JUST AS ACFT WAS ABOUT TO LEVEL OFF ON AUTOPLT; BOTH FLC BECAME DISTRACTED LOOKING OUT OF ACFT AND DIDN'T NOTICE THE ALTDEV UNTIL THE ALERT SOUNDED. RPTR CAPT RETURNED ACFT TO ASSIGNED ALT.

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ENRTE FROM VUZ TO SAV; GA; BY WAY OF MACON VOR; ACFT WAS LEVEL FL230 AND GIVEN CLRNC TO CLB TO FL250; FL250 TO BE FINAL ALT. AUTOPLT WAS ON OPERATING IN NAV MODE. I INITIATED A CLB OF ABOUT 1300 FPM AND SELECTED THE VERT SPD MODE. OUT OF FL240 I CALLED OUT 1000 FT TO GO AND FO ACKNOWLEDGED. I CONTINUED TO MAINTAIN THE CLB UNTIL THE AUTOPLT COMMUNICATOR INDICATED AT CAPTURE AT FL250 APPROX 100 FT BELOW THE ASSIGNED ALT. AT THAT POINT I BEGAN LOOKING OUTSIDE THE ACFT. IT WAS A BEAUTIFUL DAY AND I WAS ENJOYING THE VIEW. APPARENTLY SO WAS MY FO. APPROX 15-20 SECONDS LATER THE ALT ALERTER WENT OFF. THE ANNUNCIATOR INDICATED A CAPTURE OF THE PRESET ALT; THE ACFT WAS STILL CLBING AT 1300 FPM. THE ALERTER WENT OFF AT 300 FT ABOVE THE ASSIGNED ALT. I DISENGAGED THE AUTOPLT AND BEGAN A RECOVERY; BUT WE HAD CLBED 400 FT ABOVE THE ASSIGNED ALT. I IMMEDIATELY BEGAN A DSCNT BACK TO FL250. I HAD TCASII ON; AND IT SHOWED NO ACFT WITHIN 20 MI OF OUR POS. CTR NEVER QUESTIONED THE ALT. THE FLT CONTINUED WITHOUT INCIDENT. THE SIT OCCURRED BECAUSE I FAILED TO MONITOR THE LEVELOFF COMPLETELY. I HAVE NEVER HAD AN AUTOPLT MALFUNCTION IN THIS ACFT AND TRUSTED IT. ONCE I SAW THE CAPTURE LIGHTS ON THE ANNUNCIATOR; I FELT CONFIDENT WE WOULD LEVEL OFF AND DIVERTED MY ATTN OUTSIDE THE ACFT. THE FO CALLED ALT AS SOON AS THE ALERTER WENT OFF. WE AS CREW MEMBERS MUST REMEMBER THAT ANYTHING MAN- MADE CAN FAIL OR MALFUNCTION AT ANY TIME. WE MUST ALWAYS MONITOR EVERY ASPECT OF THE FLT.