ZID RADAR ASSOCIATE AND RADAR CTLR FAIL TO RECOGNIZE A B757 OVERTAKE AN MD80 ON APCH TO CVG EXPECTING THE B757 TO HAVE BEEN REDUCED TO THE SPD OF THE PRECEDING MD80.

Narrative

ACFT #1 AND ACFT #2 BOTH WERE LNDG CVG. ACFT #1 WAS ON STAR; CINCE ARR TO CVG AND SLOWED TO 250 KTS. ACFT #2 WAS COORDINATED; SLOWED TO 250 KTS BY PREVIOUS CTR. ACFT #2 WAS VECTORED TO GO BEYOND ACFT #1 AND THEN CLRED DIRECT TO BOWRR TO JOIN THE CINCE ARR. WHEN ACFT #2 JOINED THE CINCE ARR; HE WAS 7 MI BEHIND ACFT #1 AND OVER 100 KTS FASTER! IT WAS NEVER NOTICED BY RADAR CTLR AND RADAR ASSOCIATE THAT ACFT #2'S AIRSPD WAS FASTER THAN THE 250 KT ASSIGNMENT UNTIL IT WAS TOO LATE. CONFLICT ALERT ACTIVATED THAT SHOWED LOSS OF SEPARATION IMPENDING. ACFT #1 WAS ON LOW ALT SECTOR FREQ AND RADAR CTLR ASKED LOW ALT CTLR TO DSND ACFT #1. RADAR CTLR VECTORED ACFT #2 3 TIMES TO THE R TO MAINTAIN SEPARATION; HOWEVER; SEPARATION WAS LOST. WHY DID THIS HAPPEN? THE DATA BLOCKS HAVE THE ABILITY TO DISPLAY '4TH LINE INFO.' I ENTERED THE 250 KT SPD RESTR ON THE DATA BLOCK OF ACFT #2 DUE TO PREVIOUS CTR COORD. MY SCAN DID NOT INCLUDE SCRUTINY OF ACFT #2'S ACTUAL GND SPD. IT WAS ASSUMED THAT HE WAS ALWAYS AT 250 KTS. AFTER SEPARATION WAS LOST; THE RADAR CTLR ISSUED 250 KTS TO ACFT #2 AND ACFT #2 RESPONDED THAT HE WOULD SLOW TO 250 KTS. SO; HE WAS NEVER AT 250 KTS AS COORD BY THE PREVIOUS CTR CTLR. ALSO; IF THE RADAR CTLR HAD ONLY DSNDED ACFT #2 TO FL250 AND CLRED HIM TO BOWRR; VERT SEPARATION WOULD HAVE BEEN MAINTAINED WITH THE 100+ KT OVERTAKE FROM BEHIND. HOW TO PREVENT: ACCURATELY SCAN WHAT ACFT IS REALLY DOING. LOOK AT GND SPD! ALSO; ALWAYS HAVE A FORM OF SEPARATION APPLIED AT ALL TIMES. WITH ONLY 5 MI SEPARATION ALLOWED; VECTORING FOR 7 MI WAS PRETTY TIGHT. VERT SEPARATION WOULD HAVE SAVED THIS ERROR FROM OCCURRING. SUPPLEMENTAL INFO FROM ACN 616122: ACR Y WAS ISSUED 250 KTS FROM A PREVIOUS SECTOR; SO 250 KTS WAS INDICATED IN 4TH LINE OF DATA BLOCK. I FAILED TO OBSERVE THE GND SPD THAT DIDN'T COINCIDE WITH WHAT HE WAS ASSIGNED. I REQUESTED THE LOW ALT SECTOR DSCNT TO HELP ME OUT. THAT CTLR HAPPENED TO BE A FLOW CTLR GETTING HIS PROFICIENCY FOR THE MONTH. I DIDN'T GET MUCH HELP WITH A STANDARD DSCNT. POSSIBLY A FULL TIME CTLR WORKING THE OTHER ACFT COULD HAVE HELPED ME OUT.