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.