CE550 MAINTAINING FL250 CHKING CHARTS FOR DIRECT ROUTING; ACFT BEGAN A CLB RESULTING IN DEV FROM ALT OF 450 FT. CTR QUERIED AND RETURNED ACFT TO FL250.

Narrative

AFTER LEVELING OFF AT FL250; AUTOPLT WAS ENGAGED. ALL INDICATIONS WERE NORMAL. THE PLT FLYING THE ACFT ASKED ME TO SEE IF CTR WOULD CLR US DIRECT TO VOR NEAR OUR DEST (SPA VOR). I STATED I DON'T BELIEVE THE RTE WOULD BE THAT MUCH SHORTER AND PROCEEDED TO GET THE ENRTE CHART OUT TO CHK. WE WERE BOTH LOOKING AT THE CHART. AFTER A SHORT TIME (ABOUT A MIN); NOTICED ALT WAS HIGH. I LOOKED DOWN AND NOTICED AUTOPLT ALT HOLD HAD BECOME DISENGAGED. THE PLT THEN TOOK OVER MANUALLY AND RETURNED ACFT TO PROPER ALT. NEITHER OF US COULD FIGURE OUT WHAT CAUSED THE AUTOPLT TO DISENGAGE. THE CONTRIBUTING FACTOR WAS THAT; FOR A SHORT PERIOD OF TIME; NEITHER OF US WERE MONITORING OUR FLT INSTS. AS A RESULT; WE GOT APPROX 450 FT OFF OUR ALT BEFORE WE REALIZED THERE WAS A PROB. WE HAVE HAD AUTOPLT PROBS BEFORE SO SHOULD HAVE BEEN MORE ALERT THAT THIS COULD POSSIBLY HAPPEN. SUPPLEMENTAL INFO FROM ACN 349137: AT FL250 AND 360 KTS; AUTOPLT WAS KNOCKED OFF ACCIDENTALLY. PLTS WERE LOOKING AT MAPS AND DID NOT CATCH IT UNTIL ATC QUESTIONED ALT AT FL254. AUTOPLT PITCH TRIM WHEEL IS EXTREMELY SENSITIVE. MAINT HAS BEEN WORKING ON IT.