Narrative:

While at 4000 ft MSL on the F4 localizer of mdw, just outside cadon intersection air carrier X told by approach to turn left 270 degree. This transmission was very quickly given and came across very garbled. The first officer was flying and I requested approach to say again. I thought I heard the controller ask for the turn but was not sure so I looked at my first officer to see if he heard the question. As I looked toward my first officer I saw the reason for the vector. Air carrier Y type aircraft was in a climbing left turn at our close 2 O'clock position. By the time I spotted air carrier Y it was apparent that it would pass above and behind our aircraft. However, we were well into the 270 degree heading turn by the time the controller repeated the command to turn left 270. We were eventually vectored back onto the localizer to complete the approach. An unidentified aircraft asked what happened and the controller replied that the turns were for a 'NORDO' aircraft. Air carrier Y aircraft stated to approach that 'he had to climb to avoid that aircraft.' upon landing at mdw I called chicago approach and asked to talk to the watch supervisor. I asked what had occurred. He stated that air carrier Y was being worked by another controller and that an error had most likely occurred! I said that I wanted to make sure that we had not messed up in any way and he replied that we had not. He wanted to know what the WX conditions were and I told him that we were in and out of clouds between layers. I did tell him that we were in the clear long enough for me and some of my 114 passengers to see air carrier Y go by. Our last instructions before the conflict was to intercept the runway 4R localizer and maintain 4000 ft till cadon, cleared approach, contact tower at ermin, maintain 210 KTS for now. Supplemental information from 190866. Approach came up and said to turn left to a 270 degree heading. His transmission was garbled so we asked to clarify and at that time we saw air carrier Y at our 2 O'clock taking evasive action in a climbing left turn through our altitude. We immediately turned left as well. Air carrier Y passed behind us at about 1/2 mi range. According to ATC, air carrier Y was on departure from mdw and somehow was on the wrong frequency and altitude. ATC believed a controller error was made. We continued our approach and landed at mdw. The air carrier Y crew stated on the radio they saw us and had to take evasive action.

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Original NASA ASRS Text

Title: DEP CTLR UNAUTH UNCOORD PENETRATION OF AIRSPACE ACR Y HAD LTSS WITH ACR X. SYS ERROR.

Narrative: WHILE AT 4000 FT MSL ON THE F4 LOC OF MDW, JUST OUTSIDE CADON INTXN ACR X TOLD BY APCH TO TURN L 270 DEG. THIS XMISSION WAS VERY QUICKLY GIVEN AND CAME ACROSS VERY GARBLED. THE FO WAS FLYING AND I REQUESTED APCH TO SAY AGAIN. I THOUGHT I HEARD THE CTLR ASK FOR THE TURN BUT WAS NOT SURE SO I LOOKED AT MY FO TO SEE IF HE HEARD THE QUESTION. AS I LOOKED TOWARD MY FO I SAW THE REASON FOR THE VECTOR. ACR Y TYPE ACFT WAS IN A CLBING L TURN AT OUR CLOSE 2 O'CLOCK POS. BY THE TIME I SPOTTED ACR Y IT WAS APPARENT THAT IT WOULD PASS ABOVE AND BEHIND OUR ACFT. HOWEVER, WE WERE WELL INTO THE 270 DEG HDG TURN BY THE TIME THE CTLR REPEATED THE COMMAND TO TURN L 270. WE WERE EVENTUALLY VECTORED BACK ONTO THE LOC TO COMPLETE THE APCH. AN UNIDENTIFIED ACFT ASKED WHAT HAPPENED AND THE CTLR REPLIED THAT THE TURNS WERE FOR A 'NORDO' ACFT. ACR Y ACFT STATED TO APCH THAT 'HE HAD TO CLB TO AVOID THAT ACFT.' UPON LNDG AT MDW I CALLED CHICAGO APCH AND ASKED TO TALK TO THE WATCH SUPVR. I ASKED WHAT HAD OCCURRED. HE STATED THAT ACR Y WAS BEING WORKED BY ANOTHER CTLR AND THAT AN ERROR HAD MOST LIKELY OCCURRED! I SAID THAT I WANTED TO MAKE SURE THAT WE HAD NOT MESSED UP IN ANY WAY AND HE REPLIED THAT WE HAD NOT. HE WANTED TO KNOW WHAT THE WX CONDITIONS WERE AND I TOLD HIM THAT WE WERE IN AND OUT OF CLOUDS BTWN LAYERS. I DID TELL HIM THAT WE WERE IN THE CLR LONG ENOUGH FOR ME AND SOME OF MY 114 PAXS TO SEE ACR Y GO BY. OUR LAST INSTRUCTIONS BEFORE THE CONFLICT WAS TO INTERCEPT THE RWY 4R LOC AND MAINTAIN 4000 FT TILL CADON, CLRED APCH, CONTACT TWR AT ERMIN, MAINTAIN 210 KTS FOR NOW. SUPPLEMENTAL INFO FROM 190866. APCH CAME UP AND SAID TO TURN L TO A 270 DEG HDG. HIS XMISSION WAS GARBLED SO WE ASKED TO CLARIFY AND AT THAT TIME WE SAW ACR Y AT OUR 2 O'CLOCK TAKING EVASIVE ACTION IN A CLBING L TURN THROUGH OUR ALT. WE IMMEDIATELY TURNED L AS WELL. ACR Y PASSED BEHIND US AT ABOUT 1/2 MI RANGE. ACCORDING TO ATC, ACR Y WAS ON DEP FROM MDW AND SOMEHOW WAS ON THE WRONG FREQ AND ALT. ATC BELIEVED A CTLR ERROR WAS MADE. WE CONTINUED OUR APCH AND LANDED AT MDW. THE ACR Y CREW STATED ON THE RADIO THEY SAW US AND HAD TO TAKE EVASIVE ACTION.

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.