SNA LCL CTLR DESCRIBED CONFLICT EVENT WITH TWO ARR ACR'S ON VISUAL APCH'S; EVENT RECORDED AS PLTDEV; FLT CREW FOLLOWED WRONG ACFT.

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I WAS WORKING LCL AT SNA TWR. I HAD A B737 ON A 6 MI FINAL WHO WAS ALREADY CLRED TO LAND. THE B737 SAID; 'TWR; IS THERE TFC AT MY 12 O'CLOCK?' I LOOKED AT THE RADAR AND IMMEDIATELY GAVE THE B737 A TFC ALERT REGARDING AN ACFT IN SOCAL TRACON'S AIRSPACE (SNA TWR'S AIRSPACE STARTS ON A 5 MI FINAL AT 2400 FT.) THE B737 WAS DSNDING OUT OF 3100 FT AND WHEN I LOOKED; THE ERJ 135 WAS AT 2500 FT TURNING DIRECTLY INTO THE B737. IT WAS NOT UNUSUAL TO SEE AN ACFT WHERE THE ERJ WAS AT BECAUSE ACFT ARE COMMONLY VECTORED THROUGH OUR FINAL AND IT ACTUALLY APPEARED THAT THE ERJ WAS DOWNWIND. IT WASN'T UNTIL THE B737 SAID THAT WHEN MY ASSISTANT AND MYSELF NOTICED THAT THE ERJ WAS IN A TURN. WHILE I WAS GIVING THE TFC ALERT; MY ASSISTANT WAS ON THE LINE WITH SOCAL TRACON (TUSTIN SECTOR) ASKING WHAT THE ERJ WAS DOING. I COULD NOT SUGGEST A SPECIFIC AVOIDANCE MANEUVER WITHOUT KNOWING WHAT THEY WERE DOING. THE B737 EITHER HAD A TCAS ALERT OR HE HAD THE ERJ IN SIGHT AND BOTH OF THOSE COULD PROVIDE HIM WITH MORE ACCURATE INFO TO EVADE THE TFC THE WAY MY RADAR LAGS. ALL I COULD THINK OF WAS WHAT TO SAY IN ORDER TO GET THE B737 TO REALIZE THAT IT WAS AN IMMINENT TFC SIT. AS I WAS GIVING THE ALERT; MY ASSISTANT INFORMED ME THAT THE ERJ WAS WITH THE TRACON AND THAT SHE ACCIDENTALLY TURNED IN FRONT OF THE TFC SHE WAS INSTRUCTED TO FOLLOW. I PASSED THAT ON TO THE B737 AND ASKED HIM IF HE HAD THE ERJ IN SIGHT. THE B737; OBVIOUSLY SHAKEN UP DID NOT RESPOND. I WAITED A LITTLE BIT AND ASKED AGAIN. THE B737 FINALLY SAID HE DID; AND BY THAT TIME WE ALMOST HAD SEPARATION. I TOLD THE B737 TO FOLLOW THE ERJ AND THAT HE WAS CLRED TO LAND. THE CONFLICT ALERT DID NOT GO OFF UNTIL AFTER WE WERE ALREADY AWARE OF THE SIT AND SEPARATION WAS LOST. THE CLOSEST PROX THAT I WITNESSED ON THE RADAR WAS WITHIN 1 MI AND WITHIN 500 FT; ALTHOUGH I BELIEVE IT WAS CLOSER AS I WAS MORE FOCUSED ON SEPARATION. I BELIEVE THAT WHAT CAUSED THE PROB IS A CHAIN OF EVENTS. FIRST OFF; PERHAPS THE ERJ SAID SHE HAD THE B737 IN SIGHT IN ORDER TO BE CLRED FOR THE VISUAL APCH. OUR RADAR LAGS SO IT TAKES A WHILE FOR US TO SEE THAT TURN; AND WHEN ACFT ARE IN SUCH CLOSE PROX TO BEGIN WITH; A SHARP TURN CAN EASILY AND QUICKLY RESULT IN A DANGEROUS SIT. OUR CONFLICT ALERT DOES NOT DETECT TURNS WELL; AND IF IT IS UNEXPECTED IT IS HARD FOR US AS CTLRS TO DO SO AS WELL. THIS WAS A VERY SCARY EVENT FOR EVERYONE IN THE TWR AND NATURALLY I ASSUME THE SAME FOR THE CTLR WORKING THE TUSTIN SECTOR. IN HINDSIGHT; I CANNOT THINK OF ANYTHING I COULD HAVE DONE DIFFERENTLY IN ORDER TO MAKE THE SIT SAFER; AS THE ERJ APPEARED TO BE STEADILY ON THE DOWNWIND AND MADE A QUICK TURN. I THINK THE PLT OF THE ERJ DID NOT HAVE HER TFC AND PERHAPS WANTED THE VISUAL APCH CLRNC SO SHE SAID SHE DID. THERE ARE SO MANY RULES NOW; SOME OF WHICH JUST CHK A BOX. THIS GETS PLTS INTO A ROUTINE AND THE THINGS THAT ARE TRULY NECESSARY DO NOT STAND OUT AS MUCH ANY MORE. AN EXAMPLE OF THIS IS IN OUR TFC PATTERN. WE CONSTANTLY NEED PLTS TO SAY THINGS TO BE CLRED FOR THE OPTION BUT TO THEM IT IS ALL ROBOTIC. OUR ATIS IS CLOGGED WITH SOMETIMES USELESS INFO OR INFO THAT IS ALREADY KNOWN ONLY TO LEAD A PLT TO JUST 'CHK A BOX' AND NOT TRULY PAY ATTN BECAUSE THE ATIS IS EXTREMELY LONG AND IT CHANGED 3 TIMES IN 1 HR. THE MORE RULES AND VERBIAGE THAT ARE INTRODUCED TO THE NAS MAY PERHAPS PROVIDE SAFETY IN SOME WAYS BUT IN SOME WAYS IT DETRACTS FROM THE THINGS THAT ARE IMPORTANT.