The Future of Collision Avoidance – ACAS X

Introduction & Context

Traditional TCAS II often issues alerts that prove unnecessary, leading to operational inefficiencies. With evolving surveillance technologies—such as ADSB under NextGen—and more accurate aircraft tracking, smaller separation minima now possible. ACAS X is engineered to capitalize on these advancements by reducing false alerts and enabling tighter, safer traffic throughput.

Advanced Probabilistic Threat Logic

v Rather than relying on fixed rule sets, ACAS X implements a dynamic, probabilistic approach:

v State Estimation – Millions of possible aircraft states are calculated using models of sensor and dynamic uncertainties.

v Cost Lookup – Each state is evaluated via a precomputed table, estimating costs of potential advisory actions.

v Optimal Manoeuvre Selection – Dynamic programming determines the advisory with the lowest expected cost, balancing safety and efficiency.

Variants of ACAS X

ACAS X is modular and mission-adapted:

v ACAS Xa: Designed to replace TCAS II on commercial transports while maintaining interoperability with existing systems.

v ACAS Xo: Tailored for specific operations (e.g., close formations) where nuisance alerts might rise.

v ACAS Xu: Optimized for UAS, integrating their unique flight profiles.

v ACAS Xp: A passive variant for smaller general aviation aircraft that aren’t TCAS-equipped.

Benefits Over Legacy Systems

v Alert Reduction: Simulations indicate up to a 30% reduction in unnecessary Resolution Advisories (RAs), while collision risk is halved.

v Future-proof: Capable of integrating enhanced surveillance (like ADSB), facilitating denser airspace operations under NextGen.

v Wider Applicability: Supports broader aircraft classes—from light GA to cargo, unmanned vehicles, and beyond.

v Cost-effectiveness: Uses numerical tables instead of complex logic, easing future updates and reducing hardware overhead.

What Pilots Need to Know: ACAS X vs. TCAS

Familiar interface, better logic: ACAS Xa (pronounced “Ay-cas Xa”) uses the same cockpit alerts and coordination protocols as TCAS II—“Traffic, Traffic,” climb/descend RAs—but is driven by vastly more sophisticated algorithms concealed behind the scenes.

Fewer nuisance alerts: Decision-theoretic logic reduces unnecessary alarms by up to ~30%, while still delivering stronger safety performance.

Inside the Black Box: State Estimation to Advisory

A. Surveillance & Sensor Model

Multisource tracking: ACAS X leverages radar, ADS-B, Mode S, and even optical sensors, integrating them via a Kalman-style filter into a probabilistic state distribution—capturing uncertainties in range, bearing, altitude, and closure rate.

State variables include relative altitude (±1000 ft), own/intruder climb/descent rates (±2500 fpm), and pilot reaction delay.

B. Dynamic Programming & Lookup Table (LUT) Generation

The design team builds a Markov Decision Process (MDP), discretizing state variables into grids and modelling transitions per possible advisory action.

They define a utility function—strongly penalizing near-midair collisions (NMACs), lightly penalizing unwarranted advisories, and rewarding safe resolution.

Offline computation uses Bellman recursion to calculate the lowest-cost action at each state over a 40–50 s horizon; the result is a huge numeric lookup table (~300 MB raw).

C. Onboard Advisory Selection (Every Second)

Real-time sensor inputs update the belief state. The system interpolates from the LUT to find the lowest-cost action—“Climb,” “Descend,” or “Maintain”—balancing safety and operational efficiency.

Lightweight filters adjust for recent advisories or sensor spikes to avoid oscillation or overly aggressive reversals.

Where ACAS III Fits In (and Why It Didn’t)

ACAS III (TCAS III) was envisioned to issue horizontal RAs (“turn right,” “turn left”) in addition to vertical ones—but was shelved in the 1990s because TCAS antenna geometry couldn’t accurately resolve bearing for reliable horizontal advisory calculation.

ACAS X stays focused on vertical manoeuvres for manned aircraft but includes variants like ACAS Xu—including horizontal avoidance—specifically optimized for unmanned platforms where modern sensors (like ADSB In or cameras) provide higher positional fidelity.

What This Means in the Cockpit

Pilot Perspective	What You’ll See / Experience
Alert familiarity	Same TA/RA alerts as TCAS II; no new voicing or display changes .
Alert timing	RAs may trigger slightly earlier or later but remain within pilot expectations—interactions remain interoperable
Workload & drills	Fewer false alarms = less distraction and more consistent RA response behaviour
Coordination	RAs remain coordinated vertically; ACAS X retains compliance with TCAS coordination standards (e.g., CP112E reversals)
Future operations	Eventually supports reduced separation, continuous descent approaches, and unmanned vehicle integration—all with familiar cockpit workflow

Pilot Guidance & Training Points

v Fly the RA” still applies: Immediate pilot response remains critical. ACAS X presents no change to cockpit procedures ..

v Expect smoother RA behaviours: Less oscillation and fewer reversals due to cost-optimized advice—execution remains consistent with TMRA training.

v New training aids on the way: Simulator updates to capture subtle timing or directional adjustment variations are being incorporated into recurrent training.

v Why no directional commands? Horizontal commands in ACAS III were unreliable due to bearing inaccuracies—vertical-only remains safer and validated.

Looking Ahead: ACAS X Integration

Variants for unique airspace users:

v ACAS Xo – Optimized for close-formation or approach phases with fewer nuisance alerts.

v ACAS Xp – Basic, passive system for general aviation using ADSB.

v ACAS Xu – Tailored for UAS, with possible horizontal advisories.

Certification & fielding:

v Flight testing: prototypes flown from 2013–2017, evaluated by FAA and Eurocontrol.

v Standards: RTCA DO385A MOPS published for ACAS Xa/Xo in 2025.

v Introduction expected mid-2020s, with full retrofit via typical avionics upgrade cycle.

Takeaway for Pilots

You will keep seeing familiar alerts and performing familiar responses—but ACAS X gives you smarter, cleaner advisories under the hood. Fewer nuisance alerts, steadier RA behaviour, and seamless integration with modern airspace tools—while leaving your well-rehearsed "fly the RA" mindset intact.

Implementation & Timeline

v Prototype testing: FAA flight evaluations using ACAS Xa prototypes, conducted in 2013 over 120 scenarios.

v Standards development: Formal minimum performance standards slated for 2018, followed by further flight testing.

v Deployment goal: Full fielding targeted around 2020, with plans for installation in over 30,000 transport-category aircraft.

Professional Implications

For Airlines: Expect smoother flight paths, fewer costly avoidance manoeuvres, and fuel savings from reduced vertical excursions.

For GA and UAS Operators: Enhanced situational awareness and safety support in airspace previously beyond the reach of TCAS II.

For Regulators: A data-driven basis for reduced minima and updated separation standards.

Conclusion

ACAS X introduces a transformative leap in airborne safety systems. By integrating probabilistic logic with flexible platform architectures, it achieves superior threat detection, minimizes false alerts, and supports future airspace innovations. With a planned rollout in the early 2020s, ACAS X is poised to redefine collision avoidance across all classes of aviation.

Author : GR Mohan