AN IFR SMA ON INST TRAINING FLT AT 6000 FT WAS ISSUED A DSCNT BELOW THE MVA. THE ACFT GOT TO 3600 FT BEFORE THE CTLR INTERVENED AND ISSUED A CLB. THE INSTRUCTOR PLT BELIEVES THAT THE CTLR TRANSPOSED ACFT CALL SIGN AND GAVE DSCNT TO THE WRONG ACFT. THE ACFT WAS IN VMC CONDITIONS DURING THE DSCNT.

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THE EVENT OCCURRED DURING A PORTION OF A STUDENT'S LONG IFR XCOUNTRY FLT BTWN ONP AND CVO AT 6000 FT MSL. ZSE HANDED US OFF TO CASCADE APCH AND MOMENTS LATER CASCADE CLRED US TO 5000 FT AND A SHORT TIME LATER TO 3000 FT; BOTH CLRNCS WERE READ BACK. THE STUDENT BEGAN THE DSCNT AND AS WE WERE IN VFR CONDITIONS; I HAD THE STUDENT RAISE HIS FOGGLES AND LOOK AT HOW CLOSE WE WERE TO THE TERRAIN AND EXPLAINED THAT THESE HILLS DID NOT MEET THE FAA DEFINITION OF MOUNTAINOUS SO WE WOULD ONLY HAVE 1000 FT OF CLRNC. THEN CASCADE APCH ASKED OUR ALT AND THE STUDENT RESPONDED WITH 3600 FT. A MOMENT LATER; APCH INFORMED US THAT MVA IN OUR LOCATION WAS 5500 FT; AT WHICH TIME I STOPPED OUR DSCNT AND A SHORT TIME LATER WE WERE DIRECTED TO CLB TO 5000 FT. WHEN WE REACHED 4000 FT WE RECEIVED AN AMENDMENT FOR 4000 FT THEN MOMENTS LATER WE WERE CLRED TO CROSS THE VOR AT 3000 FT AND CLRED FOR THE APCH. CASCADE APCH WAS VERY BUSY AT THE TIME AND ANOTHER ACFT WITH THE SAME LAST DIGIT; WHICH WAS ALREADY IN THE VALLEY AREA; HAD REQUESTED AN APCH. THIS UNDOUBTEDLY LED TO THE APCH CLRING THE WRONG ACFT TO 3000 FT. SUBSEQUENTLY I FOUND OUT THAT ZSE'S MAIN FRAME HAD GONE DOWN AND THAT WOULD HAVE ADDED TO CASCADE'S WORKLOAD. WHEN THE EVENT OCCURRED I COULD HAVE HELPED CASCADE OUT BY INFORMING THEM WE WERE IN VFR CONDITIONS. I HAVE ALSO CAUTIONED THE STUDENT TO RPT ALT LEAVING; AS WELL AS THE ALT GOING TO; EVEN WHEN IN A RADAR ENVIRONMENT IN AN EFFORT TO CONFIRM ALT CHANGES.