The prediction of PIO susceptibility of a large transport aircraft

Flight control system design for large, fly-by-wire aircraft is aimed at providing optimal handling qualities for all mission tasks that the aircraft has to fulfill. Pilot-Induced Oscillations (PIO) are handling qualities deficiencies that are highly undesirable, and have a tendency to show up unexpectedly during developmental flight-testing. Numerous design criteria have been developed that provide guidelines in the design process that should help avoid PIO tendencies of an aircraft. Recently, several category II PIO criteria that specifically deal with nonlinearities such as rate and position limiting in the flight control system have been proposed. In this paper, these methods are appraised against an example aircraft that has experienced well-documented category II PIOs. Furthermore, two methods are examined that can help reduce PIO susceptibility. The application of a PIO suppression filter, and of phase compensation in software rate limiting elements are evaluated. The effectiveness of both methods is examined. The ideas behind some of the category II PIO criteria prove to be useful in this application.