B737-300 CREW HAD LESS THAN LEGAL SPACING IN MDW CLASS C AIRSPACE.

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ON MAR/XA/02 AT APPROX XA10; WE CONTACTED MDW APCH CTL. (FO WAS FLYING THE AIRPLANE.) AS PER THEIR INSTRUCTIONS; AND AS PER THE ATIS; WE WERE VECTORED FOR THE ILS RWY 4R CIRCLE-TO-LAND RWY 13C. (WX WAS VMC -- GREATER THAN 4000 FT CEILING; VISIBILITY 10+ MI; SURFACE WIND 150 DEGS AT 7 KTS). WE RPTED THE WIND AT 2600 FT MSL (WHICH WAS GS INTERCEPT ALT) TO APCH CTL AS 'OUT OF THE S AT 50 KTS.' THIS WAS ACKNOWLEDGED. WE WERE CLRED FOR THE ILS RWY 4R CIRCLE-TO-LAND RWY 13C AND TOLD TO CONTACT TWR AT ERMIN (FAF; NDB). AT ERMIN; I CALLED TWR AND RPTED; 'ACFT X WITH YOU AT ERMIN.' TWR WAS COMMUNICATING WITH OTHER ACFT AND DID NOT ACKNOWLEDGE MY XMISSION AT THIS TIME. WE COMMENCED THE CIRCLING MANEUVER FO RWY 13C. ABOUT 1/2 WAY THROUGH THE CIRCLING MANEUVER; AND FOCUSING ON MDW ARPT TO OUR R; TWR CTLR SAID; 'HEY ACFT X -- ARE YOU UP?' I REPLIED TO THE AFFIRMATIVE WHEREUPON HE CAME BACK QUICKLY AND SAID; 'ACFT X; TURN L IMMEDIATELY; TFC INBOUND FOR RWY 13C!' WE LOOKED STRAIGHT AHEAD AND WERE APPALLED TO SEE THE LNDG LIGHTS OF THE INBOUND TFC AND BEGAN AN IMMEDIATE EVASIVE MANEUVER BACK TO THE L -- WHICH WAS TO THE NW. TWR ALSO CALLED OUT ADDITIONAL TFC INBOUND BEHIND THE INITIAL TFC. (MY INITIAL THOUGHT WAS WHY IN THE WORLD ARE THESE ACFT COMING STRAIGHT IN TO RWY 13C WHEN WE HAVE BEEN CLRED FOR THE CIRCLING APCH TO RWY 13C? I LATER LEARNED THAT THESE ACFT HAD INDEED FLOWN SOME TYPE OF CIRCLING APCH TO RWY 13C.) BOTH ACFT WERE APPARENTLY LEAR JETS OR SOME OTHER TYPE OF CPR JETS. WE WERE SUBSEQUENTLY CLRED TO TURN BASE ABEAM THE SECOND ACFT; WHICH WE DID; FLEW INBOUND AND LANDED UNEVENTFULLY. OBSERVATIONS: 1) FLYING THE CIRCLING APCH AS A CAT D ACFT (WE ARE ACTUALLY CAT C; BUT ARE REQUIRED BY OUR COMPANY TO USE CAT D MINIMA AND CRITERIA). WE NEVER EXCEEDED THE 2.3 NM DISTANCE CRITERIA UNTIL DIRECTED TO DO SO BY TWR. 2) WE WERE NEVER ADVISED BY TWR OR APCH CTL THAT TFC WAS INBOUND FROM OUR L FOR RWY 13C; PRIOR TO THE CALL; 'ACFT X; TURN L IMMEDIATELY; ETC.' (THAT WOULD ALSO MEAN COMING INBOUND FROM WAY OUTSIDE ANY 2.3 NM CIRCLING APCH PARAMETER!) 3) THE PAPI FOR RWY 13C WAS INOP. THIS OUTAGE WAS NOT INDICATED IN EITHER OUR FLT PLAN NOTAMS; OR THE ARR ATIS NOTAMS. (I SUBSEQUENTLY LEARNED THAT THE PAPI HAD BEEN RENDERED INOP BY A SNOW PLOW AND HAD BEEN OUT FOR SOME TIME.) 4) AN AGGRAVATING FACTOR WAS THE STRONG WIND FROM THE S AT TFC PATTERN ALT. 5) WITH LNDG GEAR DOWN AND FLAPS SET FOR LNDG; WE WERE NOT IN AN ADVANTAGEOUS CONFIGN FOR DRASTIC EVASIVE MANEUVERING. 6) MDW APCH CTL ROUTINELY CLRS ACFT FOR A CIRCLING APCH; AND THEN MDW TWR EXTENDS DOWNWINDS; CALLS BASE TURN; ETC. BY THEIR OWN ADMISSION; TWR IS NOT AWARE OF THE CIRCLING AIRSPACE WE; AS PLTS; ARE RESTR TO. (IE; THE SERIES OF 2.3 NM ARCS A CAT D CIRCLING APCH IS BASED ON.) WHO THEN IS RESPONSIBLE FOR OBSTACLE CLRNC IF TWR DIRECTS US TO EXCEED PROTECTED AIRSPACE? CAN THEY REALLY DO THIS? IT SEEMS TO ME THAT EITHER WE FLY A TRUE CIRCLING APCH CONFORMING TO THE AIRSPACE RESTRS RELATIVE TO OUR CATEGORY; AS SPECIFIED IN THE AIM; OR WE FLY A VISUAL APCH. I BELIEVE THAT PART OF THE PROB IS PLT IGNORANCE OF THE AIRSPACE RESTRS PERTAINING TO THEIR CATEGORY OF ACFT. ADDITIONALLY; TWR'S RESPONSE TO THIS SEEMS TO BE ONE OF ACCEPTANCE; AND THEN VECTORING SUBSEQUENT ACFT AROUND THE SAME INCORRECT PATHWAY OR REMAIN WITHIN DISTANCE. ERRORS ON THE PART OF ONE ACFT SET-UP FOLLOWING ACFT FOR THE SAME PROB. WHEN THE THIRD ACFT IN THIS SEQUENCE IS NOT GIVEN TIMELY INFO; AS HAPPENED TO US; THEN AN INCREDIBLY DANGEROUS SCENARIO DEVELOPS. WE WERE LITERALLY SET UP FOR A POTENTIAL MIDAIR COLLISION! RECOMMENDATION: I FEEL THAT FOR A HIGH VOLUME ARPT SUCH AS MIDWAY; IT WOULD BE BETTER TO HAVE A CHARTER VISUAL APCH RATHER THAN A CIRCLING APCH. A CHARTED VISUAL APCH WOULD HAVE ALTS; POSSIBLY SPDS; AND LANDMARKS ALSO DEFINED BY DME'S OR NDB'S. A GOOD EXAMPLE OF THIS IS THE EXPRESSWAY VISUAL APCH TO RWY 31 AT NEW YORK'S LGA ARPT. THIS WOULD HELP TO ELIMINATE THE VAGARIES OF PLTS FLYING WAY OUTSIDE THE PROTECTEDAIRSPACE AND WOULD REALLY HELP TO DEFINE A SPECIFIC GND TRACK!