Backhaul solutions with large capacity and global coverage are of great importance for 6G wireless communication networks. With its ultra-large bandwidth and immunity to electromagnetic interference, free-space optical (FSO) communication has stood out as a major connectivity solution for UAV-to-UAV and UAV-to-ground links. While the performance of horizontal terrestrial FSO links is mainly limited by atmospheric-turbulence-induced fading, geometrical and atmospheric losses dominate the link budget of an airborne link between a high-altitude fixed-wing UAV and a ground station. Since the fixed-wing UAV continuously moves, the transmission distance between it and the ground station changes, causing both atmospheric and geometric losses to change, which results in the fading effect. In this paper, we statistically characterize this aggregate channel coefficient. First, we derive a probability density function for the channel coefficient; then, we obtain an expression for the instantaneous received signal-to-noise ratio (SNR). We validate our derived expressions through numerical calculations.