Narrative:

Aircraft X was nebound at 055 VFR routed aus to ZZZ. Aircraft Y was eastbound routed lbb abi jen act ZZZ1 5500 ft VFR with an intermittent transponder. Radar controller called the traffic beginning 12 mi apart and regularly as they got closer. Radar controller was very busy with other traffic. At 3 mi with no visual contact; aircraft X advised he was starting a descent to avoid traffic and descended to 4500 ft. Immediately after passing PA28; aircraft X reported passing a third aircraft at his altitude and that he was about 1/4 mi behind the third aircraft. When our computer recognized the initial confliction; the data blocks flashed conflict alert as it is supposed to. The computer never alerted to the third aircraft because the computer is set to a base of 'mode C interceptor' or mci of 5000 ft and above. There are many VFR aircraft from 5000 ft and below that our computer does not warn us about conflictions for because the mci is set too high. This mci should be reset to 2000 ft and above.

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Original NASA ASRS Text

Title: ZFW CTLR DESCRIBED CONFLICTS BETWEEN VFR ACFT; SUGGESTING THE CONFLICT ALERT PARAMETERS NEED TO BE ADJUSTED TO DISPLAY ADDITIONAL VFR TFC.

Narrative: ACFT X WAS NEBOUND AT 055 VFR ROUTED AUS TO ZZZ. ACFT Y WAS EBOUND ROUTED LBB ABI JEN ACT ZZZ1 5500 FT VFR WITH AN INTERMITTENT XPONDER. RADAR CTLR CALLED THE TFC BEGINNING 12 MI APART AND REGULARLY AS THEY GOT CLOSER. RADAR CTLR WAS VERY BUSY WITH OTHER TFC. AT 3 MI WITH NO VISUAL CONTACT; ACFT X ADVISED HE WAS STARTING A DSCNT TO AVOID TFC AND DSNDED TO 4500 FT. IMMEDIATELY AFTER PASSING PA28; ACFT X RPTED PASSING A THIRD ACFT AT HIS ALT AND THAT HE WAS ABOUT 1/4 MI BEHIND THE THIRD ACFT. WHEN OUR COMPUTER RECOGNIZED THE INITIAL CONFLICTION; THE DATA BLOCKS FLASHED CONFLICT ALERT AS IT IS SUPPOSED TO. THE COMPUTER NEVER ALERTED TO THE THIRD ACFT BECAUSE THE COMPUTER IS SET TO A BASE OF 'MODE C INTERCEPTOR' OR MCI OF 5000 FT AND ABOVE. THERE ARE MANY VFR ACFT FROM 5000 FT AND BELOW THAT OUR COMPUTER DOES NOT WARN US ABOUT CONFLICTIONS FOR BECAUSE THE MCI IS SET TOO HIGH. THIS MCI SHOULD BE RESET TO 2000 FT AND ABOVE.

Data retrieved from NASA's ASRS site as of May 2009 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.