ALTDEV ALT OVERSHOT -- CPR CREW DSNDS BELOW THEIR ASSIGNED ALT AFTER FIRST CORRECTLY LEVELING OFF AT THE PROPER ALT.

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ON A ROUTINE TRIP; ATC CLRED OUR ACFT FROM CRUISING ALT DOWN TO 5000 FT MSL. WHILE COPYING HOLDING INSTRUCTIONS; DURING A DIVERSION AROUND A TSTM; I NOTICED THAT THE PLT AT THE CTLS WAS PROPERLY EXECUTING A LEVELOFF AT 5000 FT. I BEGAN; OR CONTINUED; TO CONTACT THE RPTED TFC AT WATERTOWN; AS WELL AS MAKING FUEL ARRANGEMENTS. THE PIC WAS COMMUNICATING WITH ATC; FLYING THE ACFT; AND NAVIGATING AROUND WX. 5000 FT WAS SET IN THE ALT PRESELECT AND I OBSERVED THE ALT CAPTURE LIGHT ILLUMINATE PRIOR TO 5000 FT. ONCE I HAD COMPLETED MY DUTIES ON THE SECONDARY RADIO; I TURNED THE VOLUME BACK ON THE PRIMARY (ATC) RADIO. THE CTLR ASKED WHAT OUR ALT WAS. THE ACFT HAD DSNDED TO 3800 FT. THE PIC RESPONDED; 'CLBING BACK TO 5000 FT;' AND BEGAN INITIATING IMMEDIATE CLB. THE CTLR RE-CLRED US TO 3000 FT AT THIS TIME. THE ACFT WAS LANDED WITHOUT INCIDENT. SOME FACTORS CONTRIBUTING TO THIS VIOLATION: UNUSUALLY HIGH CREW WORKLOAD INCLUDING; WX; TURB; VFR AND IFR TFC OPERATING AT ART; COORDINATING CUSTOMS; ARRANGING FOR FUEL; COPYING HOLDING INSTRUCTIONS; ALSO; FAILURE IN CREW COORD; FAILURE TO CROSS-MONITOR OTHER CREWMEMBER'S PERFORMANCE. FAILURE TO CORRECTLY PRIORITIZE TASKS. FAILURE TO PROPERLY DISTRIBUTE WORKLOAD. OVERCONFIDENCE; SPECIFICALLY ON MY PART; IN THE ABILITIES OF SENIOR CREWMEMBERS. THIS WAS NOT A PLT ERROR; BUT A CREW ERROR. ALTHOUGH SIMPLE MISTAKES WILL ALWAYS BE PRESENT IN AVIATION; MORE EMPHASIS ON CREW COORD OR COCKPIT RESOURCE MGMNT MAY HELP ELIMINATE THE POTENTIALLY DISASTROUS RESULTS OF A SIMPLE HUMAN ERROR SUCH AS THIS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO SAID THAT THEY WERE FLYING A BEECH SUPER KING AIR 200/HURON AND THAT HE WAS VERY BUSY ON 3 DIFFERENT FREQS DURING THE DSCNT. HE DID WATCH TO MAKE SURE THAT THE CAPT DID LEVELOFF AND ONCE THAT OCCURRED HE THEN CALLED THE REFUELING FACILITY. WHILE THUS ENGAGED THE ACFT STARTED TO DSND AND HE DID NOT NOTICE THIS UNTIL HE HAD COMPLETED HIS CONVERSATION WITH THE FUELER. HE HAS NOT HEARD FROM THE FAA TO THIS DATE.