A maximum likelihood parameter estimation method for nonlinear systems has been applied to flight data of the research aircraft ATT AS (advanced technologies testing aircraft system). The emphasis is on modeling and identification of 1) dynamics and nonlinearities of actuation systems for the direct-lift-control (DLC) flaps, and 2) their aerodynamic effectiveness and other influences. These specially designed flaps provide an additional longitudinal control suitable for in-flight simulation or load alleviation investigations. The identification results indicate that under aerodynamic loads the possible maximum flap deflections are severely limited. The flightestimated aerodynamic characteristics are compared with those predicted by wind tunnel and analytical methods. It is found that the DLC flaps are somewhat less effective aerodynamically than they were designed for. More importantly, the transit time lag in the downwash generated due to flap deflections significantly affects the dynamic pitching motion. Furthermore, it leads to certain additional unsymmetrical and interference effects.