ALTDEV ALT EXCURSION DURING APCH ON AUTOPLT.

Narrative

WHILE ON A HDG TO INTERCEPT THE ILS 29R AT MSP WE CALLED THE ARPT IN SIGHT AND WE WERE CLRED FOR THE VISUAL APCH. AT THIS TIME WE WERE LEVEL AT AN ASSIGNED ALT OF 4000 FT. PRIOR TO LOC INTERCEPT; THE GS CAME INTO VIEW AND I ARMED THE APCH FUNCTION OF THE AUTOPLT. (THE LATEST VERSION OF SOFTWARE ALLOWS GS CAPTURE PRIOR TO LOC INTERCEPT.) THE GS CAPTURED AND WE BEGAN A DSCNT FOR A COUPLE HUNDRED FT. FOR AN UNKNOWN REASON (POSSIBLY A FALSE GS) WE BEGAN TO CLB; WHILE STILL IN GS CAPTURE MODE. THIS TRANSITION TO A CLB WAS NOT NOTICED IMMEDIATELY DUE TO CONFIGN CHANGES; TURBULENT CONDITIONS AND A PWR INCREASE THAT WAS NOT OBVIOUS; SINCE THE THRUST LEVERS DO NOT MOVE. I BECAME AWARE OF THE CLB AT ABOUT 4100 FT AND ATTEMPTED TO LEVEL OFF BY PULLING THE VERT SPD KNOB AND ROTATING IT TO A DSCNT. THE ACFT CONTINUED TO CLB ON THE AUTOPLT; WHICH I THEN DISCONNECTED. BY THIS TIME WE HAD BALLOONED THROUGH 4350 FT. I WAS SURPRISED TO FIND THAT THE VERT SPD SELECTOR WINDOW SHOWING A CLB. I DON'T KNOW WHY THIS WAS THE CASE -- POSSIBLY BECAUSE OF PULLING AND TURNING THE KNOB AT THE SAME TIME; AND THE SYS ACCEPTED THE ACFT CURRENT VERT SPD RATHER THAN THE REQUESTED VERT SPD. THERE IS NO ALT HOLD BUTTON. NO CONFLICT OCCURRED. THERE IS A LOT TO DISLIKE ABOUT THE DESIGN AND IMPLEMENTATION OF THIS AUTOMATED COCKPIT.