Matt Thurber, editor-in-chief at AIN Media Group, has been flying since 1975 and writing about aviation since 1978 and now has the best job in the world, running editorial operations for Aviation International News, Business Jet Traveler, and FutureFlight.aero. In addition to working as an A&amp;P mechanic on everything from Piper Cubs to turboprops, Matt taught flying at his father’s flight school in Plymouth, Mass., in the early 1980s, flew for an aircraft owner/pilot, and for two summer seasons hunted swordfish near the George’s Banks off the East Coast from a Piper Super Cub. An ATP certificated fixed-wing pilot and CFII and commercial helicopter pilot, Matt is type-rated in the Citation 500 and Gulfstream V/550. Based in the Pacific Northwest, Matt and his team cover the entire aviation scene including business aircraft, helicopters, avionics, safety, manufacturing, charter, fractionals, technology, air transport, advanced air mobility, defense, and other subjects of interest to AIN, BJT, and FutureFlight readers.

Editor-in-Chief

Matt Thurber

In the wake of the midair near Ronald Reagan Washington National Airport (KDCA) on Jan. 29, 2025, NTSB recommendations spurred two pieces of congressional legislation that have put a spotlight on collision avoidance systems: the ROTOR Act and the ALERT Act. The ROTOR Act passed the Senate in December but failed in the House of Representatives, and the ALERT Act passed in the House in March. At press time, the respective bills were undergoing a House/Senate Conference, but substantial debate swirled around how to mandate ADS-B In and/or ACAS X. Both acts address the installation of these systems, but differ in the specifics of requirements and the timelines. In the end, operators are facing a mandate to install at least ADS-B In but possibly ACAS X as well.
<h2>NTSB Recommendations</h2>
The simplest place to begin is with the NTSB’s recommendations on collision avoidance, although these are just a handful of the dozens that address many different aspects that surfaced during the investigation of the accident. Basically, the NTSB believes that aircraft flying in airspace where ADS-B Out is required should be equipped with ADS-B In and a cockpit display of traffic with audible alerts. The board also wants aircraft that require TCAS installations to be retrofitted with ACAS X, or installed in new production aircraft, and for ACAS X to provide more information. This would include exploring lowering the inhibit altitude for traffic advisories (TAs) and resolution advisories (RAs), a feature that was highlighted in the KDCA midair collision. Accordingly, the NTSB recommendations state:
<ul>
<li>Modify airborne collision avoidance system traffic advisory aural alerts to include clock position, relative altitude, range, and vertical tendency.</li>
<li>Require existing and new traffic alerting and collision avoidance system (TCAS) I, TCAS II, and airborne collision avoidance system (ACAS) X installations to integrate directional traffic symbols.</li>
<li>Require all aircraft operating in airspace where Automatic Dependent Surveillance–Broadcast (ADS-B) Out is required to also be equipped with ADS-B In with a cockpit display of traffic information that is configured to provide an audible alert to the pilot and/or flight crew.</li>
<li>Require the use of the appropriate variant of ACAS X on new-production aircraft that are subject to TCAS equipage regulations.</li>
<li>Require existing aircraft that are subject to TCAS equipage regulations to be retrofitted with the appropriate variant of ACAS X.</li>
<li>Evaluate the feasibility of decreasing the traffic advisory and resolution advisory inhibit altitudes in ACAS Xa to enable improved alerting throughout more of the flight envelope.</li>
<li>If the evaluation resulting from the safety recommendation finds that the inhibit altitudes can be safely decreased, require retrofitting of the applicable ACAS X variant incorporating the reduced traffic advisory and resolution advisory inhibit altitudes on all aircraft that are subject to TCAS and equipage regulations.</li>
<li>Require that all rotorcraft operating in Class B airspace be equipped with ACAS Xr technology once the ACAS Xr standard has been published.</li>
</ul>

<h2>TCAS and ACAS</h2>
TCAS is simply one implementation of an overarching concept called the airborne collision avoidance system (ACAS). Collision avoidance is the final layer in the conflict management model, as explained by Eurocontrol, and comes into play when the previous two layers—involving air traffic control—have failed.
It was a midair collision on Aug. 31, 1986, when a Piper Cherokee ran into an Aeromexico DC-9 over Cerritos, California, that spurred Congress to mandate TCAS II in U.S. airspace. Development of TCAS accelerated after the 1956 midair collision of two airliners over the Grand Canyon. Bendix Aviation scientist John Morrel had published a white paper on a collision avoidance concept a month before the accident, but it wasn’t until about two decades later that the technology translated into full-scale product development. Implementation, as is often the case, came about after the Cerritos accident.
