CHS CTLR DESCRIBED CONFLICT AT APPROX 2300 FT WHEN C17 ON VOR AND TACAN APCH DSNDED BELOW ASSIGNED ALT AND CONFLICTED WITH TFC ON ILS APCH.

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ILS APCH TO RWY 33 WAS ADVERTISED. THE ACR Y WAS ON A VECTOR FOR THE APCH AT 2000 FT. THE C17 REQUESTED THE FULL VOR/TACAN RWY 33 APCH. I TOLD THE C17 TO PROCEED DIRECT THE CHS VORTAC AND OUTBOUND ON THE APCH; EXPECT APCH CLRNC PROC TURN INBOUND; DSND AND MAINTAIN 3000 FT. THE PLT READ IT BACK VERBATIM. WHEN THE C17 WAS 2 MI N OF THE VORTAC I ISSUED TFC THAT THE ACR Y WAS 9 MI S OF THE VORTAC NEBOUND AT 2000 FT FOR THE ILS RWY 33 APCH. THE C17 SAID HE WAS IMC. I THEN ISSUED TFC TO THE ACR Y THAT THE C17 WAS OVER THE VORTAC TURNING SE AT 3000 FT; HE ALSO SAID HE WAS IMC. I CLRED THE ACR Y FOR THE ILS RWY 33 APCH. OUR VORTAC IS LOCATED ON THE ARPT SO I LOST THE C17 OVER THE MAIN BANG OF THE RADAR. WHEN HE REAPPEARED HE WAS 2 MI SE OF THE VORTAC SEBOUND INDICATING 2300 FT. HE WAS NOSE-TO-NOSE WITH THE ACR Y WHO WAS AT 2000 FT. I IMMEDIATELY TOLD THE C17 TO CLB AND MAINTAIN 3000 FT AND TURN R HDG 240 DEGS IMMEDIATELY. I THEN TOLD THE ACR Y TO TURN R HDG 060 DEGS IMMEDIATELY BECAUSE THE C17 TFC HAD DSNDED. I THEN ASKED THE C17 WHY HE HAD DSNDED. HE SAID BECAUSE THE APCH PLATE TOLD HIM TO DSND TO 2100 FT AFTER XING THE VORTAC OUTBOUND. I SAID BUT I TOLD YOU TO MAINTAIN 3000 FT AND EVEN ISSUED YOU THE 2000 FT TFC ON THE ACR Y. THE C17 PLT SAID OH; SORRY I MESSED UP. THE ONLY THING I COULD HAVE DONE DIFFERENTLY WAS TO TELL HIM TO MAINTAIN 3000 FT UNTIL I ADVISED; BUT I THOUGHT MY INSTRUCTIONS WERE VERY CLR.