THE FLC OF A BEECH TWIN TURBOPROP DEVIATED FROM ALT AND HDG CLRNC. CAPT WAS FALLING ASLEEP.

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ON THE EVENING STATED ABOVE; THE CAPT FLYING WAS BECOMING VERY FATIGUED AND TIRED. HE BEGAN TO 'NOD OFF' WHICH RESULTED IN AN ALTDEV. THE HDG WAS ALSO AFFECTED. THE FIRST ALT AND HDG DEV WAS 300 FT AND L 20 DEGS FROM ASSIGNED. I THEN BROUGHT THIS TO THE CAPT'S ATTN WHEN PASSING THROUGH 150 FT AND 15 DEGS OF HDG. HE IMMEDIATELY CORRECTED. WITHIN THE NEXT 10-15 MI THE SAME OCCURRENCES HAPPENED. EACH TIME I BROUGHT THIS TO THE ATTN OF THE CAPT. DEVS WERE BECOMING PLUS OR MINUS 300 FT AND HDG PLUS OR MINUS 30 DEGS FROM ASSIGNED. MY CORRECTIVE ACTION WAS TRYING TO LET THE CAPT KNOW THAT I WAS VERY MUCH AWAKE AND COULD CONTINUE THE FLT. THE CAPT SAID HE WAS BETTER. I ALSO COOLED DOWN THE CABIN TEMP AND DIRECTED AIRFLOW ONTO THE CAPT. THIS 'WOKE' HIM UP. HOWEVER I BELIEVE THAT THE CAPT STILL SUFFERED FROM SLEEP DEPRIVATION BECAUSE THE ILS RWY 15 (DET) APCH WAS NOT EXECUTED WELL. THE CAPT HAD DIFFICULTY TRACKING THE LOC. ONE TIME; THE LOC WAS 8-9 DEGS L. I KEPT BRINGING IT TO HIS ATTN THROUGHOUT THE APCH. EACH TIME THE CAPT ACKNOWLEDGED AND MADE CORRECTIVE ACTIONS. PRIOR TO THE OM; THE LOC AND GS WERE FINALLY CTRED AND THE APCH WAS SAFELY COMPLETED. CORRECTIVE ACTION: I WILL NOW ONLY WARN OF ALT AND HDG DEVS TWICE. AFTER THESE WARNINGS I WILL BE FORCED TO ASK THE CAPT IF HE OR SHE WOULD LIKE ME TO FLY AND BE VERY PERSISTENT ABOUT THE FLYING IF THE CAPT IS ILL OR EXTREMELY TIRED.