A PLT RPTS DIFFICULTY WITH AN AUTOPLT CLBING ABOVE ASSIGNED ALT NEAR GQE.

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I HAVE HAD NUMEROUS UNPREDICTABLE AND INADVERTENT PROBS IN A/S KAP-140 AUTOPLT. PROB #1: WITHOUT WARNING OR ANY DISPLAY CHANGE; AUTOPLT WILL INITIATE A SUBTLE CLB. THIS PROB APPEARS TO BE AGGRAVATED BY TURB OF ANY KIND. PROB #2: ON DSCNT AND APCHS; A PROGRAMMED DSCNT WILL BE STARTED AS DESIGNED BUT WILL BE ALTERED BY THE AUTOPLT DURING THE EXECUTION; IE; PROGRAM FOR A DSCNT AT 5000 FPM WHICH IS DISPLAYED; WILL ACTUALLY BE 200-300 FT LESS. ON NOV/XA/99; THIS AUTOPLT INITIATED ONE OF THESE UNWANTED CLBS AFTER BEING STABLE AT 7000 FT FOR 2 HRS OR MORE. THE CHANGE IN ALT WAS SUBTLE UNTIL I DETECTED THE PLANE RETRIM. I TRIED TO RETRIM THE PLANE; BUT IT TRIMMED AGAINST ME RAPIDLY AND I HAD TO DISCONNECT AUTOPLT AND RETRIM. I DEVIATED 200-300 FT FROM ASSIGNED ALT OF 7000 FT. I RESET THE AUTOPLT AND ALL WAS WELL. LATER IN SAME FLT; I PROGRAMMED FOR 500 FPM DSCNT THAT RESULTED IN ONLY ABOUT 1/2 THAT. THIS AUTOPLT IS A REPLACEMENT FOR A SIMILAR ONE WITH SAME PROBS. THIS AUTOPLT INCREASES THE WORKLOAD AND IN MY OPINION IS OF QUESTIONABLE RELIABILITY AND QUALITY. AUTOPLT NEEDS TO BE UPGRADED OR PLACARDED AS DEFICIENT FOR PRECISE CTL.