MEM TWR CTLR DESCRIBED CONFLICT EVENT BETWEEN GAR ON RWY 27 FOLLOWED BY A GAR ON RWY 18R; CITING CRDA AS CONTRIBUTORY.

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I OBSERVED A GAR INCIDENT AT THE MEMPHIS INTL ARPT. MEMPHIS WAS IN A 'S CONFIGN;' LNDG ON RWYS 18L; 18R; AND RWY 27. CONVERGING RWY DISPLAY AID (CRDA) PROCS WERE IN USE. THE WIND WAS OUT OF THE SE; 160 DEGS AT 7 KTS; A QUARTERING TAILWIND. AN SF340 CHKED IN ON APCH TO RWY 27. THERE WAS ANOTHER ACFT ON APCH TO RWY 18L; BUT I DO NOT RECALL THE TYPE OR CALL SIGN. THIS ACFT WAS #1 IN THE CONVERGING RWY DISPLAY AID SEQUENCE; WITH THE NEXT RWY 27 ARR TO CROSS THE RWY 27 THRESHOLD AFTER THIS ACFT FLEW OVER THE RWY 27 CTRLINE WHILE ON APCH TO LAND ON RWY 18L. ACR X; A CRJ; WAS FOLLOWING THE SF340 TO RWY 27. I LOOKED OUT THE WINDOW AND SAW THE SF340 ROLL PAST TXWY C. ACR X WAS ON SHORT FINAL TO RWY 27. I YELLED OUT; 'HE'S NOT GONNA MAKE IT!' MEANING THAT THE SF340 WOULD NEVER CLR THE END OF RWY 27 AT TXWY N. THE SUPVR IN CHARGE OF THE TWR TOLD LCL CTL TO SEND ACR X AROUND. LCL ISSUED GAR INSTRUCTIONS. THE ACR X WAS OVER OR PAST THE NUMBERS OF RWY 27; AND THE SF340 WAS STILL ON RWY 27 WHEN THESE INSTRUCTIONS WERE ISSUED. ACR X STAYED VERY LOW. I ESTIMATE 10-20 FT OFF OF RWY 27; AND FLEW DOWN THE ENTIRE LENGTH OF THE RWY AT THIS LOW ALT. LCL THEN ISSUED GAR INSTRUCTIONS TO ACR Y; AN EMBRAER JET; ON APCH TO RWY 18R; AND THIS ACFT STARTED AN IMMEDIATE CLBOUT AFTER ABANDONING THE APCH. IT APPEARED THAT ACR X FLEW THROUGH THE RWY 18R CTRLINE JUST PRIOR TO ACR Y OVERFLYING THE DEP CTRLINE OF RWY 29. THE FAA SAYS THAT CONVERGING RWY DISPLAY AID IS A SAFE PROC AND OP FOR USE AT MEM; BUT I DISAGREE. WHEN YOU COMBINE A QUARTERING TAILWIND SUCH AS WE HAD TODAY; MIXED ACFT TYPES WITH DIFFERENT APCH SPDS; MULTIPLE RWYS; MULTIPLE FLT PATHS; DIFFERENT GAR AND CLBOUT PERFORMANCE CHARACTERISTICS; TRAINING ON NEARLY EVERY POS IN THE TWR AND TRACON; AND INEXPERIENCED CTLRS WORKING COMPLEX SECTORS AND OPS -- AN ACCIDENT IS GOING TO HAPPEN!