For the systems installed in aircraft, the latest standard is TCAS II version 7.1. But a successor to TCAS, ACAS X, has been developed, and the baseline variant, ACAS Xa, is designed to replace TCAS II version 7.1. What ACAS Xa does is build on TCAS II with new functionality that should make the system more capable, and it should address some of the NTSB’s concerns.
The improvements of ACAS X stem from recommendations the RTCA made in 2012, and these include reducing unnecessary RAs and incorporating ADS-B In. According to Eurocontrol, “Instead of using a set of hard-coded rules, ACAS X alerting logic is based upon a numeric lookup table optimized with respect to a probabilistic model of the airspace and a set of safety and operational considerations. The ACAS X probabilistic model provides a statistical representation of the aircraft position in the future. It also takes into account the safety and operational objectives of the system, enabling the logic to be tailored to particular procedures or airspace configurations.”
However, ACAS X in its present form isn’t too much different from TCAS II in the way traffic is presented to the pilot, according to Cam Morast, product manager at ACSS, a manufacturer of TCAS systems.
“The initial impetus for designing the ACAS X variant was to address the evolving airspace where we’ve now got aircraft that do unpredictable things, such as UAVs and helicopters,” he explained, “and allow a more prescribed response to potential aircraft-to-aircraft conflicts. So instead of just a climb or descent response that you’re used to with the existing TCAS II implementation, you now have lateral maneuvers that could be injected into the pilot’s reaction options. To consider those additional options…you need a different alerting algorithm…and many more options for the resolution, for the response [by] the pilot, so that was the motivation. It is envisioned to be less nuisance-prone, so that was another motivating factor in redesigning that architecture, and that’s not a huge leap forward in functionality…but is more of an architecture that supports further growth.”
TCAS displays traffic as diamonds, circles, and squares, but there is no directional information similar to what is displayed with ADS-B In traffic. The diamonds show proximate traffic that is non-threatening, with the unfilled shape meaning no immediate danger and solid for traffic within 6 nm and plus/minus 1,200 feet. Yellow circles indicate a TA situation, where an aircraft is a potential threat, but the pilot needs to take no action other than look for the target. An RA is shown by a red square and comes with audible commands that pilots are required to follow to avoid a collision, as well as guidance on the primary flight display if equipped. The commands are vertical-only, such as “climb,” “descend,” or “adjust vertical speed.”
ACAS X will add lateral maneuvers to the mix, which should make it more flexible, but on the whole, explained Morast, the intent with the ACAS X design standards is to make the functionality not much different to minimize training impacts during the transition to ACAS X. The technology does offer the opportunity for further improvements, however, and one that may result from the KDCA midair collision would be to reduce the alerting altitude. The midair collision happened below the altitude where TCAS is inhibited, and ACAS X may change that altitude as a result of congressional action.
<h2>ACAS Timeline</h2>
ACAS X is not one system but, due to the nature of upcoming types of airborne traffic, accommodates a variety of system-level capabilities.
ACAS Xa will be the main system replacing TCAS II, where required. It is already approved for operation in European airspace, although systems are not yet deployed.
ACAS Xo is an extension of ACAS Xa that will enable certain operations, such as closely spaced parallel approaches without excessive nuisance alerts.
ACAS Xu is for uncrewed aircraft.
ACAS sXU is for small uncrewed aircraft with wingspans of fewer than 50 feet.
ACAS Xr is for rotorcraft.
ACAS Xp is passive surveillance, envisioned as a system that uses ADS-B In to track intruders and doesn’t include active interrogations of transponder signals from other aircraft. This is intended for smaller aircraft.
ACSS is developing ACAS systems, but at present, the focus is on ACAS Xu for programs with Northrop Grumman and Lockheed, according to Morast. ACAS X is effectively mandated in the commercial and general aviation segments (where TCAS II is required, for example, for business jets) because the FAA has included the ACAS X technical standard order in the applicable minimum operational performance standards, he explained. However, ACAS Xa will be required only on newly type-certificated airplanes, in other words, not a new airplane that is certified under an amendment to an earlier type certificate, but a clean-sheet, new type certificate. That effectively means that the aviation industry will not see ACAS Xa implemented until the 2035 to 2040 timeframe, Morast told AIN.
