Narrative:

We were cleared for the expressway visual to runway 31 at lga. At the maspeth tanks we turned to 085 degrees and flew the prescribed procedure. We were visual behind an medium large transport Z. First officer was flying the aircraft at 180 KTS beginning the descent and configuring to land. We heard tower ask if an air carrier Y flight had us (air carrier X) in sight. To an affirmative response he was also cleared the expressway visual to follow us. We immediately received a TCASII RA to descend which we were already doing. I checked the TCASII and it showed traffic at about 1/2 mi behind us, 600 ft above, descending. The RA increased to 400 ft separation while first officer immediately increased descent to 1500 FPM. RA changed to 'monitor descent' then descend, descend as first officer slowed descent. I called tower who immediately broke the air carrier Y medium large transport Y off of the approach. My first officer looked out his side window and saw the air carrier Y raising its gear and turning away. Tower told us we had 1000 ft separation, although our TCASII showed only 400 ft at one time during the warning. Our approach was completed to a normal landing. Apparently, approach control had cleared air carrier Y to fly the expressway approach when he confirmed us in sight. The reduced separation must have occurred because air carrier Y saw the air carrier Z ahead of us at about 3 mi and mistakenly idented him as us. If our TCASII had not alerted us to the air carrier Y descending on us, our descent would not have been as rapid and probably would have resulted in a midair. Air carrier Y TCASII evidently failed, issued incorrect information, was not operational or was not installed. Contributing to the incident was misident of our flight by air carrier Y.

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Original NASA ASRS Text

Title: FLC OF AN MLG ACR ACFT STOPPED THEIR DSCNT ON A VISUAL APCH WHEN THEIR TCASII RA ADVISED TO CLB.

Narrative: WE WERE CLRED FOR THE EXPRESSWAY VISUAL TO RWY 31 AT LGA. AT THE MASPETH TANKS WE TURNED TO 085 DEGS AND FLEW THE PRESCRIBED PROC. WE WERE VISUAL BEHIND AN MLG Z. FO WAS FLYING THE ACFT AT 180 KTS BEGINNING THE DSCNT AND CONFIGURING TO LAND. WE HEARD TWR ASK IF AN ACR Y FLT HAD US (ACR X) IN SIGHT. TO AN AFFIRMATIVE RESPONSE HE WAS ALSO CLRED THE EXPRESSWAY VISUAL TO FOLLOW US. WE IMMEDIATELY RECEIVED A TCASII RA TO DSND WHICH WE WERE ALREADY DOING. I CHKED THE TCASII AND IT SHOWED TFC AT ABOUT 1/2 MI BEHIND US, 600 FT ABOVE, DSNDING. THE RA INCREASED TO 400 FT SEPARATION WHILE FO IMMEDIATELY INCREASED DSCNT TO 1500 FPM. RA CHANGED TO 'MONITOR DSCNT' THEN DSND, DSND AS FO SLOWED DSCNT. I CALLED TWR WHO IMMEDIATELY BROKE THE ACR Y MLG Y OFF OF THE APCH. MY FO LOOKED OUT HIS SIDE WINDOW AND SAW THE ACR Y RAISING ITS GEAR AND TURNING AWAY. TWR TOLD US WE HAD 1000 FT SEPARATION, ALTHOUGH OUR TCASII SHOWED ONLY 400 FT AT ONE TIME DURING THE WARNING. OUR APCH WAS COMPLETED TO A NORMAL LNDG. APPARENTLY, APCH CTL HAD CLRED ACR Y TO FLY THE EXPRESSWAY APCH WHEN HE CONFIRMED US IN SIGHT. THE REDUCED SEPARATION MUST HAVE OCCURRED BECAUSE ACR Y SAW THE ACR Z AHEAD OF US AT ABOUT 3 MI AND MISTAKENLY IDENTED HIM AS US. IF OUR TCASII HAD NOT ALERTED US TO THE ACR Y DSNDING ON US, OUR DSCNT WOULD NOT HAVE BEEN AS RAPID AND PROBABLY WOULD HAVE RESULTED IN A MIDAIR. ACR Y TCASII EVIDENTLY FAILED, ISSUED INCORRECT INFO, WAS NOT OPERATIONAL OR WAS NOT INSTALLED. CONTRIBUTING TO THE INCIDENT WAS MISIDENT OF OUR FLT BY ACR Y.

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.