Narrative:

We were flying the unarm 1 arrival into clt. Clt approach had given us a turn to 360 degrees and a descent to 9000 ft. The autoplt was engaged and the first officer was flying the aircraft. Approach control had called out traffic below us. Both pilots were occupied searching for the traffic. When I looked back at the instruments, I noticed the aircraft descending below 9000 ft. The first officer immediately disconnected the autoplt and initiated a climb back to 9000 ft. The aircraft had descended to approximately 8700 ft before returning back to assigned altitude. Approach control meanwhile verified our assigned altitude and I acknowledged. I feel that crew fatigue and both pilots being preoccupied looking for traffic were the major factors for deviating below altitude.

Google
 

Original NASA ASRS Text

Title: MLG FLC OVERSHOT DSCNT ALT AND THEN IMMEDIATELY RETURNED.

Narrative: WE WERE FLYING THE UNARM 1 ARR INTO CLT. CLT APCH HAD GIVEN US A TURN TO 360 DEGS AND A DSCNT TO 9000 FT. THE AUTOPLT WAS ENGAGED AND THE FO WAS FLYING THE ACFT. APCH CTL HAD CALLED OUT TFC BELOW US. BOTH PLTS WERE OCCUPIED SEARCHING FOR THE TFC. WHEN I LOOKED BACK AT THE INSTS, I NOTICED THE ACFT DSNDING BELOW 9000 FT. THE FO IMMEDIATELY DISCONNECTED THE AUTOPLT AND INITIATED A CLB BACK TO 9000 FT. THE ACFT HAD DSNDED TO APPROX 8700 FT BEFORE RETURNING BACK TO ASSIGNED ALT. APCH CTL MEANWHILE VERIFIED OUR ASSIGNED ALT AND I ACKNOWLEDGED. I FEEL THAT CREW FATIGUE AND BOTH PLTS BEING PREOCCUPIED LOOKING FOR TFC WERE THE MAJOR FACTORS FOR DEVIATING BELOW ALT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.