Switch-Induced Simulated PIO: Relay Feedback as a Flight Test Technique for Pilot-in-the-Loop Stability

Abstract

Relay feedback has been used for decades as a simple method to estimate the ω180 frequency and gain for approximately linear feedback systems; characteristics fundamental to important measures of closed-loop system stability such as gain and phase margin. The switch-induced simulated pilot-induced oscillation (SISPIO) flight test technique (FTT) was developed to duplicate PIO by generating out-of-phase pilot inputs using inceptor switching 180 degrees out of phase with the feedback parameter of interest. Safety buildup is provided by starting with small inceptor bang-bang displacements and increasing their size when the results show that doing so is reasonable. SISPIO is a relay feedback method that—when provided by the pilot—captures the entire closed-loop system, including factors difficult to simulate such as stick dynamics, pilot biometrics, and cognitive limitations. This paper shows how SISPIO may be used to closely estimate PIO characteristics and consequences in a linear system without producing an actual PIO, examines the value of automatic SISPIO inputs, and introduces how SISPIO may be used to find unexpected non-linearities before a pilot goes “over a cliff.”