ALT BUST.

Narrative

APPROX 10 NM N OF GRW VOR; CTLED. ASSIGNED INITIAL DSCNT FROM 12000 FT TO 10000 FT. AS PNF; I RESPONDED WITH 'ACR XYZ CLRED TO 1-0 THOUSAND.' AT APPROX 10400 FT; CTLR ASKED PRESENT ALT AND I RESPONDED WITH '10 POINT 4 DSNDING TO 1-0 THOUSAND.' CTLR SAID CLRNC WAS FOR 11000 BUT TO CONTINUE DSCNT TO 9000. CONTINUED DSCNT TO 9000. NO FURTHER INCIDENT. CTLR WORKLOAD WAS HIGH AND READBACK WAS NOT CHALLENGED OR CORRECTED. READING BACK ALT ASSIGNMENTS TWICE SUCH AS '1-0; 10 THOUSAND' IS A GOOD WAY TO HELP ELIMINATE THE PROBLEM. ALTHOUGH WE ARE NOT TRAINED TO USE THIS METHOD; AND 'TEACHING OLD DOGS NEW TRICKS' IS SOMETIMES DIFFICULT; I WOULD RECOMMEND THAT THIS METHOD BE THE APPROVED FAA METHOD OF CLRNC READBACKS. SUPPLEMENTAL INFO FROM ACN 210688: FLT WAS ENRTE TO MEM AT 12000; RECEIVED WHAT WAS PERCEIVED AS CLRNC TO 10000. I WAS PF AND HEARD PNF (CAPT) READBACK 10000. PASSING 10400 IN DSCNT; CTR SAID WE WERE CLRED TO 11000; THEN ISSUED A CONTINUED DSCNT TO 9000. I REMEMBERED HEARING CAPT READ BACK 10000.