The DC-9-80 aircraft is equipped with an integrated digital flight guidance system that provides autopilot, flight director, thrust management, speed control, autothrottle, and stability augmentation within a single computer complex. The system architecture is based on two essentially autonomous computer complexes, each able to provide all of these functions, with complete fail-passive monitoring for all flight-critical functions, including category III A autoland. This paper describes how the multiplicity of hardware and software monitors are implemented, and how their effectiveness has been analyzed and verified. The system monitors are designed so that fault detection and appropriate shutdown of the failed elements can meet the quantitative safety criteria required by the civil aviation certificating agencies.