A DSCNT BELOW MVA IS PERFORMED BY A FLT CREW OF A DEPARTING E145 IN RESPONSE TO A TCASII RA FROM A PA28 WHO HAD ALREADY PASSED THE FLT WITH MORE THAN ADEQUATE CLRNC 2 NM NE OF CHS; SC.

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DAYTIME VFR -- MODERATE TFC. PA28 3 MI NE OF CHS HEADING 210 DEGS AT 3000 FT FOR R DOWNWIND ENTRY FOR VISUAL APCH TO RWY 3. E145 DEPARTS RWY 3 ON RWY HEADING CLBING TO 2000 FT. E145 DID NOT HAVE XPONDER ON. I RADAR IDENTED E145 AND TOLD HIM TO RECYCLE XPONDER; THEN ISSUED TFC ON PA28 (AT E145'S 2 O'CLOCK POS; AND 2 MI AT 3000 FT; SWBOUND). IMMEDIATELY AFTER; E145'S XPONDER ACTIVATED WITH AN ALT OF 1200 FT. I AGAIN CALLED TFC ON PA28 (NOW 3 O'CLOCK POS; 1 1/2 MI; LEVEL 3000 FT). E145 REPLIES NEGATIVE TFC. PA28 RPTS E145 IN SIGHT. I INSTRUCT PA28 TO MAINTAIN VISUAL SEPARATION WITH E145 AND THEN INSTRUCT E145 TO CLB TO 10000 FT; AND THAT PA28; NOW AT 5 O'CLOCK POS; AND 2 MI; HAD HIM IN SIGHT AND WAS NO FACTOR. E145 THEN STATES THAT HE'S RESPONDING TO AN RA ON TFC AND IS DSNDING. I ADVISE E145 THAT THE ONLY TFC WITHIN 10 MI OF E145 WAS THE PA28; NOW 6 O'CLOCK POS; AND 4 MI. E145 CONTINUES TO DSND BELOW MVA (1600 FT) AND AGAIN STATES HE'S RESPONDING TO RA. I AGAIN STATE THAT THERE WAS NO TFC WITHIN 10 MI OF E145 (OTHER THAN PA28). E145 THEN SAID RA WAS OVER AND THAT HE WAS CONTINUING CLB. I BELIEVE THE CAUSE OF THIS WAS THAT THE E145 DID NOT HAVE XPONDER ON INITIALLY; IN THAT TCASII RELIES ON BASELINE ALT INFO FROM THE PLANES OWN MODE C AS RPTED BY XPONDER TO ANALYZE POTENTIAL THREATS FROM OTHER XPONDER-EQUIPPED ACFT. FOR THOSE FEW SECONDS WHILE PROCESSING INFO FROM THE E145 ON MODE C AFTER INITIALIZATION; THE TCASII ISSUED THE RA ON THE PA28 BASED ON INCOMPLETE DATA. HAD THE E145 ACTIVATED THE XPONDER AT COMMENCEMENT OF TKOF ROLL; THERE WOULD NOT HAVE BEEN AN INCIDENT. WHILE I REALIZE THAT PRESENTLY; PLTS ARE MANDATED TO FOLLOW AN RA; IN THIS CASE; HAD THE RA CAME SOONER; IT WOULD HAVE PUT THE E145 IN DANGEROUS PROX TO THE GND. THERE WERE NO OTHER ACFT NEAR THE E145; AND THE PLT WAS INFORMED OF THIS; BUT STILL HE DSNDED BELOW THE MVA TO COMPLY WITH THE RA. HOW SAFE IS THAT? MY RECOMMENDATION WOULD BE TO ALTER THE TCASII LOGIC SO THAT IN A CASE SIMILAR TO THIS; A TA WOULD BE ISSUED UNTIL MODE C ALT INFO IS STABILIZED AND VERIFIED PRIOR TO ISSUING AN RA. I REALIZE THAT A TA WITHOUT ALT INFO PUTS AN INCREASED WORKLOAD ON PLTS; BUT IN THIS CASE; THAT TA; ALONG WITH MY TFC INFO; WOULD HAVE ENABLED THE PLTS TO REALIZE THAT THE TFC WAS NOT A FACTOR.