An integrated digital flight control system has been designed for the terrain following task. The control system based on the use of cubic spline paths and energy management techniques, controlling flight path and engine thrust. A nonlinear reference model based on the desired path, complete reference state and command were computed. Deviations from the reference model form a linear regulator problem, solved by discrete optimal control theory. This system consider the operational constraints (speed, acceleration), the speed adjusts the desired energy level for the vehicle, which allows the interchange between kinetic and potential energy improving fuel consumption. This flight control system can be used also in remotely piloted vehicles in performing similar tasks.