MKE CTLR CHRONICLES OPERROR AT 12000 FT AS SECOND DEP OVERTOOK PREVIOUS DEP; BOTH ACFT ON ZAU FREQ.

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I WAS WORKING THE E FEEDER AND DEP POS FROM XA34 LCL UNTIL XB47 LCL. XB42:22 ACR X DEPARTED RWY 7R AND CALLED ME AT 2200 FT CLBING TO 5000 FT ON A 090 DEG HDG. I SAID RADAR CONTACT; CLB AND MAINTAIN 13000 FT. ACR X ACKNOWLEDGED. XB43:58 ACR Y DEPARTED RWY 7R AND CALLED ME AT 2300 FT CLBING TO 5000 FT ON A 090 DEG HDG. HAVING MORE THAN THE 5 MI AND 1000 FT SEPARATION REQUIRED FOR THE CTR; I SAID RADAR CONTACT; CLB AND MAINTAIN 13000 FT. ACR Y ACKNOWLEDGED. XB44:48 ACR X WAS AT 7300 FT; 220 KTS AND ACR Y WAS AT 4900 FT; 200 KTS WHEN ZAU TOOK THE HDOF ON ACR X. BOTH ACFT STILL MORE THAN THE 5 MI AND 1000 FT APART AND ACR X INCREASING HIS LEAD. XB45:25 37 SECONDS LATER ZAU TOOK THE HDOF ON ACR Y. SEPARATION WAS STILL MORE THAN 5 MI AND 1000 FT WITH ACR X AT 9200 FT 240 KTS AND ACR Y AT 6800 FT 250 KTS. ACR Y HAD GAINED 30 KTS IN 37 SECONDS AND WAS NOW 10 KTS FASTER AND CLOSING. I EITHER DO NOT REMEMBER NOTICING THE OVERTAKE OR WAS NOT CONCERNED ABOUT IT AT THE TIME. THE FASTER PERFORMING ACFT WAS AHEAD AND ALMOST THROUGH 10000 FT WHERE IT WOULD PICK UP SPD. THAT WAS AROUND THE TIME THAT THE NEXT CTLR CAME IN TO RELIEVE ME. I ALSO HAD A VFR GUARD HELI CALL ME 1 MI OUTSIDE THE CLASS C AIRSPACE; LOW LEVEL; WANTING TO OVERFLY NBOUND. AS I ATTEMPTED TO IDENT HIM; I WAS TRYING TO COORDINATE THE OVERFLT WITH THE TWR AND VERIFY THE PLT'S INTENTIONS. I LET THIS DISTRACT ME. XB45:29 I TOLD ACR X; FLY HDG 080 DEG VECTOR TO BUTRR AND CONTACT ZAU. ACR X WAS AT 9300 FT 230 KTS. ACR Y WAS AT 7000 FT 250 KTS. SEPARATION WAS 5.36 MI AND 2300 FT. ACR X CHKED IN WITH THE CTR 23 SECONDS LATER AND WAS ISSUED A CLB TO FL230. XB46:00 I TOLD ACR Y TO CONTACT ZAU. ACR X WAS AT 10400 FT 250 KTS AND ACR Y WAS AT 8800 FT 280 KTS. DISTANCE BTWN THE ACFT NOW 1600 FT AND 5.09 MI. I STILL DID NOT NOTICE ACR Y'S 30 KT OVERTAKE. XB47:06 ACR Y CHKED ON THE ZAU FREQ CLBING THROUGH 12200 FT FOR 13000 FT AND 310 KTS. ACR X WAS AT 12600 FT AND 300 KTS. ZAU CTLR INSTRUCTED THE PLT OF ACR Y TO 'STAND BY FOR HIGHER' AND ASSIGNED A 250 KT SPD RESTR. SEPARATION WAS LOST. DISTANCE WAS 4.78 MI AND 600 FT. XB47:17 I BEGAN BRIEFING THE RELIEVING CTLR. XB47:25 RADAR PLAYBACK SHOWED CLOSEST PROX IS 400 FT AND 4.74 NM. I WAS NOTIFIED OF THE LOSS OF SEPARATION APPROX 30 MINS AFTER THE RELIEF BRIEFING; BY MY SUPVR; WHO RECEIVED A PHONE CALL FROM ZAU. I REMEMBER THE ACFT BEING CLOSE; BUT I HAD PLENTY OF ALT AND SPACING AT THE TIME. I DO NOT REMEMBER EVER LOOKING AT THE SPDS AND CLB RATES OF THESE ACFT. I KNOW THAT I AM NOT USUALLY WORRIED ABOUT THIS SITUATION WITH THE DC94 BEING THE LEAD ACFT. THERE ARE ANY NUMBER OF REASONS FOR THIS EVENT. I LET MYSELF GET DISTR BY THE VFR HELI. I WAS IN A HURRY TRYING TO 'CLEAN UP' MY POS FOR THE NEXT CTLR. I LET MYSELF GET COMPLACENT; THINKING I KNEW HOW THESE ACFT WOULD PERFORM. AT THE SAME TIME; I HAVE TO QUESTION HOW THE PLTS WERE FLYING THEIR ACFT. WAS THE ACR Y PLT BREAKING THE 250 KT BELOW 10000 FT RULE? WHY DID IT TAKE HIM 1 MIN 6 SECONDS TO CHK ON WITH THE CTR CTLR? WHY DID HE GAIN 3400 FT IN THE SAME TIME? ARE THESE NORMAL CLB RATES FOR THESE ACFT AT THESE ALTS? WAS HE TRYING TO CATCH THE DC94? WHY WAS THE ACR X PLT FLYING AND CLBING SO SLOW? FUEL CONSERVATION? IN THE END; I HAVE TO TAKE RESPONSIBILITY FOR NOT USING ALL THE INFO THAT I HAD AVAILABLE; AND NOT TAKING ACTION TO CTL A SITUATION THAT I KNEW WAS CLOSE.