A B727 CARGO FLT CREW OVERSHOT THEIR ASSIGNED ALT WHEN THE ALT AUTO CAPTURE FAILS TO LEVEL THE ACFT AT 6000 FT 25 NE OF MEM; TN.

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WE WERE CLRED TO DSND TO 6000 MSL WHILE TALKING TO MEM APCH CTL WHEN COPLT FAILED TO MAKE MANDATORY CALL OF '7000 FT FOR 6000 FT.' WHILE ON DSCNT; I MADE THE CALL; AND HE ACKNOWLEDGED. THEN; AS ALL 3 OF US (CAPT; FO; SO) SAT IN SILENCE; WE MISSED LEVEL OFF AT 6000 FT; AND I CAUGHT IT AT 5600-5700 FT MSL. I DISCONNECTED THE AUTOPLT AND RETURNED TO 6000 FT MSL. WE HAD BEEN FLYING AN ALT AUTO CAPTURE AIRPLANE ALL WK; UNTIL THIS NIGHT. OBVIOUSLY; WE HAD BECOME SOMEWHAT DEPENDENT ON ITS LEVELING THE AIRPLANE. BUT; MOST IMPORTANTLY; 3 PROFESSIONAL AND COMPETENT PLTS WERE HIGHLY FATIGUED AT THE END OF A LONG WK OF BACK SIDE OF THE CLOCK FLYING; AND FAILED TO NOTICE. SUPPLEMENTAL INFO FROM ACN 580944: THE ALT ALERT AURAL/LIGHT WARNING DID NOT ACTIVATE AT 6300 FT. HOWEVER; IT DID ACTIVATE AT 5300 FT MSL; AND THE CAPT IMMEDIATELY CORRECTED THE MOMENTARY DEV. THE RECOVERY INTO MEMPHIS IS VERY BUSY AND DEMANDING. THIS BUSY ENVIRONMENT COUPLED WITH SOME FATIGUE AND THE ALERTER PROBS RESULTED IN THE MOMENTARY DEVS.