DSCNT BELOW ASSIGNED ALT DURING APCH TO TYS. AN UNDETECTED AUTOPLT DISCONNECT; AN ALT ASSIGNMENT CHANGE AND CREW FATIGUE ARE CITED AS FACTORS.

Narrative

DURING DSCNT FOR APCH TO KNOXVILLE; TN; ACFT WAS ALLOWED TO DSND BELOW ASSIGNED ALT OF 4000 FT. ON VECTORS FOR ILS RWY 23R TYS; WERE INITIALLY CLRED TO 5000 FT. AUTOPLT WAS ENGAGED AND ARMED TO LEVEL AT 5000 FT MSL. APCH CTL GAVE CLRNC TO 4000 FT. 4000 FT WAS SET IN ALT ALERTER WINDOW. ALT CAPTURE MADE OF AUTOPLT DID NOT TRANSITION TO NEW ALT AND DISENGAGED. CREW WAS BUSY GETTING SET UP FOR APCH; RADIOS; ILS COURSE; CONFIGURING AND DID NOT CATCH FACT THAT ALT CAPTURE HAD DISENGAGED. ACFT DSNDED BELOW 4000 FT AND APCH QUERIED US ABOUT WHAT OUR ALT WAS. WE REPLIED THAT WE WERE CORRECTING BACK TO 4000 FT. APCH THEN CLRED US TO CONTINUE DSCNT TO 2500 FT; R TURN 210 DEGS TO INTERCEPT THE RWY 23R LOC; CLRED FOR APCH. CREW DUTY STARTED AT XA18L; MEMPHIS; TN. INCIDENT OCCURRED AT XH15L (XH15Z) TYS TIME. CREW HAD BEEN ON DUTY 6 HRS; EXPERIENCED A DELAY OUT OF MEM; ANOTHER DELAY AT GRR BEFORE DEPARTING FOR TYS. CREW HAD SPENT A LOT OF EFFORT AT MEM AND GRR CHKING AND CORRECTING PAPERWORK DEFICIENCIES IN FLT DOCUMENTS GIVEN TO CREW. CREW HAD ALSO COPED WITH ADVERSE WX IN GRR AND HAD GONE AROUND WHEN APCH VECTORS DID NOT ACCOUNT FOR A VERY STRONG XWIND AND ACFT WAS POSITIONED SUCH THAT IT WOULD/COULD NOT INTERCEPT ILS LOC. CREW WAS FATIGUED AND STRUGGLING TO STAY ALERT WHILE FLYING. IT WAS THE 4TH DAY CAPT WAS FLYING THIS TRIP; 3RD DAY FOR SO; AND 2ND DAY FOR FO. TRIP WAS A 3 LEG XA00 AM OUT/BACK FROM MEM.