CL65 CREW HAD AN ALT OVERSHOOT IN DAY CLASS E.

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WE WERE GIVEN A CLRNC TO CLB AND MAINTAIN 10000 FT; WHICH WAS ALSO OUR CRUISING ALT ON THIS SHORT FLT FROM DAY-CVG. I WAS THE PNF. THE FO WAS FLYING. UPON NEARING 10000 FT THE FO THOUGHT HE HAD ENGAGED THE AUTOPLT. SUBSEQUENTLY THE PLANE CLBED TO 10400 FT. DURING THIS TIME I WAS CALLING OUR OPS WITH OUR OUT AND OFF TIME. I NOTICED THE DEV AT APPROX 10300 FT (WHEN THE ALT ALERT HORN GOES OFF -- AGAIN) AND IMMEDIATELY TOLD HIM TO PUSH THE NOSE DOWN. AFTER THE ALTDEV INCIDENT; THE FO TOLD ME THE AUTOPLT WAS ENGAGED. I TOLD HIM I DIDN'T THINK IT WAS SINCE THERE WASN'T ANY INDICATION ON MY PFD IT WAS ENGAGED. HE SAID HE CLICKED OFF THE AUTOPLT AT ABOUT 10400 FT AND PUSHED THE NOSE BACK DOWN. I DON'T RECALL HEARING THE AUTOPLT DISCONNECT WARNING DURING THIS PHASE. I BELIEVE I SHOULD HAVE BEEN MORE VIGILANT WITH THE NEW FO AND NOT AS HASTY TO CALL OUR 'OUT AND OFF' TIMES; BUT TO MONITOR THE FLT FIRST. THE FO SHOULD HAVE BEEN MORE AWARE OF THE 'FLT CTL PANEL' AND PFD ANNUNCIATIONS WHEN ENGAGING THE AUTOPLT TO ENSURE IT HAD INDEED ENGAGED. I DON'T RECALL WHETHER IT WAS ZID OR DAYTON DEP WE WERE IN CONTACT WITH AT THIS TIME. HOWEVER; AT THE HT OF OUR DEV; THE CTLING AGENCY TOLD US TO 'MAINTAIN 10000 FT.' I REPLIED 'ROGER; 10000 FT' AND OUR CALL SIGN. NO TFC CONFLICT WAS NOTED ON OUR TCASII.