The field of aerial autonomy is an expanding engineering field with applications ranging from sea exploration searches to environmental monitoring tasks. From technological perspective, aircraft is intrinsically a nonlinear system and its aerodynamic characteristics vary widely with altitude, speed and atmospheric conditions. Flight Control System (FCS) is responsible for tracking inputs from navigation and guidance algorithm by controlling actuators on aircraft. In this book a nonlinear FCS system based on the concept of Nonlinear Dynamic Inversion (NDI) is discussed. Longitudinal and lateral modes of aircraft are handled separately for ease of design and analysis. The FCS consists of controllers on each of aircraft axes i.e., an altitude and airspeed controller on x-axis, a roll rate controller on y-axis and a yaw rate controller on z-axis. These controllers are the result of explicit model following, NDI and nonlinear transformations of state variables. A practical methodology is used to analyze the stability and robustness of the control system in presence of model parameter and sensor uncertainties. In last, implementation strategy of the controllers is discussed.