Narrative:

At approximately PM35L, after having been cleared for a visual approach to 17R at laredo, and descending through 2400 ft MSL, we received an RA (resolution advisory) on our TCAS for an aircraft climbing, 100 ft below our altitude, and within our '2 mi' radius. We received a TCAS instruction to 'climb, climb, climb'. A climb was initiated to approximately 2800' MSL. The laredo tower controller then advised of a light twin who had departed laredo approximately 4 mins earlier northbound. This was the first information or advisory of this traffic. We were not able to visually confirm the location of this traffic, although the aircraft stated afterward that he had us in sight. Upon completing the climb, the TCAS message was 'clear of conflict.' we initiated a right turn in order to realign with final to the runway, and advised them we would make a right 270 degree. This put us 1/2 mi to the right of the final approach path. Laredo then advised us to turn back to the south, cross over the airport midfield and enter a left downwind for 17R. The TCAS was operational; and delivered a message of conflicting traffic which we had not received by local controllers. The aircraft showed up on TCAS as an RA. We did not receive prior warning for 'traffic, traffic' as a TA (traffic alert), which leads us to believe that perhaps the departing twin had just then turned on his transponder. Our perception of this situation is that a potential conflict was avoided by initiating a 'climb' response to the TCAS command. Even though the other aircraft said he had us in sight, we did not see him. The twin aircraft was not mentioned by the controller until we inquired about the aircraft in our vicinity as depicted on the TCAS. In my opinion, a possible serious conflict was avoided.

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Original NASA ASRS Text

Title: TCASII RA AND EVASIVE ACTION CLB DURING A VISUAL APCH TO ARPT MAKES RPTR WONDER AS TO THE VISUAL SEPARATION PROC USED BY ATCT LCL CTLR, AND WHY NO TA ON THE TCASII, ONLY RA.

Narrative: AT APPROX PM35L, AFTER HAVING BEEN CLRED FOR A VISUAL APCH TO 17R AT LAREDO, AND DSNDING THROUGH 2400 FT MSL, WE RECEIVED AN RA (RESOLUTION ADVISORY) ON OUR TCAS FOR AN ACFT CLBING, 100 FT BELOW OUR ALT, AND WITHIN OUR '2 MI' RADIUS. WE RECEIVED A TCAS INSTRUCTION TO 'CLB, CLB, CLB'. A CLB WAS INITIATED TO APPROX 2800' MSL. THE LAREDO TWR CTLR THEN ADVISED OF A LIGHT TWIN WHO HAD DEPARTED LAREDO APPROX 4 MINS EARLIER NBND. THIS WAS THE FIRST INFO OR ADVISORY OF THIS TFC. WE WERE NOT ABLE TO VISUALLY CONFIRM THE LOCATION OF THIS TFC, ALTHOUGH THE ACFT STATED AFTERWARD THAT HE HAD US IN SIGHT. UPON COMPLETING THE CLB, THE TCAS MESSAGE WAS 'CLR OF CONFLICT.' WE INITIATED A R TURN IN ORDER TO REALIGN WITH FINAL TO THE RWY, AND ADVISED THEM WE WOULD MAKE A R 270 DEG. THIS PUT US 1/2 MI TO THE R OF THE FINAL APCH PATH. LAREDO THEN ADVISED US TO TURN BACK TO THE S, CROSS OVER THE ARPT MIDFIELD AND ENTER A L DOWNWIND FOR 17R. THE TCAS WAS OPERATIONAL; AND DELIVERED A MESSAGE OF CONFLICTING TFC WHICH WE HAD NOT RECEIVED BY LCL CTLRS. THE ACFT SHOWED UP ON TCAS AS AN RA. WE DID NOT RECEIVE PRIOR WARNING FOR 'TFC, TFC' AS A TA (TFC ALERT), WHICH LEADS US TO BELIEVE THAT PERHAPS THE DEPARTING TWIN HAD JUST THEN TURNED ON HIS TRANSPONDER. OUR PERCEPTION OF THIS SITUATION IS THAT A POTENTIAL CONFLICT WAS AVOIDED BY INITIATING A 'CLB' RESPONSE TO THE TCAS COMMAND. EVEN THOUGH THE OTHER ACFT SAID HE HAD US IN SIGHT, WE DID NOT SEE HIM. THE TWIN ACFT WAS NOT MENTIONED BY THE CTLR UNTIL WE INQUIRED ABOUT THE ACFT IN OUR VICINITY AS DEPICTED ON THE TCAS. IN MY OPINION, A POSSIBLE SERIOUS CONFLICT WAS AVOIDED.

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.