A flight control design methodology has been presented using the hierarchy-structured dynamic inversion (HSDI) approach, which is based on multiple time-scale separation of a general fixed-wing aircraft dynamics. HSDI approach integrates traditional guidance and control structures to realize simple, systematic, and universal design of flight control systems with a small number of design parameters. In this paper, HSDI is modified using command rate feedforward to improve control performance. Robustness of HSDI approach is then evaluated through the root sum square (RSS) analysis and Monte Carlo simulations (MCS). The design parameters are finally optimized to enhance robustness using the downhillsimplex method and MCS evaluation. The numerical simulations are based on the highly reliable 6DOF nonlinear flight simulation model of ALFLEX experimental vehicle, where the distribution data for more than 100 uncertain parameters are defined. The detailed numerical analysis discussed in this paper has shown that robustness performance of the optimized HSDI flight controller is comparable to that of the baseline linear robust controller, which is also similarly optimized.