This advisory circular (AC) describes an acceptable means for showing compliance with certain requirements of Title 14 of the Code of Federal Regulations (14 CFR) part 25, § 25.1329, Flight guidance systems. While part 25 contains the airworthiness standards applicable to transport category airplanes, the guidance in this AC pertains to the functions of autopilots, flight directors, and automatic thrust control as well as any interactions with stability augmentation and trim functions.