AN ACR MLG DSNDED IN RESPONSE TO A TCASII COMMAND.

Narrative

WE WERE INBOUND TO DTW ON THE MIZAR ARR AT 12000 FT AND OVERTAKING VISUALLY ANOTHER ACFT ON THE ARR AT 11000 FT. WE HAD JUST PASSED HIM WHEN WE HAD A TCASII RA TO DSND AS AN ACFT SBOUND AT 12400 FT CAME ON OUR TCASII ON A COLLISION COURSE. WE COMPLIED WITH RA AND DSNDED TO APPROX 11700 FT. THE CAPT WAS FLYING AND THE FO RPTED TO DTW APCH WE HAD AN RA. CAPT CALLED DTW APCH ON THE GND. THE SUPVR INFORMED US THE CTLR WAS OCCUPIED WITH HVY TFC LOAD INCLUDING ANOTHER ACR FLT. THE DTW SUPVR INFORMED ME THE 12500 FT TFC WAS 'LEVEL;' VFR; AND NOT WORKING DTW ATC. THE FO SAW THE LOW WING; SINGLE ENG ACFT PASS OVER US. CTLR HAD INFORMED PREVIOUS INBOUNDS BUT NOT US DUE TO WORKLOAD AND THE OTHER ACFT APPEARED VFR; LEVEL AT 12500 FT. WE HAD THE CURRENT DTW ALTIMETER SETTING SET AND WE WERE LEVEL 12000 FT. AT FIRST; WE THOUGHT THE OTHER ACFT WAS SLOWLY CLBING AND WOULD NOT BE A FACTOR; SHOWING PLUS 300 FT; THEN PLUS 400 FT ABOVE USE. WHEN HE APPEARED TO STAY AT PLUS 400 FT; WE DSNDED KEEPING IN MIND THE IFR TFC AT 11000 AND SOMEWHAT BEHIND US. ONLY AFTER CALLING DTW ATC DID WE KNOW HE WAS 'LEVEL.' WE WERE COMMITTED; HOWEVER; TO DSCNT DUE TO TCASII COMMAND. TOO OFTEN; VFR TFC TRY TO FLY OVER TCA'S AT MAJOR ARPTS WITHOUT KNOWLEDGE OF ARR AND DEP CORRIDORS AND NO CONTACT WITH ATC. ATC SHOULD ENCOURAGE AND OPERATORS OF VFR ACFT SHOULD SOLICIT RADAR TA'S AND RECOMMENDED RTES OVER OR AROUND TERMINAL AREAS.