JUST PRIOR TO REACHING CRUISE ALT; THE CREW OF AN SA226 BECOME DISTRACTED WITH RETRIEVING APCH CHARTS FOR THEIR NEARBY DEST AND CLB ABOVE THE ASSIGNED ALT.

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WE WERE DEPARTING DSM; THE COPLT WAS FLYING. WERE CLRED TO 10000 FT. WE JOINED THE DSM 254 DEG RADIAL OUTBOUND CONTINUING TO CLB. AT 9000 FT MADE 1000 FT TO GO CALL. I THEN REACHED DOWN TO GRAB THE CHART BETWEEN SEATS (THE APCH PLATE) AND STARTED SETTING THE PAGE TO OFF. DEP CALLED; TOLD US TO MAINTAIN 10000 FT. READ BACK MAINTAIN 10000 FT. WE WERE AT 10600 FT. WE IMMEDIATELY DSNDED TO 10000 FT. DEP ASKED OUR ALT AND I RESPONDED 10000 FT. I BELIEVE WHAT CAUSED THE PROB IS THAT I WASN'T WATCHING THE FO CLOSE ENOUGH. HE BECAME DISTRACTED AND MISSED HIS ALT. IN ORDER TO PREVENT THIS FROM HAPPENING IN THE FUTURE; I WILL WATCH THE FLYING PLT CLOSER; AND NOT GET OUT APCH PLATES UNTIL WE ARE LEVEL IN CRUISE AND STABLE AT OUR ALT. SUPPLEMENTAL INFO FROM ACN 654512: WE WERE DEPARTING DSM ON THE WAY TO OFF. I; THE FO; WAS FLYING PLT AND WE WERE CLBING ON COURSE TO 10000 FT. AT 1000 FT PRIOR; THE 1000 FT CALL WAS MADE. DURING THAT TIME; WE REFERENCED THE ENRTE CHART AND WE RECEIVED A CALL FROM ATC TO MAINTAIN 10000 FT. WE THEN LOOKED AT OUR ALT AND SAW WE WERE AT 10600 FT. WE IMMEDIATELY INITIATED A DSCNT BACK TO 10000 FT. AFTER REACHING 10000 FT; DEP SAID; 'SAY ALT;' AND WE SAID LEVEL 10000 FT. THEY THEN GAVE US A NUMBER TO CALL FOR A POSSIBLE PLT DEV. THE THING THAT REALLY CAUSED THE PROB WAS THAT I; THE FLYING PLT; LET MYSELF BE DISTRACTED BY DUTIES THAT SHOULD BE PERFORMED BY THE NON-FLYING PLT. IN THE FUTURE; I WILL BE MORE FOCUSED ON MY DUTIES AND NOT LET MYSELF BECOME DISTRACTED.