Narrative:

Air carrier X on the milton arrival into la guardia and on a level flight on our assigned altitude. Our TCASII showed traffic coming at us at our altitude and showed up 4 1/2 - 5 mi away. TCASII immediately started saying 'traffic, traffic.' we asked the ZNY controller if he showed any traffic. TCASII started saying, 'descend, descend.' descend while we still didn't see the traffic and still showing 00 or right at our altitude and getting closer. At this time I started a smooth but positive descent to 500 ft below our altitude while the first officer told ATC what we were doing. By the time I reached about 500 ft below assigned altitude I spotted air carrier Y whom appeared to be climbing and about 1 mi away crossing diagonally in front of us. At this time, TCASII showed plus 1000 ft and the 'descend, descend' stopped. Then a few seconds later a clear of conflict was announced by TCASII. We quickly returned to our assigned altitude. I spoke to the ZNY lga sector's supervisor. He informed me a lack of communication between the 2 controllers of the sector adjacent to us was handling the area departures from newark. The air carrier Y was issued a clearance to climb through our altitude with maintaining visual contact on us. However, our sector's controller was not informed of this, thus, we did not know or expect it.

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Original NASA ASRS Text

Title: ACR Y UNCOORD UNAUTH AIRSPACE PENETRATION HAD LTSS FROM ACR X TCASII RA EVASIVE ACTION DSND. SYS ERROR.

Narrative: ACR X ON THE MILTON ARR INTO LA GUARDIA AND ON A LEVEL FLT ON OUR ASSIGNED ALT. OUR TCASII SHOWED TFC COMING AT US AT OUR ALT AND SHOWED UP 4 1/2 - 5 MI AWAY. TCASII IMMEDIATELY STARTED SAYING 'TFC, TFC.' WE ASKED THE ZNY CTLR IF HE SHOWED ANY TFC. TCASII STARTED SAYING, 'DSND, DSND.' DSND WHILE WE STILL DIDN'T SEE THE TFC AND STILL SHOWING 00 OR R AT OUR ALT AND GETTING CLOSER. AT THIS TIME I STARTED A SMOOTH BUT POSITIVE DSCNT TO 500 FT BELOW OUR ALT WHILE THE FO TOLD ATC WHAT WE WERE DOING. BY THE TIME I REACHED ABOUT 500 FT BELOW ASSIGNED ALT I SPOTTED ACR Y WHOM APPEARED TO BE CLBING AND ABOUT 1 MI AWAY XING DIAGONALLY IN FRONT OF US. AT THIS TIME, TCASII SHOWED PLUS 1000 FT AND THE 'DSND, DSND' STOPPED. THEN A FEW SECONDS LATER A CLR OF CONFLICT WAS ANNOUNCED BY TCASII. WE QUICKLY RETURNED TO OUR ASSIGNED ALT. I SPOKE TO THE ZNY LGA SECTOR'S SUPVR. HE INFORMED ME A LACK OF COM BTWN THE 2 CTLRS OF THE SECTOR ADJACENT TO US WAS HANDLING THE AREA DEPS FROM NEWARK. THE ACR Y WAS ISSUED A CLRNC TO CLB THROUGH OUR ALT WITH MAINTAINING VISUAL CONTACT ON US. HOWEVER, OUR SECTOR'S CTLR WAS NOT INFORMED OF THIS, THUS, WE DID NOT KNOW OR EXPECT IT.

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.