PLT NEWLY CHKED OUT IN EQUIP OPERATES AUTOPLT INCORRECTLY. ALT DEV.

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THIS WAS ONLY MY SECOND TRIP IN AN SMT. THIS WAS A DIFFERENT MODEL THAN THE ONE FLOWN 2 YRS PREVIOUS. AS AN AIRLINE PLT I THOUGHT I HAD IT ALL COVERED. MY CHKOUT WAS 1.1 HRS THE DAY BEFORE. VFR T AND G AND SINGLE ENG. VERY QUICK BRIEFING ON XX MODEL AUTOPLT. APCHING YQT I WAS UNDER HIGH TASK LOADING; GETTING ATIS; BALANCING FUEL; SWITCHING FREQS; NAVIGATING; INTERRUPTIONS BY NON-PLT PAX. THE AUTOPLT (WHILE LISTENING TO ATIS) DID AN UNEXPECTED 30 DEG TURN; SO I TURNED IT OFF; CHKED THE NAV AND TURNED IT BACK ON. SHORTLY THEREAFTER I NOTICED THAT I WAS UP 300 FT. I TURNED OFF THE AUTOPLT AGAIN AND ANALYZED THAT I HAD TOUCHED ALT HOLD BUT NOT AUTOPLT ON; OOPS; SO I TURNED ON THE AUTOPLT. I WENT BACK TO CHKING ATIS; APCH CALLED FOR ME TO 'IDENT' A SECOND TIME; OH NO! I WAS 900 FT HIGH. I 'FESSED UP' THAT I HAD AUTOPLT PROBLEMS. I GOT A DSCNT CLRNC AND DSNDED MANUALLY. ON FURTHER ANALYSIS; I FOUND THAT I HAD INCORRECTLY ACTIVATED THE AUTOPLT MODES. WHEN THE XX MODEL IS TURNED ON (ALL PUSH BUTTONS) IT IS AUTOMATICALLY IN HDG (HEADING) AND ATT (ATTITUDE). I CORRECTLY SELECTED 'NAV' INSTEAD OF HDG BUT DID NOT SELECT 'ALT' ALT HOLD. THUS I KEPT CLBING IN 'ATT'; DESPITE 2 MODE ANNUNCIATOR PANELS. HAVING 2 MODE SWITCHES NEXT TO EACH OTHER SO CLOSE IN NAME 'ATT'; 'ALT' IS A POOR DESIGN; BUT; MY OVERCONFIDENCE EXACERBATED THE PROBLEM!