE OF THE UNRPTED TARGET, WITH ITS SYMBOL VERY CLOSE TO THE KNOWN TFC SYMBOL. AT THE POINT OF CLOSURE, WE HAD 2 RED SYMBOLS, BUT HAD SEEN THE TARGET RPTED BY ATC, AND FELT IT WAS POSSIBLE A PHANTOM IMAGE MIGHT HAVE OCCURRED. PHONE CONVERSATION BTWN THE CAPT AND ATC CONFIRMED 2 TARGETS, WITH ATC TRYING TO PUT OUR LGT AT 5000 FT ON APCH BTWN 2 VFR ACFT, ONE AT 4500 FT TRANSITING THE AREA, AND THE 2ND AT 5500 FT.

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.