Unmanned aerial vehicle (UAV) communications could offer flexible scheduling, improved reliability, enhanced capacity over much wider range, and has become a key part of the space-air-ground integrated network. In this letter, we consider a communication system in millimeter wave band, where UAV serves as an airborne base station with multiple antennas, and propose a new flight control system-based channel tracking method. Specifically, the 3-D geometry channel model is formulated as a combination of the UAV movement state information and the channel gain information, where the former can be obtained by the sensor fusion of the flight control system, while the latter can be estimated through the pilot transmission. Simulation results are provided to verify the effectiveness of the proposed method.