CENRAP IN USE WHILE ASR-9 DOWN FOR MAINT. SEE AND AVOID SIT INVOLVING ACR X RESPONDING TO A TCASII RA ON ACFT Y. ACFT Y WAS NOT OBSERVED ON CENRAP UNTIL AFTER THE ACFT HAS PASSED.

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IT'S FRI AFTERNOON. WE HAD BEEN ON CENRAP (CTR RADAR ARTS PRESENTATION/PROCESSING) THE ENTIRE SHIFT; AS MAINT HAD OUR ASR-9 DOWN FOR MAINT. THE WX IS EXCELLENT VFR. I'M WORKING ACR X; A SAAB-340 ENRTE AT 10000 FT PROCEEDING SWBOUND OVER APE VOR; DEST CINCINNATI. THE PLT SUDDENLY ASKS WHETHER I SAW THE TFC THAT JUST WENT BY. I HAD NOT. BUT IMMEDIATELY AFTER HIS INQUIRY I SAW A VFR TARGET CLBING OUT OF 10300 FT; NWBOUND; AT HIS 3 O'CLOCK POS. IT IS OBVIOUS THAT THIS VFR ACFT CAME VERY CLOSE TO THE ACR X. THE PLT ADVISED THAT HE FOLLOWED A TCASII RA TO AVOID WHAT HE DESCRIBED AS A BUSINESS JET. I WAS PAYING CLOSE ATTN TO THE SCREEN; BUT I NEVER SAW THE VFR TFC IN TIME TO GIVE THE ACR X CREW A WARNING. AFTER A LITTLE INVESTIGATION; WE FOUND A PROPOSED IFR FLT PLAN ON A CESSNA CITATION FROM NEWARK ARPT (2I8) TO TOLEDO; OH (TOL). 2I8 IS LOCATED 10 MI SE OF APE VOR. WE CALLED AND VERIFIED THAT THE CITATION HAD DEPARTED AROUND THAT TIME; WHICH IS WHY I FEEL CONFIDENT THAT THE OTHER ACFT WAS A C550. I QUESTION IF THE TARGET HAD EVEN APPEARED ON MY SCREEN PRIOR TO HIS QUERY. ON THE OTHER HAND; I HAVE 100 PERCENT CONFIDENCE THAT THE TARGET WOULD HAVE BEEN ON MY SCREEN HAD WE BEEN UTILIZING THE ASR-9. ALSO; HAD WE BEEN ON THE ASR-9; I HAVE CONFIDENCE THAT THE CONFLICT ALERT WOULD HAVE SOUNDED. AS IT IS; WHEN ON CENRAP; THERE IS NO CONFLICT ALERT FUNCTION AVAILABLE TO BACK US UP IF WE GET DISTRACTED. EVEN WHEN A XPONDER IS WORKING; THE ARTS TAGS OFTEN 'DROP OFF' OR GO INTO 'COAST.' IT HAPPENS ON A REGULAR BASIS WHEN USING CENRAP. IN FACT; IT WASN'T BUT A FEW MINS AFTER THIS INCIDENT THAT I RADAR IDENTED AN ACFT IN THIS SAME GENERAL VICINITY AT 3000 FT; ISSUED AN IFR CLRNC WITH A CLB TO 7000 FT; AND THEN THE ARTS TAG DROPPED. THIS ACFT'S TARGET VANISHED AS IT CLBED THROUGH APPROX 4000 FT; AND IT DIDN'T REAPPEAR UNTIL JUST BEFORE THE ACFT LEVELED AT 7000 FT. CENRAP; AS DEFINED IN THE PLT/CTLR GLOSSARY IS 'A COMPUTER PROGRAM DEVELOPED TO PROVIDE A BACKUP SYS FOR ARPT SURVEILLANCE RADAR IN THE EVENT OF A FAILURE OR MALFUNCTION. THE PROGRAM USES AIR RTE TFC CTL CTR FOR THE PROCESSING AND PRESENTATION OF DATA ON THE ARTS IIA OR IIIA DISPLAYS.' TODAY; AS WELL AS YESTERDAY; THERE WAS NO FAILURE OR MALFUNCTION OF OUR ARPT SURVEILLANCE RADAR; YET OUR ASR-9 WAS NOT AVAILABLE DUE TO MAINT. AS I UNDERSTAND IT; THE TECHNICIANS WERE TAKING THE RADAR DOWN TO UPGRADE THE BEACON SYS TO MODE- S. WHILE THE UPGRADE OF THE BEACON SYS IS IMPORTANT; IN MY OPINION SUCH NON-EMER WORK SHOULD BE ACCOMPLISHED DURING PERIODS OF LIGHT TFC (IE; WEEKEND MIDNIGHT SHIFTS); NOT ON A VFR FRIDAY AFTERNOON. CENRAP IS CERTAINLY BETTER THAN NO RADAR AT ALL; BUT ITS USE IN A NON-FAILURE SIT IS NOT IN THE BEST INTEREST OF SAFETY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED HE WAS UNSURE IF THE CENRAP SYS HAD BEEN FLT CHKED WITH FACILITY EQUIP PRIOR TO ACTIVATION AS THE BACKUP FOR THE ASR-9 RADAR SYS. CTLR INDICATED THE XPONDER OF THE CLBING ACFT MENTIONED IN HIS RPT MAY NOT HAVE BEEN TURNED ON UNTIL AROUND 10000 FT. HE INDICATED THAT THE TAG DROP OF THE OTHER ACFT WHICH WAS CLBING FROM 3000 FT TO 7000 FT WAS POSSIBLY DUE TO THE ACFT BEACON ANTENNA BEING SHIELDED DURING THE CLB AWAY FROM THE CTR RADAR. RPTR STATED THAT RECENTLY WHEN THE CENRAP WAS AGAIN USED; A LOG OF TAG DROPS WAS KEPT BY THE FACILITY. HE INDICATED THAT THEY RECORDED AN ESTIMATED 20 TAG DROPS AT RANDOM LOCATIONS AND ALTS DURING THE 2 HR PERIOD. RPTR INDICATED THAT THE ASR-9 SYS WAS BEING GIVEN TO MAINT AT INAPPROPRIATE TIMES FOR WORK OTHER THAN NECESSITY WHICH HE FELT COULD BE DONE ON A MIDSHIFT.