Analysis of pilot-in-the-loop oscillations due to position and rate saturations

In this paper we deal with the analysis of category II (nonlinear) pilot-in-the-loop oscillations (PIO). PIO phenomena are originated by a misadaptation between the pilot and the aircraft that causes sustained or uncontrollable oscillations, which especially occur during some tasks where tight closed loop control of the aircraft is required from the pilot. Category II PIO are those oscillations that can strictly be correlated with the activation of rate and position limiter elements in the closed loop pilot-vehicle system. This kind of nonlinearity is unavoidably present in every aircraft, because of physical constraints of elements such as stick/column deflections, actuator position and rate limiters, limiters in the controller software and so on. In this paper we propose an approach, based on the describing function technique, to evaluate the nonlinear effects of the simultaneous presence of position and rate saturations in the control loop. The X-15 landing flare PIO is used as test case to demonstrate the effectiveness of the method.