ACR-MDT LEFT ALT WITHOUT CLRNC AND CAUSED LESS THAN STANDARD SEPARATION WITH SMT.

Narrative

MDT X WAS IN LEVEL CRS AT 11000' HDG NE DIR LAN. SMT Y WAS IN LEVEL CRS AT 10000' HDG W ON V170. ABCD; A FLT OF 2 FGT'S; CHKED ON THE FREQ HDG N OFF OF BTL LEAVING 5000 FOR 8000'. THE RADAR TRNEE ACKNOWLEDGED THE CALL ON AND CLBED THE FGT'S TO 9000' BECAUSE HE WAS CLOSE TO 5 MI FROM SMT Y. A FEW MOMENTS LATER WE NOTICED THE MODE C OF MDT X AT 10700'. AT THIS TIME WE ASSUMED IT WAS A MODE C SWAP SITUATION; BECAUSE MDT X WAS WITHIN 1 MI OF ANOTHER ACFT AT 12000' (THIS IS A NORMAL OCCURRENCE). MDT X'S MODE C THEN WENT OUT FOR A FEW SWEEPS. THE NEXT MODE C FOR MDT X SHOWED 10300'; AT WHICH TIME THE TARGET WAS ONLY 2-3 MI FROM SMT Y'S PRIMARY TARGET (WE HAD LOST THE BEACON TARGET A FEW MI BEFORE). MDT X WAS THEN ASKED WHAT HIS ALT WAS AND HE REPLIED; 'OUT OF 10 FOR 9.' I THEN IMMEDIATELY TURNED MDT X TO A 360 DEG HDG AND TOLD HIM TO EXPEDITE TO 9000'. SMT Y WAS TURNED TO A TIGHT 180 DEG HDG. APPARENTLY WHAT HAPPENED WAS MDT X TOOK THE CLRNC TO 9000' THAT WAS ISSUED TO FGT Z; BUT WAS STEPPED ON WHEN HE READ IT BACK BY FGT Z ALSO READING BACK THE CLRNC. A SEPARATE SPEAKER FOR VHF RECEIVING MAY HAVE ENABLED US TO HEAR BOTH READBACKS INSTEAD OF JUST FGT Z'S. ALSO; ELIMINATION OF THE MODE C SWAP AND LOSS OF MODE C ON THE RADAR WOULD HAVE ENABLED US TO DETECT THE ERROR SOONER. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE FAC CONDUCTED A FULL INVESTIGATION OF THE LOSS OF SEP. IT WAS DETERMINED THAT THE ACR MDT TOOK THE CLRNC TO 9000' THAT WAS GIVEN TO ANOTHER ACFT. THE CTLR WAS UNABLE TO HEAR THE INCORRECT READBACK BECAUSE HE WAS HEARING THE READBACK FROM THE ACFT ON UHF FOR WHICH THE CLRNC WAS MEANT. IT WAS DETERMINED THAT THE LOSS OF SEP WAS CAUSED BY THE FLT CREW DEVIATION.