Congress has the option to accelerate this timeline if politicians decide to vote for something like the ROTOR or ALERT Acts, or a combined version of them. Otherwise, the FAA could begin the rulemaking process, which takes many years but is the only other way new rules are implemented. AIN asked the FAA if anyone has submitted a petition for rulemaking to make ADS-B In mandatory, but the agency suggested that AIN use the Freedom of Information Act process to find that information. AIN also asked Tim Lilley, a pilot and safety advocate whose son Sam was one of the two pilots killed in the DCA midair, if he had submitted a petition for rulemaking, and he said, “We haven’t gone that route yet.”
The FAA, which could begin the rulemaking process on its own, told AIN, “The FAA supports ADS-B In and technologies like it that enhance situational awareness and safety.”
It appears that ACSS is the sole major avionics company making ACAS X-type equipment at this point. None of the other avionics manufacturers that make TCAS equipment responded to AIN’s query about their ACAS efforts. There are others, however, such as Reliable Robotics, which has developed ACAS equipment for its work on uncrewed general aviation airplanes for cargo operations.
This work gives ACSS a head start in developing various forms of ACAS, according to Morast, and the company plans to certify ACAS Xu (for uncrewed aircraft) in 2027 and ACAS Xr (rotorcraft) around 2029. “We’re well-positioned to certify ACAS Xa, when that [timeframe] is required,” he said.
For some time, it seemed as though ACAS X plans were dormant. “But now with the ALERT Act, our phones have been ringing. [Asking], ‘What’s this ACAS X they’re talking about? Who has it? When’s it going to be available?’”
<h2>ACAS and ADS-B</h2>
There is a fundamental difference between ACAS and ADS-B In, and that is the RA instruction feature of ACAS (currently TCAS II). “TCAS II has been around for a long time,” said Scott O’Brien, v-p of legislative affairs at Reliable Robotics. “It’s a great safety-enhancing technology. Whereas ADS-B In is providing traffic alerting, TCAS II is actually providing a traffic resolution advisory, telling the pilot they have to climb or make an aversion move to avoid traffic. On its face, TCAS II is a more robust technology than ADS-B In, and that’s why it’s required on larger aircraft.”
It should be noted that, contrary to some reports, both the ROTOR and ALERT Acts call for implementation of ACAS X and ADS-B In. The ALERT Act would require ADS-B In for all turbine aircraft that already must be equipped with ADS-B Out and all civil aircraft flying in Class B and C airspace by Dec. 31, 2031. The ROTOR Act has a similar timeline, but for a broader class of aircraft types.
As for ACAS X, the ALERT Act calls for a rulemaking committee to address whether more aircraft types would require ACAS Xa, while the ROTOR Act would require the FAA to develop an ACAS X action plan.
Realistically, nothing is going to happen soon with ACAS X, regardless of the congressional language. As ACSS’ Morast pointed out, ACAS X isn’t expected to enter service until sometime in the 2030s at the earliest.
O’Brien agrees that the ACAS X will be beneficial, especially the lower-altitude RA capability, which would have been helpful in the KDCA accident. However, he also noted, “Our position is that having broader equipment with ADS-B In and Out is beneficial. We as a country should be working towards having broader ADS-B-In equipage.”
But there are other aerial vehicles, such as balloons, gliders, ultralights, and drones, that might not carry the transponders that trigger ACAS/TCAS and ADS-B In, and Reliable Robotics needs to worry about these because its uncrewed aircraft will be flying in the same airspace. “So [our airplanes will have] an air-to-air radar that’s paired with ACAS Xu, and that is enabling us to have a capability where, if it has ADS-B Out, if it has TCAS, if it has nothing, we—to an extremely high level of safety—are going to, with our radar, detect that traffic and then avoid it.”
Nevertheless, he added, “For new entrants, ACAS X is a really important technology, because this is how eVTOLs and other technologies are going to integrate into busy airspace. So having legislation that promotes ACAS X is beneficial for safety of the current aircraft we have, and it’s helping invest in a technology that is going to enable new entrants to better integrate into the [national airspace].”

<h2>Why Not ADS-B In Now?</h2>
There appears to be widespread acknowledgment of the benefits of ADS-B In.
