A320 FLC HAS ALT EXCURSION DURING APCH TO ATL.

Narrative

ATC CLRED US TO FLY 230 DEGS; INTERCEPT AND DSND TO 4000 FT FOR THE ILS RWY 27L; 7 MI FROM DEPOT. WE HAD ALT CAPTURE AT 4000 FT; LOC ARMED. THE APCH MODE WAS ARMED BEFORE LOC CAPTURE AND WE GOT AN UNCOMMANDED PITCH UP FROM THE AUTOPLT. WE GAINED 300 FT BEFORE THE AUTOPLT WAS DISCONNECTED AND THE PROB CORRECTED FOR A NORMAL APCH. THIS CHARACTERISTIC ANOMALY IS IN THE A320 BULLETINS AND WE ARMED THE APCH TOO EARLY. THE REST OF THE APCH WAS PERFECTLY FLOWN AND WAS ABSOLUTELY NORMAL FOR A LOW VISIBILITY HVY RAIN APCH IN CAT I CONDITIONS. HUMAN FACTORS: NO CALLS WERE MISSED; ALL CORRECTIVE ACTION WAS TAKEN IN A TIMELY MANNER; NO TFC CONFLICT EXISTED; NO CALL FROM ATC. THE PROB WAS LIMITED TO VERY FULL BLADDERS SINCE WE HAD NOT ANTICIPATED HOLDING AT ALTS BELOW STERILE COCKPIT ALT; WITH THE FLT ATTENDANTS SEATED FOR THE HOLD; APCH. BASICALLY; WITH THE CURRENT SECURITY PROCS; WE WERE IN A JEOPARDY SIT; SINCE FULL BLADDERS OR NOT; WE COULDN'T LEAVE THE COCKPIT. BOTH PLTS WERE UNCOMFORTABLE; BUT ABLE TO FLY AND RESPOND TO INPUT. THIS LITTLE AIRBUS NUANCE ESCAPED OUR ATTN BECAUSE OF THE STRONG DESIRE TO COMPLETE A SAFE; STABILIZED APCH AND GET OUR JET ON THE GND SAFELY. WE WERE ABOUT 15 MINS FROM DIVERTING.