ACR CREW OVERSHOOTS CLRED ALT ON DSCNT IN ZTL AIRSPACE.

Narrative

WHILE DSNDING IN A LARGE TURN; I CALLED 'FL290 FOR FL280' AS STATED IN OUR OPS MANUAL. THE CAPT ACKNOWLEDGED. THE ALT ALERTER WENT OFF. THE CAPT ASKED ME TO GET THE APCH PLATE TO RWY 26R AS WE HAD BRIEFED FOR RWY 27L. ATL ASSIGNED US RWY 26R BUT TOLD US TO EXPECT RWY 27L. WHILE I REACHED DOWN TO GRAB THE BOOK TO FIND THE PLATE; UNKNOWN TO ME; THE CAPT PASSED THROUGH FL280 TO FL270 AND NO ALT WARNING NOTIFIED US. AS THE CAPT LEVELED OFF; ZTL CALLED US TO TELL US WE WERE AT FL270. AS THE CAPT STARTED TO CLB; ATC CLRED US DOWN TO FL240 AND NOTHING MORE HAPPENED. THE CAPT TOLD ME HE HAD BEEN 'CONCENTRATING ON THE TURN AND NOT THE ALT.' THIS WAS HIS FIRST TIME TO HAVE AN ALTDEV. TO PREVENT THE OCCURRENCE: I SHOULD HAVE HELPED MONITOR THE ALT ALL THE WAY TO LEVELOFF. WE KEPT GETTING SMALL DSCNTS AND I HAD FELT I NEEDED TO GET THE APCH PLATE OUT FOR TIME TO BRIEF BOTH APCHS. I DO NOT KNOW WHAT THE FE WAS DOING AT THE TIME; BUT HE COULD HAVE NOTICED IF HE WAS WATCHING; OR MAYBE HE WAS DOING SOMETHING ELSE. THE CAPT JUST FORGOT AND WOULD NEED TO BE MORE AWARE OF HIS ALT. ATC NEVER CALLED US UNTIL WE LEVELED OFF AT FL270. A HEADS UP AS WE PASSED FL280 WOULD HAVE HELPED.