Narrative:

We were approaching mdw from the southeast. A WX system (heavy rain, snow, etc) was affecting the airport from the west. We were asked if we saw the preceeding aircraft. We responded 'negative.' we were cleared for the 'ILS runway 31C, circle to runway 22.' in the vicinity of the FAF, we were instructed to begin circling for runway 22L. We began a standard rate right turn to a heading of 040 degrees for a left downwind for runway 22L. (At this point, we were at 1180 ft MSL.) we were again asked if we saw an aircraft 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 heading of 040 degrees while trying to keep the airport in sight, which is required for a circling approach. While on a downwind heading of 040 degrees, we might have drifted toward the airport a little due to the crosswind and the necessity to remain in constant visual contact with the airport. At a point about 45 degrees from the approach end of runway 22L, we began a standard rate left turn to base leg for runway 22L, which was a point that would allow the runway to remain in sight (required by far instrument approach procedures) and the aircraft to remain in a continuous confign to land. At this point, the tower controller ordered us to make an immediate hard right turn for traffic. We saw a B737 flying directly at us, at about our 12:30 O'clock position (same altitude and less than 1/2 mi), and immediately received a TCASII warning. Due to his relative position to us, I elected to not follow ATC's directive and banked slightly to the left, thereafter proceeding straight ahead to avoid the B737, which moved behind us. (Had we turned right as directed by ATC, we would have surely had a midair collision.) we then heard the B737 announce and execute a missed approach. ATC then instructed us to make a right turn (behind the B737) and land on runway 22R. After a normal landing, I informed ground control that I would be calling the control tower. This entire event took approximately 45 seconds according to the 'tape' at mdw tower. After a lengthy conversation with ATC and the B737 crew, this is what we discovered. The B737 crew called a 4 mi final approach to runway 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 approach to runway 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 airport was a bad assumption on the part of ATC, especially when an aircraft is executing a circling approach, which requires them to remain in visual contact with the airport and in a continuous position to make a normal and safe landing.) the B737 confirmed that when they finally had a visual on us, and likewise received a TCASII traffic alert, that it was certainly best not to follow ATC's directive to turn right to avoid the conflict. The problem was a 'controller procedural error.' ATC should not have assumed an aircraft would remain on a downwind leg while on a circling approach, unless ATC specifically indicates that they will 'call' the base turn. Likewise, it is ATC's sole responsibility to maintain aircraft separation unless a visual contact by at least 1 pilot has been called. Contributing factors were rapidly deteriorating WX, congested airspace, and ATC work overload. Both of the flight crew's involved, as well as mdw ATC, agreed that since nobody did anything intentionally wrong and nobody got hurt, that no FAA near midair collision report would be generated by either flight crew or ATC, but that both flight crew's would voluntarily submit a NASA report. Callback conversation with reporter revealed the following information: reporter, B737 and supervisor conference called. The reporter agreed with the B737 captain that if the other didn't care, they would not file an near midair collision report, which the reporter alleges was supported by the supervisor. The supervisor advised that he would establish a control procedure that controllers would have to apply when advising the pilot to turn base leg, or not to turn base, in similar events. The reporter has received no further communication concerning this eventfrom any party to this event.

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Original NASA ASRS Text

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

Narrative: 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.

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.