  Aircraft failures alter the aircraft dynamics and cause maneuvering flight
envelope to change. Such envelope variations are nonlinear and generally
unpredictable by the pilot as they are governed by the aircraft's complex
dynamics. Hence, in order to prevent in-flight Loss of Control it is crucial to
practically predict the impaired aircraft's flight envelope variation due to
any a-priori unknown failure degree. This paper investigates the predictability
of the number of trim points within the maneuvering flight envelope and its
centroid using both linear and nonlinear least-squares estimation methods. To
do so, various polynomial models and nonlinear models based on hyperbolic
tangent function are developed and compared which incorporate the influencing
factors on the envelope variations as the inputs and estimate the centroid and
the number of trim points of the maneuvering flight envelope at any intended
failure degree. Results indicate that both the polynomial and hyperbolic
tangent function-based models are capable of predicting the impaired fight
envelope variation with good precision. Furthermore, it is shown that the
regression equation of the best polynomial fit enables direct assessment of the
impaired aircraft's flight envelope contraction and displacement sensitivity to
the specific parameters characterizing aircraft failure and flight condition.
