Accurate estimation of aerodynamic parameters of an aircraft is a matter of great importance in aviation industry as well in aircraft design and development programs. In this paper, Gaussian sum extended information filter (GSEIF) is studied for aircraft parameter estimation. The convergence condition for the GSEIF has been obtained using the normalized Lyapunov energy functional by proposing a nonlinear observer for the filter and studying its asymptotic behavior, this is a novel approach for deriving stability results for corresponding nonlinear filter. The performance of extended information filter (EIF) and GSEIF is evaluated for aircraft parameter estimation using simulated flight data generated using the MATLAB implementations. The estimation results using GSEIF are better than that obtained using EIF, especially when the initial state is not known (well), which shows that the proposed technique is really feasible for aerodynamic parameter estimation.