B727 CREW OVERSHOT CLRED ALT IN CLB IN ZTL AIRSPACE.

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IFR FLT FROM BIRMINGHAM TO ATLANTA; GA. RADAR VECTORS TO LGC VOR CLBING TO FL190 ON HDG 120 DEGS. AUTOPLT COUPLED AND FL190 SET IN ALT ALERT SYS FOR BLOCK V AUTO LEVELOFF. AT APPROX LEVELOFF ALT; RECEIVED CTR INSTRUCTIONS 'TURN L TO HDG 090 DEGS TO INTERCEPT LGC ARR TO ATLANTA; CROSS 40 MI SW OF ATLANTA AT 12000 FT; 250 KTS; ALTIMETER 29.99.' PF DIALED 12000 FT INTO ALT ALERT SYS AND ASSUMED THAT AUTOPLT WOULD LEVEL OFF; DSND TO AND CAPTURE 12000 FT. ACFT CONTINUED TO CLB UNTIL CREW CAUGHT ERROR AT FL196. AUTOPLT WAS IMMEDIATELY DISENGAGED AND ALT CORRECTED BACK TO FL190. THE TIMING OF THE ATC CALL WAS UNFORTUNATE IN THAT THE CREW WAS ASSIGNED SEVERAL TASKS RIGHT AT LEVELOFF AND BECAME PREOCCUPIED READING BACK THE CLRNC; ATIS INFO AND SETTING COURSES; HDG BUGS; ALT LIMITS; ETC. THE FO INCORRECTLY ASSUMED THAT RESETTING THE ALT ALERT REDIRECTED THE AUTOPLT TO THE NEW ASSIGNED ALT WHILE IT MERELY DISABLED THE ALT ALERT AND CAPTURE FOR THE OLD ALT (FL190). THE CAPT (PNF) ALLOWED HIMSELF TO GET PREOCCUPIED RESETTING INSTS AND RESPONDING TO THE CLRNC. AS WORKLOAD INCREASES; ATTN AND VIGILANCE MUST ALSO INCREASE.