ALT BUST BY ACR FLC IN ATR-72.

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CLRED TO 10000 FT; WHICH WAS TO BE OUR FINAL; WE DEPARTED TVC FOR MQT. WE NOTICED ON CLBOUT WE COULD NOT ENGAGE THE IAS MODE ON THE VERT MODE OF THE FLT DIRECTOR/AUTOPLT. WE ELECTED TO USE THE 'BASIC' MODE WHICH CTLS STRICTLY A PITCH ATTITUDE WITH THE HORIZON. THIS IS COMPANY APPROVED AND DOESN'T NORMALLY EFFECT THE ALT WARNING/CAPTURE CAPABILITY OF THE AUTOPLT. WHILE INVESTIGATING THE PROB WITH THE IAS MODE (LOOKING FOR POPPED CB'S ETC) WE FLEW THROUGH OUR ASSIGNED ALT OF 10000 FT. UPON DISENGAGING THE AUTOPLT THE ALT SELECT INDICATOR AND EADU WOULD NOT INDICATE SELECTED ALT OR ANY MODE OF THE FLT DIRECTOR. (LATER ON THE GND IN MQT THE EADU INDICATED 'DADC FAIL' AND 'CPL FAIL'). PASSING 12000 FT; AND TRYING TO LEVEL OFF; WE REQUESTED 14000 FT FROM ZMP; WHICH WE WERE CLEARED TO (WITH NO COMMENT FROM CTR). WE CONTINUED TO MQT WITH NO FURTHER INCIDENT. CONTRIBUTING FACTORS: PARTLY DUE TO FATIGUE; WE PUT WAY TOO MUCH TRUST IN THE EQUIP TO WARN US OF ALT AND CAPTURE OF ASSIGNED ALT; THEREFORE; BOTH THE EQUIP AND THE HUMAN BACKUP ELEMENTS FAILED. WE WERE TIRED. WE HAD NORMAL REST PREVIOUS TO DUTY; HOWEVER WE WERE ON THE SECOND OF THE 3 CONSECUTIVE 'CONTINUOUS DUTY' OVERNIGHTS; WHICH AVERAGED ABOUT 5 (?) HRS SLEEP BTWN OUR ARR AND MORNING DEP. TRYING TO SLEEP DURING THE DAY DOES NOT MAKE UP FOR THE DISRUPTION OF CIRCADIAN RHYTHMS. SUPPLEMENTAL INFO FROM ACN 270553: THE ADU WAS SET PROPERLY AND ALL GAUGES AND INDICATIONS WERE NORMAL. THE SKY WAS VERY CLEAR AND NORTHERN LIGHTS WERE OUT. WE FAILED TO NOTICE THE FAILURE OF THE ALT CAPTURE PORTION ON THE ADU. THERE WERE NO ALT ALERTS. THIS DEV WAS CLEARLY THE RESULT OF COMPLACENCY AND DIVERSION OF OUR SCAN TO THE OUTSIDE ONLY.