IMPAIRMENT OF THE PILOT-IN-COMMAND (PIC) DUE TO DRUGS/MEDICATION, CHEMOTHERAPY AND FATIGUE; FAILURE OF THE PIC TO ASSURE THAT THE IFR (INSTRUMENT) APCH PROCEDURE WAS FOLLOWED; AND HIS FAILURE TO MAINTAIN THE MINIMUM DESCENT ALTITUDE (MDA). INADEQUATE SURVEILLANCE OF THE OPERATION BY COMPANY/OPERATOR/MANAGEMENT PERSONNEL WAS A RELATED FACTOR.