Issues in flight control design for robustness to failures and damage

Significant efforts have been made in designing fighter flight control systems that are robust to aircraft impairments such as actuator failures and surface damage. The approach consists of designing baseline control laws that are robust to a large set of impairments. The remainder of the impairment cases are explicitly isolated and new gains are switched in (reconfigured) to compensate. This paper discusses some of the key issues encountered in designing highly robust control laws using mathematical models of the AFTI/F-16, Control Reconfigurable Combat Aircraft (CRCA), and F-15 Highly Integrated Digital Electronic Control (HIDEC) as examples. Issues discussed include inability to design a stabilizing controller, degraded nominal performance, and reconfigured control law conflicts. These issues have forced a dependence on identification, with its penalties, to maintain performance in the presence of impairments.