ACR FLC DSNDED ACFT BELOW CLRNC ALT DURING DSCNT TO ARPT.

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CLRED TO INTERCEPT THE RWY 8L LOC AT ATL AND DSND AND MAINTAIN 6000 FT MSL. DSNDING THROUGH 5700 FT MSL ON THE GLIDE PATH; I NOTICED MY ALTDEV JUST BEFORE THE ALT ALERTER SOUNDED. DSCNT STOPPED AT 5600 FT MSL AND CLBED BACK TO 6000 FT MSL. UPON REACHING 6000 FT MSL; APCH CTLR CLRED US FOR THE ILS RWY 8L. NO OTHER COMMENTS FROM HIM. BUSY TIME -- LOTS OF RADIO TFC; HDG; AIRSPD CHANGES. I WAS INTENT ON BEING ON COURSE AND ON GLIDE PATH AND MOMENTARILY FORGOT OUR ALT CLRNC. CANNOT RECALL IF THE CAPT GAVE ALT CALL-OUTS AS PER COMPANY SOP. NEXT TIME I'LL TRY NOT TO BE SO CONTENT WITH NEEDLES CTRED! SUPPLEMENTAL INFO FROM ACN 313030: WE WERE CLRED TO 6000 FT WHILE BEING VECTORED FOR APCH TO ATL. WE WERE ALSO SLOWED BY CTLR TO 180 KTS AND FLAPS WERE CONFIGURED TO 15 DEGS. I WAS WORKING RADIO AND LOOKING OUT WINDOW TO THE E FOR TFC AND ARPT. I GLANCED BACK INSIDE AND SAW OUR ALT AT 5600 FT. THE PROB SEEMS TO BE INATTN OR LOSING TRACK OF CLRNC INSTRUCTIONS OR MOMENTARY 'DAY DREAMING' BY PF; AND AS USUAL FOR THIS KIND OF EPISODE; IT HAPPENED JUST AS I DIVERTED MY ATTN ELSEWHERE.