Real World Flight Test of a Three-Dimensional Collision Avoidance Control Barrier Function Algorithm

Urban air mobility (UAM) operations in dense, obstacle-rich airspace demand collision-avoidance methods that remain safe under tight path constraints and real-world disturbances. Control barrier functions (CBFs) offer a principled framework for safety-critical control, but existing fixed-wing CBF collision-avoidance methods have only been demonstrated in simulation, leaving their guarantees untested under actuator limits, hardware constraints, sensor noise, and atmospheric disturbances. This paper presents the first documented flight-test validation of a higher-order CBF quadratic program (HOCBF-QP) safety filter for fixed-wing UAS collision avoidance. Flight tests are conducted on a SkyHunter aircraft executing a complex figure-8 trajectory in the presence of static obstacles under both low-wind and high-wind conditions. A 3D engagement-dynamics formulation reduces the problem to one-dimensional range dynamics, enabling real-time onboard implementation. A reduced 2D formulation is also derived and compared with the 3D implementation. In low wind, both formulations maintain the required separation throughout all avoidance maneuvers. In high wind, with sustained winds exceeding 40% of cruise speed, two safety-boundary violations occur due to gusts, revealing where nominal theoretical guarantees break down in flight. This work establishes a practical pathway for deploying CBF-based safety filters and highlights the importance of flight-test validation in safety-critical autonomy.