AN ACR FLT DSNDS TOWARDS FL230 WHEN THE CLRNC WAS TO DSND TO FL330. POTENTIAL CONFLICT OCCURS.

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ACFT AT FL390 CRUISE ALT 50 NM E OF MOD. FO (PNF) HEARD CLRNC FROM ATC TO DSND AT PLT'S DISCRETION TO FL230 AND READ BACK; 'CLRED AT PLT'S DISCRETION TO DSND TO FL230.' CAPT (PF) CONFIRMED READBACK AND ALT SET IN MCP; BUT DID NOT HEAR ORIGINAL ATC CLRNC. (PNF WAS NOT AWARE PF DID NOT HEAR ORIGINAL CLRNC.) AS ACFT DSNDED THROUGH FL320; ATC REQUESTED ACFT'S ALT AND WAS TOLD ACFT WAS PASSING FL320 FOR FL230. ACFT WAS IMMEDIATELY GIVEN A 90 DEG COURSE DEV TO THE S WHILE AIR CREW STOPPED THE DSCNT AS IT SAW TCASII TFC BELOW. TFC BELOW WAS ASKED TO INCREASE ITS RATE OF DSCNT. ACFT MAINTAINED ITS PRESENT ALT AFTER ITS COURSE DEV; APPROX FL310. ONCE CLR OF CONFLICT; ACFT WAS CLRED DIRECT TO MOD AND DSND TO FL240. CAUSES FOR INCIDENT: 1) PNF HEARD WRONG ALT AND THUS READ BACK WRONG ALT. 2) ATC MISSED INCORRECT READBACK. 3) PF DID NOT HEAR ORIGINAL CLRNC. 4) PNF ASSUMED PF HEARD INITIAL CLRNC. PREVENTIVE MEASURES: 1) ENSURE BOTH PF AND PNF HEAR ORIGINAL CLRNC. 2) IMPLEMENT A SYS SIMILAR TO ALT CLRNCS SUCH AS '10000 FT' (ONE-ZERO THOUSAND; TEN THOUSAND) AND '11000 FT' (ONE-ONE THOUSAND; ELEVEN THOUSAND). FOR EXAMPLE: FL230 (FLT LEVEL TWO-THREE-ZERO; TWENTY-THREE). FL330 (FLT LEVEL THREE-THREE-ZERO; THIRTY-THREE).