1. Home
  3. Next-generation collision avoidance system

Next-generation collision avoidance system


ACAS X has better algorithms and added ADS-B data  to improve airborne traffic avoidance in the future.

By David Bjellos
Contributing writer


This graphic illustrates the intended integration of multiple users of airspace, with the requisite protection features afforded by the application of ACAS X.

Less than 70 years ago, a midair collision between 2 airliners changed the trajectory of aircraft safety forever. A Douglas DC-7 and a Lockheed L-1049 collided over the Grand Canyon, claiming 128 lives. Aviation authorities then recognized the need for improved nationwide radar coverage, enhanced traffic separation and a 3rd independent collision avoidance system. Dr John Morrell of the Bendix Aviation Corporation had devised such a system in 1965. Originally called beacon collision avoidance system, it evolved into our present-day traffic collision avoidance system (TCAS). The most current version is TCAS II V7.1.

It’s been mandatory in the US NAS since 1993 and internationally since 2001, and it has been remarkable. The reduction in midair collisions has been reduced by an order of magnitude if just one aircraft is equipped  with TCAS. And when 2 aircraft are equipped with it, that reduction increases an additional order of magnitude (see Fig 1).

The US NAS has not had a midair collision since 1993, an outstanding record and achievement. But the technology of the 1970s and 80s has improved dramatically, and the airspace continues to evolve and absorb more aircraft every year, especially with the introduction of unmanned and remotely piloted aircraft systems (UAS/RPAS). ICAO’s mandate for ACAS X has been critical to maintain this exceptional safety record.


The benefits of TCAS- to TCAS-equipped aircraft is shown here, with an improvement of 2 orders of magnitude from no equipage. The challenge in densely populated airspace is nuisance alerts and resolution advisories. ACAS X logic and accuracy will reduce TA/RA issues dramatically through improved refresh rates and very accurate position/trajectory projections. The addition of passive ADS-B inputs and better processor functions will enhance current functionality.


Although ACAS and TCAS are terms often used interchangeably, there is a modest technical difference. ACAS is typically used when referring to the technical standard or concept, while TCAS is typically used when referring to a current implementation of the technical standards, which is widely fitted throughout the world.

Currently, TCAS II version 7.1 is the only implementation that fully meets the ACAS ICAO Standards and Recommended Practices (SARPs), as documented in Annex 10, Aeronautical Communications. The common misconception that TCAS is an FAA term and ACAS one for EASA/ICAO is mostly semantics, but noteworthy. For the purposes of this presentation, TCAS will be used to refer to the current system, and ACAS X to the imminent replacement.

Three types of technical standards for ACAS have been specified in ICAO Annex 10:

1. ACAS I (implemented as TCAS I) provides information as an aid to “see and avoid” action, but does not include the capability for generating Resolution Advisories (RAs).

2. ACAS II (TCAS II) provides vertical RAs in addition to TAs. Our current system.

3. ACAS III (ACAS X) is intended as the future system of ICAO Annex 10-12.

Graphical illustration of ACAS X applications as planned by ICAO. ACAS Xa and Xo will operate autonomously, depending on airspace (much like our FMS sensing enroute, terminal, final approach, and missed approach RNP categories).

Defining ACAS X

ACAS II/TCAS II is constructed upon enterprise-based computational algorithms. They are “hard-coded” rules and had pre-programmed solutions for a finite number of near-mid-air collision (NMAC) scenarios. Instead of using a set of hard-coded rules, ACAS X is based upon a numeric look-up table optimized to a probabilistic projection model of the airspace and a set of safety and operational considerations. These algorithms use dynamic programming to formulate a predicted path and provide the pilot the best proposed solution to avoid an NMAC.

As operators and airmen, the transition will be transparent – the existing TCAS cockpit displays will remain identical with the installation of ACAS X. All TA and RA avoidance commands, alerts, guidelines, and proximate traffic symbology will remain.

The ACAS X probabilistic model provides a statistical 4D representation of the aircraft position as it approaches a conflict (converging) scenario. It also considers the safety and operational objectives of the system, enabling the logic to be tailored to particular procedures (think PRM or closely-spaced parallel approaches) or airspace configurations (radius to fix transitions or RNP 0.1 procedures).

A vital system of measurement for operational suitability and pilot acceptability includes minimizing the frequency of alerts that result in reversals/intentional intruder altitude crossings or disruptive advisories in both critical and non-critical encounters. The ACAS X refresh rate is exceptional (approximately every second) and is designed to avoid such scenarios.

ACAS Xa (active surveillance) is the general-purpose ACAS X that conducts active Mode S and passive ADS-B interrogations to detect intruders.


It is very important to emphasize that the displays for TA/RA will remain the same with ACAS X as it was for TCAS. Extensive human factors studies found that multiple generations of airmen have adapted extremely well to the TCAS alerting models. Research by Airbus, Boeing, FAA, and academia have found that maintaining these models enhances the growing challenges of human-machine interface (HMI) issues. Additional automation is a risk/reward endeavor. The solution is finding the correct balance to avoid overloading the manned aircraft crews with data.

