Flight control law clearance using optimization-based worst-case search

The flight control law clearance problem can be formulated as a robustness analysis problem, where a set of suitably defined clearance criteria must be checked to lie within certain limits for all admissible variations of aircraft parameters, pilot inputs and all flight conditions. The idea of optimisation based worst case clearance is to use available and efficient optimisation methods to find those parameters/inputs/flight conditions for which the criteria are violated or poorly satisfied. The potentials of this approach are its general usability for both frequency-domain and time-domain analysis, and for both linear and non-linear models and control laws including protections. Moreover, there is no limitation on the number of parametric uncertainties that can be investigated and the method does not itself add conservatism to the clearance problem as many other alternatives do, mainly those based on approximations or simplifications. The most challenging problem with the optimisation based approach is not to find hidden weaknesses if they exist, but to confidently assert that no weakness exists for all parameters/inputs/flight conditions. To assert this a global optimum solution has to be found usually resulting in a huge amount of computational work. For highly nonlinear clearance problems regarding manoeuvrability in the peripheral flight envelope of a civil transportation aircraft the suitability of the optimisation based approach is demonstrated. Different global optimisation algorithms and strategies are investigated with respect to its reliability in finding worst cases, computational efficiency and its potential for distributed computation in order to reduce time to clearance.