ACAS X update: A small, big change
Updated: Jun 28
TCAS II is a well-known form of an Airborne Collision Avoidance System (ACAS) and has been in use for decades. Over the years, it certainly prevented numerous collisions, but as every experienced aviator will know, it also created countless “spurious warnings”. In fact, about 80% of TCAS-related crew interactions are dealing with advisories that are “undue”  i.e., the involved aircraft are following their assigned trajectories and no collision hazard exists. Why is that? Well, mainly because of the way TCAS II is designed to work: A fixed set of rules, using a concept involving the time to the “Closest Point of Approach” (CPA), topped-off with a choice of “sensitivity levels” depending on the altitude of the aircraft. The parameters are tuned to transport category aircraft, and this makes the system ill-suited for other airspace users, such as smaller UAV’s. There has been an industry-wide effort to improve this over recent years.
Enter: ACAS X
Back in 2009, the FAA launched a research campaign to develop a TCAS II successor that would remedy the known shortcomings and be scalable to different airspace users. This resulted in a new system called ACAS X. Related MOPS have been published a couple of years ago .
In 2023, EASA published a Notice of Proposed Amendment (NPA), listing ACAS Xa as an acceptable means of compliance in the regulatory framework . The good news: The system is compatible with TCAS II and for the flight crew, there is little difference in terms of the Human Machine Interface (HMI).
ACAS X variants
ACAS X comes in different “flavors”, adjusted for specific applications. See Figure 2 below:
The following description is based on :
ACAS Xa (active surveillance)
Is meant to be the successor of TCAS II, making active interrogations. Compatible with TCAS II, but fundamentally different in the avoidance logic.
ACAS Xo (operation specific)
A special option to enable operation-specific tailoring, such as closely spaced approaches.
ACAS Xu (unmanned aircraft)
Aimed at remotely piloted aircraft, also incorporating horizontal avoidance.
ACAS sXu (small, unmanned aircraft)
A special version of ACAS Xu, tailored for small remotely piloted vehicles involving a wingspan of less than 15 m.
ACAS Xr (rotorcraft)
A helicopter-specific version. No standards available yet.
ACAS Xp (passive surveillance)
A version of ACAS X, that does not perform active interrogation, but relies on ADS-B. Mainly aimed at general aviation.
ACAS X/TCAS II differences
So, what is the big deal with ACAS X? In a nutshell, it comes down to this:
Instead of hard-coded rules, ACAS X employs a probabilistic state-estimation using real-time lookup tables. The optimum solution is then chosen using a cost/reward system.
This fundamental change in philosophy should get rid of most “false alerts” and provide more adequate advisories. It also enables tailoring to specific real-world scenarios. A comparison between TCAS II and ACAS Xa is shown in Figure 3.
The ACAS X workflow is depicted below. There are two key “modules” in ACAS X: The Surveillance and Tracking Module (STM) and the Threat Resolution Module (TRM). The STM uses a variety of sensors to establish situational awareness. The TRM contains a lookup table that enables the system to predict possible future system states and assign a “cost” at different outcomes. The optimum course of action is the selected by comparing the “cost”. The process accounts for uncertainties in the sensor data and pilot actions. ACAS X does not use a fixed set of sensitivity levels like TCAS II .
ACAS X will also incorporate “hybrid surveillance”, a concept available as an option for TCAS II (v7.1). Here, the number of active interrogations is significantly reduced if there is an ADS-B signal available. The targets are verified using active interrogation, if necessary .
One of the key questions regarding cyber-security will be if Resolution Advisories (RA’s) should be generated based on ADS-B targets only. So far, this has not been incorporated, and even ACAS Xa requires ADS-B targets to be verified by interrogation for an RA to occur. This is obviously to prevent fraudulent activity using fake ADS-B targets.
Often, visual approaches or closely spaced parallel approaches have led to undue advisories with TCAS II. Therefore, ACAS X incorporates a special option, called ACAS Xo, permitting special operations as described below and shown in Figure 6.
Designated No Alert (DNA) mode
Here, the flight crew can “tag” a specific target they intend to maintain own separation from, and the system will therefore not issue advisories against that specific target, while maintaining full availability for other targets. The system will automatically “un-tag” targets, if they move away more than 6 NM from the ownship.
Closely Spaced Parallel Operations 3000 (CSPO-3000) mode
This mode “shrinks” the advisory envelope for parallel approaches with less than 3000 ft runway separation. Also here, this can be selected for specific targets, while other targets will be treated with normal thresholds.
At the time of writing, there are no applications of ACAS Xo in the European regulatory framework, this might change in the future.
ACAS X / TCAS II similarities
After the differences, we shall look at the similarities between ACAS X and TCAS II. It turns out, for the flight crew, there is very little that changes. Some advisories have a slightly different wording, or slightly different way they might appear. The main point is this:
Pilots are expected to react to ACAS X advisories, the same way they do for TCAS II advisories.
The HMI for ACAS X and TCAS II are pretty much identical. The well-known VSI or pitch cue are the way to signal to the crew what to do.
TCAS II and ACAS X limitations
It is important to be aware of system limitations for both TCAS II and ACAS X. See below a summary of typical system limitations :
TCAS II limitations
- Closing speeds of up to 1200 kt above FL100 and 500 kt below FL100
- Relative vertical rates of up to 10,000 ft/min.
- Within 14 NM of own aircraft
- Within ±3000 feet and whenever possible within ±10,000 feet altitude relative to own
aircraft. (Display ranges may vary).
ACAS Xa limitations
- No fixed limits for closing speeds
- No fixed limits for relative vertical rates
Sidenote: Tiebreak and geometric reversal
Have you ever wondered what happens, if two aircraft issue a resolution advisory in the same sense i.e.: both “climb”? See below for this situation:
Here is the trick: The so-called “tiebreak reversal”. The aircraft with the lower mode S address becomes the “master” and therefore has “the right of way”. This is the same for TCAS II and ACAS X.
A similar rule exists, if one aircraft maneuvers “against” the RA. This is known as “geometric reversal”. See Figure 9 below:
The geometric reversal is initiated by the “master aircraft” in TCAS II. This is limited to one reversal per intruder per encounter. In ACAS X, it may also be initiated by the “slave aircraft”, if the “master” has not yet done so . This may lead to two geometric reversals on rare occasions for ACAS X.
The regulatory framework is about to be updated regarding ACAS X and we shall see how well the ACAS X concept blends in with the current environment. It certainly represents a step in the right direction towards a universally applicable collision avoidance system.
 W. Rosenkrans, «ACAS X,» Flight safety, 2014. [Online]. Available: https://flightsafety.org/asw-article/acas-x/. [Zugriff am 19 6 2023].
 Eurocontrol, «ACAS Guide,» 2022.
 EASA, «NPA 2023-04 Introduction of ACAS Xa for operations in the single European sky (SES) airspace & PBN specifications for oceanic operations,» 2023.