ALTDEV ALT UNDERSHOT EXCURSION.

Narrative

THIS WAS OUR FIRST NIGHT OF A 5-DAY TRIP FLYING NIGHTS CLE-IND-CLE. I WAS VERY TIRED DUE TO TIME OF EVENT AND LACK OF SLEEP FOR 21 HRS. ALSO I WAS A LITTLE SLOW DUE TO RECENT 3 WK BREAK FROM FLYING. THIS WAS MY FIRST LEG AS PF. ENRTE; 100 MI SW OF CLE; LEVEL AT 25000 FT; WE WERE CLRED PLT'S DISCRETION TO FL240. WE DIALED 240 INTO OUR ALT REMINDER/WARNING. WE WERE DISTRACTED WITH AREA TSTMS; LIGHT TURB; ICING CONDITIONS AND FATIGUE. I WAS PLANNING ON FLYING 30 MORE MI OR SO; PRIOR TO DSNDING TO FL240. CAPT NOTICED WE SOMEHOW WERE NOW AT 25800 FT; NOT 25000 FT. NO WORD FROM ATC. I GENTLY DSNDED TO FL240. NO KNOWN TFC IN OUR AREA. WE DIDN'T CALL ATC OTHER THAN TO RPT LEAVING 25000 FT FOR 24000 FT. HOW WE GOT TO 25800 FT REMAINS A MYSTERY. SUPPLEMENTAL INFO FROM ACN 298987: ACFT WAS ON AUTOPLT WITH FO FLYING. THE CAPT WAS REVIEWING THE APCH TO CLE. THE FO WAS OPERATING THE RADAR FOR TSTM AVOIDANCE. THE SO WAS PREPARING THE CABIN FOR DSCNT AND MONITORING THE ICING SYS. AN IMMEDIATE DSCNT USING FLT SPOILERS WAS INITIATED TO FL240. WE DETERMINED AFTER THE FACT; ON A SUBSEQUENT FLT; THAT THE CAUSE OF THE DEV WAS THAT 24000 FT WAS SET INTO THE ALT ALERTER PRIOR TO THE AUTOPLT'S COMPLETION OF LEVELOFF. IN THAT CASE; IT CONTINUES TO CLB WITH NO ALERT WARNING. CONTRIBUTING FACTORS: HIGH WORKLOAD PERIOD; ICING AND TSTMS; FATIGUE -- FIRST ALL NIGHT FLT OF THE WK. SUPPLEMENTAL INFO FROM ACN 298566: ON EVALUATION; IT WAS REALIZED THAT THE AUTOPLT FAILED TO HOLD AT FL250 AFTER ALT SELECT (FL240) WAS ENGAGED. ON A SUBSEQUENT FLT; THE SAME FAILURE WAS NOTED BUT ALTDEV WAS MINIMAL. THIS CONFIRMED A SYS FAILURE AND WAS IMMEDIATELY NOTED IN ACFT MAINT LOG AND DISCUSSED WITH MAINT PERSONNEL.