Narrative:

Air carrier X was descending into lga at 11000 MSL on nanci 4 arrival. Contacted ny approach control. Controller pointed out 2 VFR traffic on initial contact. Both were visually idented. Shortly thereafter, we were given a descent clearance to 10000 MSL. Out of 10800 ft controller stated 'IFR traffic Y 11 O'clock 8 mi 10000.' we did not visually see traffic, but did see traffic on TCASII now level at 10000. Both pilots observed at 5 mi, traffic on TCASII appeared to be approaching us at our 12 O'clock position for a 'head-on.' TCASII sounded 'traffic, traffic.' at approximately 2 mi TCASII gave us an RA climb. Captain was PF. We followed TCASII until clear of conflict then descended back to 10000. Maximum altitude deviation 11300 MSL for 22 seconds. I, as PNF, told controller we were following TCASII RA. He acknowledged with a 'roger.' unknown as to our vertical or horizontal separation. Traffic was never in sight visually. We were also very confused as to why the controller cleared us to 10000 when he had traffic converging straight at us also at 10000.

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Original NASA ASRS Text

Title: ACR X DSCNT TO OCCUPIED ALT TCASII RA HAD LTSS FROM Y. SYS ERROR. EVASIVE ACTION TAKEN.

Narrative: ACR X WAS DSNDING INTO LGA AT 11000 MSL ON NANCI 4 ARR. CONTACTED NY APCH CTL. CTLR POINTED OUT 2 VFR TFC ON INITIAL CONTACT. BOTH WERE VISUALLY IDENTED. SHORTLY THEREAFTER, WE WERE GIVEN A DSCNT CLRNC TO 10000 MSL. OUT OF 10800 FT CTLR STATED 'IFR TFC Y 11 O'CLOCK 8 MI 10000.' WE DID NOT VISUALLY SEE TFC, BUT DID SEE TFC ON TCASII NOW LEVEL AT 10000. BOTH PLTS OBSERVED AT 5 MI, TFC ON TCASII APPEARED TO BE APCHING US AT OUR 12 O'CLOCK POS FOR A 'HEAD-ON.' TCASII SOUNDED 'TFC, TFC.' AT APPROX 2 MI TCASII GAVE US AN RA CLB. CAPT WAS PF. WE FOLLOWED TCASII UNTIL CLR OF CONFLICT THEN DSNDED BACK TO 10000. MAX ALTDEV 11300 MSL FOR 22 SECONDS. I, AS PNF, TOLD CTLR WE WERE FOLLOWING TCASII RA. HE ACKNOWLEDGED WITH A 'ROGER.' UNKNOWN AS TO OUR VERT OR HORIZ SEPARATION. TFC WAS NEVER IN SIGHT VISUALLY. WE WERE ALSO VERY CONFUSED AS TO WHY THE CTLR CLRED US TO 10000 WHEN HE HAD TFC CONVERGING STRAIGHT AT US ALSO AT 10000.

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.