WW24 FLT CREW EXECUTE EVASIVE TCASII MANEUVER IN CONFLICT WITH A B737; EXECUTING A TCASII GAR MANEUVER WHEN ON APCH TO MDW RWY 22L.

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WE WERE APCHING MDW FROM THE SE. A WX SYS (HVY RAIN; SNOW; ETC) WAS AFFECTING THE ARPT FROM THE W. WE WERE ASKED IF WE SAW THE PRECEEDING ACFT. WE RESPONDED 'NEGATIVE.' WE WERE CLRED FOR THE 'ILS RWY 31C; CIRCLE TO RWY 22.' IN THE VICINITY OF THE FAF; WE WERE INSTRUCTED TO BEGIN CIRCLING FOR RWY 22L. WE BEGAN A STANDARD RATE R TURN TO A HDG OF 040 DEGS FOR A L DOWNWIND FOR RWY 22L. (AT THIS POINT; WE WERE AT 1180 FT MSL.) WE WERE AGAIN ASKED IF WE SAW AN ACFT IN THE AREA. WE AGAIN RESPONDED 'NEGATIVE' AFTER A VISUAL SEARCH. (NOTHING DISPLAYED ON TCASII.) MAKING OUR STANDARD RATE TURN; WE WERE INSTRUCTED TO TURN A 'SOLID DOWNWIND.' WE INCREASED OUR BANK ANGLE HARD TO OUR HDG OF 040 DEGS WHILE TRYING TO KEEP THE ARPT IN SIGHT; WHICH IS REQUIRED FOR A CIRCLING APCH. WHILE ON A DOWNWIND HDG OF 040 DEGS; WE MIGHT HAVE DRIFTED TOWARD THE ARPT A LITTLE DUE TO THE XWIND AND THE NECESSITY TO REMAIN IN CONSTANT VISUAL CONTACT WITH THE ARPT. AT A POINT ABOUT 45 DEGS FROM THE APCH END OF RWY 22L; WE BEGAN A STANDARD RATE L TURN TO BASE LEG FOR RWY 22L; WHICH WAS A POINT THAT WOULD ALLOW THE RWY TO REMAIN IN SIGHT (REQUIRED BY FAR INST APCH PROCS) AND THE ACFT TO REMAIN IN A CONTINUOUS CONFIGN TO LAND. AT THIS POINT; THE TWR CTLR ORDERED US TO MAKE AN IMMEDIATE HARD R TURN FOR TFC. WE SAW A B737 FLYING DIRECTLY AT US; AT ABOUT OUR 12:30 O'CLOCK POS (SAME ALT AND LESS THAN 1/2 MI); AND IMMEDIATELY RECEIVED A TCASII WARNING. DUE TO HIS RELATIVE POS TO US; I ELECTED TO NOT FOLLOW ATC'S DIRECTIVE AND BANKED SLIGHTLY TO THE L; THEREAFTER PROCEEDING STRAIGHT AHEAD TO AVOID THE B737; WHICH MOVED BEHIND US. (HAD WE TURNED R AS DIRECTED BY ATC; WE WOULD HAVE SURELY HAD A MIDAIR COLLISION.) WE THEN HEARD THE B737 ANNOUNCE AND EXECUTE A MISSED APCH. ATC THEN INSTRUCTED US TO MAKE A R TURN (BEHIND THE B737) AND LAND ON RWY 22R. AFTER A NORMAL LNDG; I INFORMED GND CTL THAT I WOULD BE CALLING THE CTL TWR. THIS ENTIRE EVENT TOOK APPROX 45 SECONDS ACCORDING TO THE 'TAPE' AT MDW TWR. AFTER A LENGTHY CONVERSATION WITH ATC AND THE B737 CREW; THIS IS WHAT WE DISCOVERED. THE B737 CREW CALLED A 4 MI FINAL APCH TO RWY 22L. NEITHER THE B737 CREW NOR WE COULD SEE EACH OTHER. ATC INSTRUCTED US TO GIVE THEM A 'SOLID DOWNWIND TURN;' WHICH WE DID DURING OUR CIRCLING APCH TO RWY 22L; BUT WE WERE NEVER TOLD THAT ATC WOULD 'CALL OUR BASE TURN.' (ASSUMING WE WOULD CONTINUE ON AN INDEFINITE DOWNWIND LEG AND LOSING SIGHT OF THE ARPT WAS A BAD ASSUMPTION ON THE PART OF ATC; ESPECIALLY WHEN AN ACFT IS EXECUTING A CIRCLING APCH; WHICH REQUIRES THEM TO REMAIN IN VISUAL CONTACT WITH THE ARPT AND IN A CONTINUOUS POS TO MAKE A NORMAL AND SAFE LNDG.) THE B737 CONFIRMED THAT WHEN THEY FINALLY HAD A VISUAL ON US; AND LIKEWISE RECEIVED A TCASII TFC ALERT; THAT IT WAS CERTAINLY BEST NOT TO FOLLOW ATC'S DIRECTIVE TO TURN R TO AVOID THE CONFLICT. THE PROB WAS A 'CTLR PROCEDURAL ERROR.' ATC SHOULD NOT HAVE ASSUMED AN ACFT WOULD REMAIN ON A DOWNWIND LEG WHILE ON A CIRCLING APCH; UNLESS ATC SPECIFICALLY INDICATES THAT THEY WILL 'CALL' THE BASE TURN. LIKEWISE; IT IS ATC'S SOLE RESPONSIBILITY TO MAINTAIN ACFT SEPARATION UNLESS A VISUAL CONTACT BY AT LEAST 1 PLT HAS BEEN CALLED. CONTRIBUTING FACTORS WERE RAPIDLY DETERIORATING WX; CONGESTED AIRSPACE; AND ATC WORK OVERLOAD. BOTH OF THE FLT CREW'S INVOLVED; AS WELL AS MDW ATC; AGREED THAT SINCE NOBODY DID ANYTHING INTENTIONALLY WRONG AND NOBODY GOT HURT; THAT NO FAA NMAC RPT WOULD BE GENERATED BY EITHER FLT CREW OR ATC; BUT THAT BOTH FLT CREW'S WOULD VOLUNTARILY SUBMIT A NASA RPT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR; B737 AND SUPVR CONFERENCE CALLED. THE RPTR AGREED WITH THE B737 CAPT THAT IF THE OTHER DIDN'T CARE; THEY WOULD NOT FILE AN NMAC RPT; WHICH THE RPTR ALLEGES WAS SUPPORTED BY THE SUPVR. THE SUPVR ADVISED THAT HE WOULD ESTABLISH A CTL PROC THAT CTLRS WOULD HAVE TO APPLY WHEN ADVISING THE PLT TO TURN BASE LEG; OR NOT TO TURN BASE; IN SIMILAR EVENTS. THE RPTR HAS RECEIVED NO FURTHER COM CONCERNING THIS EVENTFROM ANY PARTY TO THIS EVENT.