THE ACFT COLLIDED WITH TREES APRX 6 MILES EAST OF MARION CTY ARPT AND APRX 17 NM SW OF THE FLT DEPAT PT AND ALSO THE PLT'S HOME ARPT. THE PLT HAD EARLIER THAT DAY TOLD THE HALEYVILLE ARPT MGR THAT HE WOULD RETURN TO HALEYVILLE THE SAME DAY AS DEPT. THE ACC SITE IS APRX 17 NM. SW OF THE HALEYVILLE ARPT. THE OUTBOUND FLT HAD DEPT AT APRX 1720 BOUND FOR JACKSON MS. DEPT FROM JACKSON TOOK PLACE AT 1904 CST WITH A DEST OF BIRMINGHAM, AL NAMED ON THE ATC DEPT STRIP. WITNESSES SAW THE ACFT MANEUVERING AT LOW ALT IN THE VICINITY OF THE HAMILTON ARPT JUST PRIOR TO THE CRASH. THE WX WAS DESCRIBED BY THESE SAME WITNESSES AS FOGGY WITH DRIZZLE AND A VISIBILITY OF 1/2 TO 3/4 MILE. THERE IS NO WX REPORTING FACILITY AT HIMILTON. WX SHOWN OF THIS REPORT IS FOR MUSCLE SHOALS, 40 NMI AWAY AND ALSO BIRMINGHAM AL AND TUPELO MS. POST ACC INVEST REVELAED EVIDENCE THAT THE AUTO PLT HEADING MODE WAS BEING USED AT TIME OF ACC. THE VACUUM PUMP HAD SHEARED PRIOR TO ENG STOPPAGE. THE PUMP HAD LOGGED 730 OPERATIONAL HRS SINCE INSTALLATION IN 1978.