Abstract: For the method of classical single channel measure of MIMO systems stability margin is not applicable to evaluate the stability margin when gain and phase change simultaneously in every loop. So in the paper a new method for stability margin analysis of multiloop flight control systems is presented. In terms of the minimum singular value of the system return difference matrix and the universal diagram of the multiloop gain phase margin evaluation, the simultaneous gain and phase changes in all loops of a multiloop system can be obtained. The method provides an index of robustness evaluation and remedies the defect of classical single channel measure of MIMO systems stability margin; then in order to decrease conservation, two improving methods are adopted and an illustrative example of a two input/two output flight lateral control system is given.

Key words: mult iloop fligh t cont ro l system, stability margin, the m inimum singular value of return difference mat rix, universal diagram of gain-phase margin evaluat ion