Abstract: In this paper an adaptive flight control system for the longitudinal axis based on energy balance theory is described. A practical improved scheme and design details are presented, and preliminary evaluation from simulation and flight tests is made. The system exhibits better convergence in adjusting parameters and better performance of following up without testing signal. The system's architecture is simple and practicable. Selection of the scheme, design analysis, and brief results of the flight test for the open loop system, especially convergence of adjusting parameters and how to improve the effect of the input signal types are discussed.