ALTDEV. MISCOM ON ASSIGNED ALT.

Narrative

TURNED DOWNWIND AT 6000 FT HDG 180 DEGS 170 KTS FOR VECTORS TO ILS RWY 36R. HVY TFC ARRIVING AND DEPARTING. IMC WITH MODERATE TO OCCASIONAL HVY RAIN THROUGH AREA. CLRNC RECEIVED AND ACKNOWLEDGED BY CAPT TO DSND TO 4000 FT. FO FLYING ACFT ON AUTOPLT. FO SET ALT ALERTER TO 4000 FT AND CONFIRMED. PASSING 4300 FT CTLR RADIOED AND SAID ALT WAS 4600 FT. IMMEDIATE REVERSAL MADE BACK TO 4600 FT AS CAPT REPLIED THAT FLT WAS CLRED TO 4000 FT. TCASII ISSUED 'MONITOR DSCNT' AS THE REVERSAL WAS MADE. FO THEN SAID I THOUGHT I HEARD 4600 FT FOR THE CLRNC BUT WHEN CAPT ACKNOWLEDGED 4000 FT AND CTLR DIDN'T CORRECT; HE ASSUMED HE MISHEARD AND SET 4000 FT IN ALERTER. DURING EARLIER PORTION OF FLT FO MISHEARD A CLRNC SO CAPT RE- CONFIRMED CLRNC WITH ATC. THIS OCCASION BEING RPTED; FO DID NOT ASK TO CLARIFY THE CLRNC; PERHAPS BECAUSE HE MISHEARD EARLIER CLRNC. AS ALWAYS; IF IN DOUBT; ASK. COMS AND COCKPIT ATMOSPHERE WAS FRIENDLY AND SUPPORTIVE THROUGHOUT FLT. I CANNOT EXPLAIN THE COMS FAILURE IN THIS CASE EXCEPT POSSIBLE HESITATION DUE TO EARLIER MISHEAR BY FO AND NOT WANTING TO REPEAT SAME TYPE OF ERROR.