A Unifying Point of View in the Problem of PIO - Pilot In-the-loop Oscillations

The paper starts from the problem of PIO (Pilot-In-the-loop Oscillations), a major problem in aircraft handling and control, where the idea of the feedback as hidden technology is basic. The real phenomenon called PIO is modeled by a feedback structure where the pilot acts as one of the components of the loop and has to be modeled accordingly. PIO are in fact self-sustained oscillations and usually are divided into three convenient categories that are based on the nature of the pilot and vehicle dynamics behavior models and analysis needed for their explanation. Category I PIO are essentially linear while Category II PIO are quasi-linear and typically associated with rate limiting. Category III PIO are fully nonlinear and non-stationary. Since PIO II are mostly tackled via various robustness approaches starting from linear models, the paper strives for a unifying approach which is illustrated accordingly. 1 BASICS AND PROBLEM STATEMENT According to the standard terminology of the field, PIO (Pilot Induced Oscillations, Pilot In-the-loop Oscillations, Pilot Involved Oscillations) are sustained or uncontrollable oscillations resulting from the effort of the pilot to control the aircraft, hence they can be considered as a closed loop destabilization of the aircraftpilot loop (Anon., 2000),(McRuer et al., 1996). Even from this remarkably short definition it appears that the real phenomenon called PIO can be modeled by a feedback structure where the pilot acts as one of the components of the loop and has to be modeled accordingly. As mentioned in (McRuer et al., 1996) the study of the aeronautical history reveals a remarkably diverse set of severe PIOs as exemplified by the listings of “famous PIOs” (McRuer, 1994),(Klyde et al., 1995). The feedback control character of PIOs was recognized almost from the outset because the aircraft left alone did not exhibit such oscillations. Once recognized as oscillations within a feedback system context, mathematical models were developed and used to describe the pilot’s dynamic actions as a controller and active participant in PIOs. A. Detailed analytical studies of past PIO incidents (see e.g. the references from (McRuer et al., 1996)) relied on pilot behavioral models and closed loop analysis procedures to understand and rationalize phenomena. Moreover in some cases pilot vehicle behavioral models were applied to design and assess changes to the effective vehicle to alleviate the PIO potential. Based on these results it is useful to divide PIOs into categories that reflect the analytical and pilot modeling tools. There were identified three categories of PIO as follows: Category I Essentially Linear Pilot Vehicle System Oscillations: the element characteristics are essentially linear and the pilot behavior is linear (except, possibly, for simple gain shaping in series with the pilot). Category II Quasi-Linear Pilot Vehicle Systems with Series Rate or Position Limiting. Rate limiting, either as a series element or as a rate limited surface actuator modifies the Category I situation by adding what is called (non-rigorously) an amplitude dependent lag and by setting the limit cycle magnitude. Category III Essentially Non-Linear Pilot Vehicle System Oscillations with transitions: they fundamentally depend on nonlinear transitions in either the effective control element or in the pilot behavioral dynamics. B. Most of the available information shows that mainly PIO I and PIO II were considered and analyzed due to the complexity of PIO III which never-