A SINGLE PLT EXCEEDS ASSIGNED ALT BY 500 FT.

Narrative

I DEPARTED HIO ON A DEP PROC THAT HAD AN INITIAL ALT ASSIGNED OF 5000 FT. AFTER BEING HANDED OVER TO DEP; I ENTERED IMC (RAIN; ICE AND LIGHT TURB) AT ABOUT 3400 FT. I HAD ENGAGED THE ALT PRE-SELECT FUNCTION ON MY FLT DIRECTOR WHICH SHOULD COMMAND A LEVELOFF AT THE 5000 FT ALT WHICH I SET INTO THE ALT ALERTER. I HEARD THE ALT ALERTED 'DING' PASSING THROUGH 4000 FT (1000 FT TO GO); CLBING AT ABOUT 1800 FPM. AT THIS POINT I NOTICED ICE STARTING TO BUILD. I LOOKED UP TO THE OVERHEAD PANEL TO TURN ON THE ANTI-ICE SYS (IGNITION OVERRIDES ON; INLET HEATS ON -- ONE AT A TIME; PROP HEAT ON; WINDSHIELD HEATS ON) AND JUST AS I FINISHED SETTING ALL THIS UP ATC CALLED AND ASKED ME TO VERIFY LEVEL AT 5000 FT. I LOOKED AT THE ALTIMETER AND SAW I WAS CLBING THROUGH ABOUT 5500 FT; STILL AT 1500 FPM! I DISABLED THE AUTOPLT AND RETURNED TO ALT BEFORE RE-ENGAGING ALT PRESELECT AND AUTOPLT BUT THAT TOOK LONGER THAN I EXPECTED AS I WAS IN SOLID IMC WITH ICE BUILDING; LIGHT TURB AND THE NEGATIVE G'S ASSOCIATED WITH A PITCH CHANGE FROM +1500 FPM TO -1000 FPM WERE VERY DISCONCERTING AND I WAS ON THE EDGE OF VERTIGO. FINALLY; AFTER RETURNING TO 5000 FT; ATC CALLED BACK AND ASSIGNED ME FURTHER CLB. SOMEHOW THE ALT PRESELECT MODE HAD FAILED OR NOT BEEN ENGAGED. AS A FURTHER COMPLICATION; THE PANEL LIGHTS WERE IN 'DIM' AND THAT MADE IT VERY HARD TO VERIFY THAT THE MODE WAS ENGAGED. FURTHER; IN THIS AIRPLANE; ONE BUTTON 'TOGGLES' THE PS MODE ON OR OFF; SO IT'S NOT A SIMPLE MATTER TO ENGAGE IT WITHOUT LOOKING AT THE INDICATOR LIGHT; AS A PUSH MIGHT DISENGAGE IT RATHER THAN ENGAGE IT. I THINK THE LESSON IS TO REMAIN FOCUSED ON LEVELOFF AND FLYING THE AIRPLANE DURING THAT CRITICAL '1000 FT TO GO;' EVEN IF IT MEANT HOLDING OFF ON THE ANTI-ICE FOR ANOTHER 60 SECONDS.