A B747-200 FLYING IN ZID AIRSPACE OVERSHOT HIS ASSIGNED ALT BY 300 FT.

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ATC CLRED US TO FL350; AUTOPLT WAS ENGAGED; IAS HOLD AND ALT CAPTURE SELECTED. AUTOTHROTTLE AND FULL FLT REGIME AUTOTHROTTLES WERE ENGAGED IN THE CLB MODE. AS WE APCHED FL350 THE AUTOPLT WENT INTO THE ALT CAPTURE MODE. AT THAT TIME; I SWITCHED THE AUTOPLT SPD MODE FROM IAS HOLD TO MACH HOLD AND SWITCHED THE AUTOTHROTTLE FULL FLT REGIME AUTOTHROTTLES TO THE MACH HOLD MODE ALSO. I BELIEVE THAT WHEN I SWITCHED THE AUTOPLT SPD MODE FROM IAS HOLD TO MACH HOLD DURING THE ALT CAPTURE MODE OF THE AUTOPLT; I MAY HAVE INADVERTENTLY DISARMED THE ALT CAPTURE MODE. (I DID NOT NOTICE IF THE ALT CAPTURE MODE WAS STILL ENGAGED AT THAT TIME OR NOT; I PRESUMED IT WAS.) THE ACFT CLBED THROUGH FL350 TO FL353 WHILE I WAS LOOKING DOWN AND BACK TO THE FULL FLT REGIME AUTOTHROTTLES ON THE CTR CONSOLE (IN ORDER TO SET MACH HOLD). THE FO ALERTED ME TO THE FACT THAT WE WERE STILL CLBING. AT THAT POINT; I DISENGAGED THE AUTOPLT AND AUTOTHROTTLE AND DSNDED THE ACFT FROM FL353 TO FL350. LESSONS LEARNED: 1) MONITOR THE AUTOPLT CLOSELY WHILE IN THE ALT CAPTURE MODE. 2) SWITCH FROM AUTOPLT IAS HOLD TO MACH HOLD PRIOR TO ALT CAPTURE. 3) RUN THE SAME SCENARIO ON MY NEXT FLT TO SEE IF THE ALT CAPTURE MODE DISENGAGES WITH A SWITCH FROM IAS TO MACH HOLD. (I WILL MONITOR VERY CLOSELY.) 4) NOWHERE IN OUR ACFT OPERATING MANUAL OR SYS MANUAL DOES IT ADDRESS THIS SCENARIO. I WILL DISCUSS IT WITH PEOPLE IN OUR TRAINING DEPT AND PUT THE WORD OUT TO OUR PLTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE ACFT BEING FLOWN WAS A B747-200. HE NORMALLY FLIES B747-100. HE SUSPECTS THAT THE CHANGE IN PITCH MODE FROM IAS TO MACH WAS THE CHANGE THAT DISENGAGED THE ALT SELECT MODE. HE IS UNCERTAIN THAT IS THE CAUSE; AS HE WAS NOT LOOKING AT THE ANNUNCIATOR LIGHTS TO KNOW IF THE ALT SELECT WAS ARMED OR EVEN IF ANOTHER PITCH MODE WAS ARMED. RPTR WANTS TO TRY CHANGING THE MODE AGAIN TO SEE IF THE EVENT WOULD REPEAT; BUT HAS NOT HAD A CHANCE TO FLY ANOTHER B747-200. THERE ARE VERY FEW OF THIS TYPE ACFT IN THEIR FLEET. RPTR SUSPECTS THE AUTOPLT DISENGAGEMENT IS DESIGNED TO RESPOND THAT WAY ON THE B747-200. HE FELT THERE IS NO HAZARD; JUST A REASON TO BE AWARE OF THE DIFFERENCES BTWN DIFFERENT MODELS OF ACFT.