AN EXPERIENCED CAPT; NEW TO THE HS125; RPTED THAT HIS FO WAS UNABLE TO MAINTAIN THE ASSIGNED ALT WHEN UTILIZING THE AUTOPLT.

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WHILE ON A DEADHEAD FLT FROM SALEM TO PORTLAND; OR; THE FO WAS FLYING. WE WERE TOLD TO CONTACT CTR. WE WERE CLRED IN OUR CLRNC TO 9000 FT. CTR CALLED TFC FOR US. WE TOLD THEM WE WERE IMC. THEY TOLD US TO MAINTAIN 8000 FT. I THEN WENT TO COM #2 TO GET ATIS AT PORTLAND. WHILE I WAS DOING THAT; THE FO HAD THE ACFT ON AUTOPLT AND THEN PUSHED THE XFER BUTTON TO SHIFT THE AUTOPLT TO HIS SIDE. THE ALT SELECT BUTTON WAS PUSHED AND THE FO SAID HE THOUGHT THE AUTOPLT WOULD CAPTURE THE ALT; OF 8000 FT. HE SAID IT WENT TO 8300 FT; THEN HE PUSHED IT DOWN AND BACK TO 8000 FT MSL. HE SAID HE WAS SURPRISED HOW FAST IT WAS CLBING AND IT SURPRISED HIM WHEN THE ALT SELECT DIDN'T STOP THE CLB AT 8000 FT. MY ATTN WAS DIVERTED WHILE I WAS GETTING ATIS. MY FIRST CLUE THAT WE HAD A PROB WAS WHEN CTR GAVE ME A PHONE NUMBER TO CALL. I THINK ANOTHER THING THAT COULD HAVE BEEN A PROB IS WHEN THE FO HIT THE 'XFER' BUTTON HE ALSO PUSHED THE 'TURB' BUTTON; WHICH WOULD MAKE THE ALT CAPTURE MODE LESS SENSITIVE; I AM TOLD. I'M NOT SURE IF THE 'TURB' BUTTON WAS PUSHED.