Narrative:

ILS approach to runway 33 was advertised. The air carrier Y was on a vector for the approach at 2000 ft. The C17 requested the full VOR/TACAN runway 33 approach. I told the C17 to proceed direct the chs VORTAC and outbound on the approach; expect approach clearance procedure turn inbound; descend and maintain 3000 ft. The pilot read it back verbatim. When the C17 was 2 mi north of the VORTAC I issued traffic that the air carrier Y was 9 mi south of the VORTAC nebound at 2000 ft for the ILS runway 33 approach. The C17 said he was IMC. I then issued traffic to the air carrier Y that the C17 was over the VORTAC turning southeast at 3000 ft; he also said he was IMC. I cleared the air carrier Y for the ILS runway 33 approach. Our VORTAC is located on the airport so I lost the C17 over the main bang of the radar. When he reappeared he was 2 mi southeast of the VORTAC sebound indicating 2300 ft. He was nose-to-nose with the air carrier Y who was at 2000 ft. I immediately told the C17 to climb and maintain 3000 ft and turn right heading 240 degrees immediately. I then told the air carrier Y to turn right heading 060 degrees immediately because the C17 traffic had descended. I then asked the C17 why he had descended. He said because the approach plate told him to descend to 2100 ft after crossing the VORTAC outbound. I said but I told you to maintain 3000 ft and even issued you the 2000 ft traffic on the air carrier Y. The C17 pilot 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 clear.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of January 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.