An approximate method is formulated for analyzing the performance of nonlin
ear systems controlled by mode-to-mode fuzzy controllers. It is assumed tha
t an approximate model of the nominal plant is available and the nominal mo
de-to-mode trajectory converges asymptotically from the start mode to the t
arget mode of operation. Stability of the nominal mode-to-mode trajectory i
n the presence of small, bounded variations of the system's parameters or t
he initial conditions is considered. The method is based on formulating a p
erformance measure as a Lyapunov function of the error between the nominal
and perturbed mode-to-mode trajectories. The sensitivity of the deviations
from the nominal mode-to-mode trajectory with respect to parametric or init
ial condition variations is incorporated into the total differential of thi
s performance measure. Using a Lyapunov stability condition, the robustness
of the closed-loop system is analyzed by observing a definiteness conditio
n of a time-varying matrix. A measure of robustness is then formulated usin
g the largest singular value of a time-varying matrix. A hover mode to forw
ard flight mode fuzzy controller is used to illustrate the methodology.