Narrative:

Air carrier X was being vectored for a visual approach by bwi approach over entrance to bwi harbor, level 3000 ft, hazy day, in-flight visibility approximately 4 mi. Had numerous targets on TCASII -- all were proximate. Was looking outside for traffic when controller issued descent to 2000 ft. As I looked down into cockpit, noticed TCASII traffic 12 O'clock, 2.5 - 3 mi, 400 ft lower. Read back clearance and told controller about that TCASII traffic. He replied it was air carrier Y we were following (new to both of us). There was just enough irritation in his voice to make me want to discuss this incident. After securing aircraft, called approach control and talked to supervisor. Explained what had happened and factors which helped me make my decision to tell controller about traffic. Traffic was not moving on display, was 400 ft, not 500 ft, lower (unusual for airlines). Bwi has a fair amount of GA traffic which is usually on that side of the airport, and controller had not said anything about traffic we were to follow (unusual for bwi). I felt there was a possible RA in the making and simply wanted to avoid that. Controller must have felt I was questioning his judgement. I also know that controllers do not like TCASII. I know we're not supposed to use TCASII for our own ATC, but what do you do when you feel you have information that points to a potential conflict? Not asking is dumber than asking.

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Original NASA ASRS Text

Title: ACR X TCASII TA WITH ACR Y ON VECTOR FOR VISUAL APCH.

Narrative: ACR X WAS BEING VECTORED FOR A VISUAL APCH BY BWI APCH OVER ENTRANCE TO BWI HARBOR, LEVEL 3000 FT, HAZY DAY, INFLT VISIBILITY APPROX 4 MI. HAD NUMEROUS TARGETS ON TCASII -- ALL WERE PROXIMATE. WAS LOOKING OUTSIDE FOR TFC WHEN CTLR ISSUED DSCNT TO 2000 FT. AS I LOOKED DOWN INTO COCKPIT, NOTICED TCASII TFC 12 O'CLOCK, 2.5 - 3 MI, 400 FT LOWER. READ BACK CLRNC AND TOLD CTLR ABOUT THAT TCASII TFC. HE REPLIED IT WAS ACR Y WE WERE FOLLOWING (NEW TO BOTH OF US). THERE WAS JUST ENOUGH IRRITATION IN HIS VOICE TO MAKE ME WANT TO DISCUSS THIS INCIDENT. AFTER SECURING ACFT, CALLED APCH CTL AND TALKED TO SUPVR. EXPLAINED WHAT HAD HAPPENED AND FACTORS WHICH HELPED ME MAKE MY DECISION TO TELL CTLR ABOUT TFC. TFC WAS NOT MOVING ON DISPLAY, WAS 400 FT, NOT 500 FT, LOWER (UNUSUAL FOR AIRLINES). BWI HAS A FAIR AMOUNT OF GA TFC WHICH IS USUALLY ON THAT SIDE OF THE ARPT, AND CTLR HAD NOT SAID ANYTHING ABOUT TFC WE WERE TO FOLLOW (UNUSUAL FOR BWI). I FELT THERE WAS A POSSIBLE RA IN THE MAKING AND SIMPLY WANTED TO AVOID THAT. CTLR MUST HAVE FELT I WAS QUESTIONING HIS JUDGEMENT. I ALSO KNOW THAT CTLRS DO NOT LIKE TCASII. I KNOW WE'RE NOT SUPPOSED TO USE TCASII FOR OUR OWN ATC, BUT WHAT DO YOU DO WHEN YOU FEEL YOU HAVE INFO THAT POINTS TO A POTENTIAL CONFLICT? NOT ASKING IS DUMBER THAN ASKING.

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.