Advancing the AI-Based Realization of ACAS X Towards Real-World Application

In recent years, artificial intelligence (AI) has been applied to a wide range of safety-critical domains, such as automotive, robotics, and aviation. Especially the automotive and robotics domains have seen a rapid increase in the number of AI-based systems that are being deployed in real-world applications. However, real-world applications in the aviation domain are still sparse, given the challenges of AI engineering in combination with strict safety requirements.

A first possible application of AI in the aviation domain might be the future collision avoidance system Airborne Collision Avoidance Systems X (ACAS X). The goal of collision avoidance systems is to issue advisories to the pilot to avoid near mid-air collisions (NMACs). The two important variants of ACAS X for this work are ACAS X_A, providing vertical advisories and meant as a drop-in replacement for current systems in commercial air flight, and ACAS X_U, providing horizontal advisories for the ever-growing unmanned aircraft systems market. This work brings both variants closer to real-world deployment by implementing a vertical collision avoidance system, based upon ACAS X_A, and a horizontal collision avoidance system, based upon ACAS X_U, for the research flight simulator FlightGear. Using advisories given by this implementation, this work furthermore provides an auto-avoid function that can command an airplane in FlightGear to safely avoid NMACs.

Finally, this work will show that the ACAS X implementation can avoid collisions in a simulated environment. For this task, an Operational Design Domain will be defined serving as a basis for safety considerations and evaluating the implementation of the ACAS X. In the end, simulation-based testing will be used separately for VCAS and HCAS showing the successful utilization of advisory predictions as autopilot inputs.

Summarizing, this work not only presents an open-source implementation of ACAS X_A and ACAS X_U for FlightGear but also shows how the generated advisories can be used to successfully avoid NMACs.

Recommended citation: Christensen, J. M., Anilkumar Girija, A., Stefani, T., Durak, U., Hoemann, E., Köster, F., Krüger, T. and Hallerbach, S. "Advancing the AI-Based Realization of ACAS X Towards Real-World Application", in 36th IEEE International Conference on Tools with Artificial Intelligence (ICTAI), Oct. 2024.
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