ACAS Xo (operation specific) is an operational extension to ACAS Xa designed for particular operations (eg, closely spaced parallel approaches), for which ACAS Xa is less suitable because it may generate a large number of nuisance alerts.

ACAS Xu/sXu (unmanned/small unmanned aircraft) is designed for RPAS operating beyond visual line of sight (BVLOS).

ACAS Xr (manned rotorcraft) is self-explanatory.

Readers may question why the need to replace something that has proven remarkably efficient and practical? The 2-part answer is technological improvements and airspace congestion. Enterprise computing (current TCAS II) is outdated and not sustainable.

The average laptop, for example, is far more powerful than many enterprise operating systems. Adaptive learning (much like we already enjoy in our airframes and engines through learning modules and health usage monitoring systems [HUMS]) will allow ACAS X to succeed in airspace that will become increasingly dense, thanks to the dual proliferation of PBN laterally/longitudinally and RVSM vertically.

The rapid and exponential addition of RPAS into NAS and global airspace convinced ICAO and air navigation service providers (ANSPs) to evolve ACAS II (TCAS II) into ACAS III (ACAS X). Increasing levels of automation – even in manned aircraft – make the need for more precise predictive collision avoidance mandatory. Expect the US NAS to be the 1st-adopter, followed by EASA and others.

FAA has defined these benefits from ACAS X as shown. Clearly, Xa will reduce nuisance TAs/RAs tremendously, and Xu will provide manned aircraft a significant safety margin from RPAS aircraft. Manned rotary-wing aircraft will benefit greatly from Xr while operating in the low-altitude environment. Key to success is near 100% ACAS X equipage for all airspace users.

Transitioning into 4D

Readers have already become very familiar with enhanced SID and STAR procedures, notably those into larger airports with high levels of commercial traffic. Following the “descend via” and “climb via” clearances we have become accustomed to since 2014, these new SID/STAR speed and altitude constraints are designed for impending 4D operations, with time as the 4th dimension.

This represents the exceptional quality of surface hazards presented to the pilot-selected display during exocentric view. Hot spot and runway incursions should be reduced dramatically – and eventually eliminated – when combined with runway awareness system alerts and these high-resolution graphics. ACAS X precision will enhance ground movements, reduce incursions, and assist airmen during low-visibility operations. The red arrow illustrates very clearly a runway/taxiway hot spot to the pilot.

In addition to existing supporting software in many cockpits (eg, required time of arrival [RTA]), these new procedures are designed to meter traffic into the departure and arrival corridors while allowing for significant differences in aircraft performance by applying generic speed and altitude constraints. This allows for a very predictable traffic flow from takeoff to landing, reducing enroute spacing and maximizing optimal runway occupancy times at larger airports.

ACAS X will support the necessary level of precision required for SID/enroute/STAR procedures implementing applications such as:

• Cockpit display of traffic information (CDTI) assisted visual separation (CAVS).

• Interval management (IM). An in-trail aircraft can match ground speed with a preceding (lead) aircraft, allowing for very accurate spacing in terms of either time or distance.

• In-trail procedures (ITP). Used very successfully over the North Atlantic for more than a decade, ITP will be adapted for use in the US NAS once certain FANS milestones are met.

• Surface area movement management (SAMM). This one is especially important given the number of runway incursions and hot spot transgressions we have seen in recent years. Some avionics fitted to business aircraft already display a far higher graphical resolution of hot spots, ground vehicles, and runway/taxiway markings, which is especially useful during low-visibility ops.

FAA, ICAO, and EASA are preparing for the impending, large-scale introduction of UAS/UAM. Ensuring comparable safety for all aircraft categories and applications will be greatly enhanced by the introduction of ACAS X, and paves the way for RPAS automated flight beyond visual line of sight (BVLOS).


ACAS X will become a great tool, but obstacles exist. Cooperative traffic (ie, transponder-equipped aircraft) can be avoided safely, but non-cooperative traffic (aircraft with no transponder) continue to be a threat, and the “known unknowns” of how to deal with that potential threat will remain. Given the size, weight, and relatively low cost of digital devices, mandating that all aircraft be equipped with transponders seems like a simple solution.

An obvious need from the UAS/UAV industry will be for avionics companies to embrace this novel technology (ACAS Xu/sXu) so that their airframes are using the same protocols and RA trajectories as do manned aircraft, ensuring that we all play from the same rulebook. FAA will have to mandate and enforce a new regulation, which requires a Notice of Proposed Rulemaking (NPRM) that could take years to finalize.

The concern is that we should be very proactive in advising our regulator that an accident today will be far more devastating than the tragedy over the Grand Canyon in 1956. Understanding the risks and rewards of this new technology and advocating 100% equipage for all aircraft – RPAS, small UAS, and recreational private planes – is good airmanship at its finest.

DavidDavid Bjellos has been writing for PP since 2004. He is an active airman flying a G650 based in south Florida, a former Board of Directors liaison to the UAS Committee for the Helicopter Association International, and a subject matter expert in VTOL operations.