Reliable Robotics Advances ACAS Xu Flight Testing for FAA Certification

This article is based on an official press release from Reliable Robotics.

Introduction to ACAS X Flight Testing

As the aviation industry pushes toward the integration of uncrewed aircraft systems (UAS) into the National Airspace System (NAS), advanced collision avoidance technologies are becoming critical. Reliable Robotics recently announced significant progress in this area, detailing a comprehensive flight test campaign for its Detect and Avoid (DAA) system. According to a company press release, this initiative is part of an ongoing contract with the Federal Aviation Administration (FAA) to provide validation data for certifying DAA systems based on the Airborne Collision Avoidance System X (ACAS X).

The testing focuses on ACAS Xu, a variant specifically designed for autonomous fixed-wing aircraft. By integrating this algorithm with air-to-air radar and other sensors, Reliable Robotics aims to create an FAA-certifiable DAA solution capable of operating under Instrument Flight Rules (IFR) across all airspace classes. This development marks a crucial step in ensuring that uncrewed aircraft can safely share the skies with traditional, piloted traffic.

Advancing ACAS X for Uncrewed Systems

The FAA has been developing the ACAS X family of algorithms to modernize and improve upon the legacy Traffic Alert and Collision Avoidance System II (TCAS II). The new suite includes several versions tailored to different aircraft types: ACAS Xu for fixed-wing UAS, ACAS Xr for rotorcraft, and ACAS Xa as a direct replacement for TCAS II in transport-category aircraft. The primary advantage of ACAS X is its ability to reduce unnecessary alerts, particularly in terminal areas, while enhancing overall safety and separation.

Reliable Robotics is leveraging the ACAS Xu variant as it works toward a Supplemental Type Certificate (STC) to convert the Cessna 208B Caravan into a fully uncrewed aircraft system. In their official statement, the company noted that their DAA solution will provide surveillance against both cooperative and non-cooperative traffic, a vital requirement for safe airspace integration.

Flight Test Campaign Details

Simulating Real-World Encounters

To validate the system, Reliable Robotics has transitioned from Hardware In The Loop (HITL) testing to a rigorous, month-long flight test campaign. The company reports that the campaign involves over 50 scripted encounters in and around the terminal area of the Hollister public airport (KCVH) in California.

During these tests, Reliable’s Cessna 208B (registration N927FE) flies RNAV approaches under the control of an advanced automation system, commanded by a remote pilot located 50 miles away. Simultaneously, an instrumented Cessna 182 acts as an “intruder” aircraft, flying converging paths to create specific encounter geometries. The encounters are meticulously planned to simulate a loss of safe separation while maintaining strict safety margins, particularly near the closest point of approach (CPA).

“Successful completion of each encounter requires meticulous planning, close communication between the remote pilot and intruder pilot, and precise execution,” Reliable Robotics stated in their release.

Testing Minimum Equipage Scenarios

A key component of the flight test campaign is evaluating the DAA system’s performance against aircraft with minimal cooperative equipment. The Cessna 182 intruder is outfitted with the minimum viable equipment set required for compliance with current FAA ADS-B OUT mandates, a configuration common among smaller, older general aviation aircraft.

Because ADS-B OUT relies on GPS data, its integrity must be independently validated. Reliable Robotics explains that Mode C omnidirectional interrogation is insufficient for this task. Instead, their system uses an integrated non-cooperative track source, such as radar, to validate the ADS-B tracks and provide complete positional data. This ensures that accurate collision avoidance alerts can be issued even when encountering minimally equipped traffic.

Regulatory and Financial Backing

The push for advanced collision avoidance technology has strong backing at the federal level. The FAA has been funding research and development for ACAS X since 2008. Furthermore, Reliable Robotics highlighted that the Senate Transportation, Housing and Urban Development, and Related Agencies’ draft appropriations bill for Fiscal Year 2026 allocates $16 million specifically for continued ACAS X development.

This sustained financial support underscores the growing focus on modernizing airspace safety technologies to accommodate new entrants without compromising the safety of existing NAS stakeholders.

AirPro News analysis

We view the successful validation of ACAS Xu through real-world flight testing as a major milestone for the UAS industry. By proving that uncrewed systems can reliably detect and avoid both cooperative and non-cooperative traffic, even those with minimal ADS-B equipage, companies like Reliable Robotics are dismantling one of the most significant technical barriers to routine beyond visual line of sight (BVLOS) operations. We note that the $16 million allocation in the FY 2026 draft appropriations bill further signals that lawmakers view ACAS X not just as an experimental project, but as foundational infrastructure for the future of the National Airspace System.

Frequently Asked Questions

What is ACAS X?
ACAS X (Airborne Collision Avoidance System X) is a family of collision avoidance algorithms developed by the FAA to replace and improve upon legacy TCAS II systems. It includes variants for transport aircraft, rotorcraft, and uncrewed systems.

What aircraft is Reliable Robotics using for these tests?
Reliable Robotics is using a Cessna 208B Caravan (N927FE) equipped with their DAA system, and a Cessna 182 acting as the intruder aircraft.

Where are the flight tests taking place?
The flight tests are being conducted in and around the terminal area of the Hollister public airport (KCVH) in California.

Sources

• Reliable Robotics