RADAR CTLR IN TRAINING MISSED INCORRECT READBACK OF ALT ASSIGNMENT BY ACR LGT AND HAD LTSS OCCUR WITH AN SMT.

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LGT X WAS ISSUED DSCNT CLRNC TO 13000. LGT X READ BACK 12000 AND THIS WAS MISSED BY THE INSTRUCTOR AND DEVELOPMENTAL. THE DEVELOPMENTAL ISSUED TFC TO LGT X AS TFC 12 O'CLOCK; 8 MI; WBOUND SMT Y AT 12000. LGT X ADVISED TFC IN SIGHT! YET; HE SAID NOTHING ELSE. SMT Y WAS ISSUED TFC BY THE DEVELOPMENTAL. RIGHT AFTER HE FINISHED; I OBSERVED THE LGT X FLT BELOW 13000; THE PRESUMED ALT I/WE THOUGHT HE WAS DSNDING TO. I KEYED THE FREQ TO RESTATE HIS ALT CLRED TO WAS 13000 AND LGT X REPLIED HE WAS CLRED TO 12000 AND HE HAD THE TFC IN SIGHT STILL. CAUSE OF PROB: I; THE CTLR MISSED THE LGT X READBACK OF THE WRONG ISSUED ATTITUDE. MY CONCERN IN THIS SIT WAS THE PLT DID NOT QUESTION HIS CLRNC WHEN TFC WAS ISSUED AND HE HAD IT IN SIGHT. LGT X AT THIS TIME WAS AT 14300 AND DSNDING. SUPPLEMENTAL INFO FROM ACN 256426: LGT X PLT CALLED ON; I GAVE DSCNT TO 13000 WITH THE DSM ALT AND HE READ BACK 12000 AND I MISSED IT. I ISSUED TFC TO LGT X 12 O'CLOCK; 8 MI; SMT Y AT 12000 WBOUND. HE SAID HE HAD IT IN SIGHT WHEN I ISSUED IT TO HIM. LGT X WAS PRESENTLY AT 14300 WHEN THE TFC WAS ISSUED. WE DIDN'T KNOW LGT X WOULD KEEP GOING DOWN TO 12000 UNTIL THE CONFLICT ALERT WENT OFF AND TOLD HIM HE WAS TO MAINTAIN 13000 AND HE SAID WAS GIVEN 12000. MY TRAINER STEPPED IN AND TOOK OVER TO CORRECT THE ACTION. LGT X SAID HE HAD TFC IN SIGHT AND THE TFC PASSED AND LGT X WENT DOWN TO 11000 THEN. I WOULD LIKE TO KNOW WHY LGT X DIDN'T COME BACK AND QUESTION US ON HIS DSCNT WHEN I ISSUED TFC. TFC WAS ISSUED TO SMT Y ALSO. SUPPLEMENTAL INFO FROM ACN 256526: SEPARATION WAS SEVERAL MI AND A CONFLICT WAS NOT APPARENT. WE THEN EXPLAINED TO CTR THAT WE HAD UNDERSTOOD THAT 12000 FT WAS THE ASSIGNED ALT AND THAT WE HAD READ BACK 12000 FT; WITH NO SUBSEQUENT QUESTION FROM CTR. SUPPLEMENTAL INFO FROM ACN 256423: WE ARRESTED OUR RATE OF DSCNT AT 12500 FT. WHILE LEVELING OFF; WE ACQUIRED THE TFC VISUALLY AND DETERMINED THAT IT WAS WELL CLR AND NO FACTOR. SEPARATION WAS SEVERAL MI WITH NO CONFLICT. PRIOR TO NEXT FREQ CHANGE CTR REQUESTED WE CONTACT QUALITY ASSURANCE THE FOLLOWING DAY.