Applications of Linear and Nonlinear Robustness Analysis Techniques to the F/A-18 Flight Control Laws

The F/A-18 Hornet aircraft with the original flight control law exhibited an out-ofcontrol phenomenon known as the falling leaf mode. Several aircraft were lost due to the falling leaf mode and which led NAVAIR and Boeing to redesign the flight control law. The revised flight control law successfully suppressed the falling leaf mode during flight tests with aggressive maneuvers. This paper compares the robustness of the original (baseline) and revised control laws using linear analyses, Monte Carlo simulations, and nonlinear region-of-attraction analyses. The linear analyses indicate the revised controller only marginally improves the closed-loop robustness while the nonlinear analyses indicate a substantial improvement in robustness over the baseline controller. This example demonstrates the potential for nonlinear analyses to detect out-of-control phenomenon that are not evident from linear analyses.