On February 26, less than a month after the KDCA midair collision, the NTSB urged the FAA to “require all aircraft operating in airspace where ADS-B Out is required to also be equipped with ADS-B In with a cockpit display of traffic information that is configured to provide alerting that is audible to the pilot and/or flight crew. To provide the same situational awareness advantages to military flight crews, the NTSB recommends that the Pentagon require all military aircraft operating in the national airspace be equipped with ADS-B In with a cockpit display that is configured to provide alerting audible to the pilot and/or flight crew, and that such a requirement apply wherever the FAA requires any aircraft to operate with ADS-B Out.”
According to Tim LeBaron, director of NTSB’s office of aviation safety, “We conducted a year-long investigation, published over 19,000 pages of evidence in our public docket, and issued 74 findings and 50 recommendations aimed at preventing another tragedy and saving lives. Efforts to now water down our evidence-based recommendations are counter to safety and dishonor the lives of 67 people who died on Jan. 29, 2025.”
ACSS parent company Acron Aviation conducted research to assess the current state of ADS-B In awareness. Given the potential for the ALERT Act to become law, which would mandate ADS-B In equipage by Dec. 31, 2031, Acron surveyed 100 U.S. airline leaders about ADS-B In.
The survey revealed that 47% “report their airlines have already equipped some of their fleet with ADS-B In, with a further two in five (40%) actively evaluating it; 86% “say they are prepared to comply with a mandate within three years; “Yet over half (51%) of the professionals responsible describe their knowledge of ADS-B In as general or less.”
One-third of the respondents did not understand the difference between ADS-B Out and In, according to the survey. “Awareness of the specific benefits ADS-B In brings is also weak. The headline benefit, real-time traffic awareness up to 180 nm, is known by six in 10 (61%). Half (50%) are aware of interval management for en-route spacing.
“Fewer know about the operational gains: two in five (42%) on reduced fuel burn and CO2 decreases, and just over a third (37%) on the reduction of go-arounds. To be expected, “Cost is the leading barrier to investment, cited by 38%, followed by competing capital priorities (33%) and the absence of a regulatory mandate (26%). A quarter (25%) cite insufficient awareness of the technology itself.”
While airlines tend to balk at upgrades that are not regulatory mandates, the benefits of ADS-B In have been well proven by American Airlines, which conducted an extensive test of the technology. “ADS-B In can be installed in as little as one to two shifts,” said ACSS’ Morast, “so there’s no lengthy downtime for aircraft. In addition, fleets equipped with the technology will see meaningful operational returns. The two-year FAA evaluation at Dallas-Fort Worth with American Airlines logged an average 12-second reduction in arrival spacing, equating to four to five extra landings per hour, per runway, with 490,000 pounds of fuel saved in the first year alone, and zero separation-related safety incidents.”
<h2>Changing Appetites</h2>
When ADS-B Out was mandated by 2020, there wasn’t an appetite for including ADS-B In in those rules. For ADS-B Out, even though it was a mandate, “There was a lot of pushback by the airlines,” he said, “because while it helped modernize our air traffic control management, it was really only beneficial to ATC. It didn’t bring the airlines anything; they were forced to spend money, install new equipment, but they didn’t get any real benefits out of it.”
Morast believes that airlines and other operators would have seen much more of a benefit had ADS-B In been part of the 2020 mandate. “Use of GPS as a position point for traffic is a leap forward from the ground radar tracking of aircraft. ADS-B just mandated that aircraft now broadcast their precise position and intent at one-second intervals.
“ACSS has been developing ADS-B In solutions since 2008, where we put that receiver in the flight deck, and now we give the pilots that information. Our intent, admittedly, was more on operational efficiencies, so there were ADS-B In applications that…in concept save fuel and time, and have more efficient arrival operations. It wasn’t really seen as a safety tool until the NTSB investigation around the KDCA accident highlighted the situational awareness improvement that ADS-B In and the display of ADS-B traffic in the flight deck brought to pilots.”
So while many people and groups are urging the government to mandate ADS-B In, Morast said, “It’s an industry that doesn’t move really quickly, and it’s a safety-focused industry, so there are some challenges in getting something deployed that quickly. I think that’s the path we’re on right now, to look at more broader ADS-B In equipage, and that, in my opinion, is the leap forward that either the ROTOR or the ALERT Act would bring to the industry.”
However, while the ROTOR Act seems to focus more on ADS-B In, the ACAS X display of directional traffic symbology comes only with ADS-B In information. “There’s no feature of ACAS X that does that without ADS-B,” he said. “It is ADS-B information that’s brought to the flight deck to allow directional display of traffic symbology, and that again is the leap forward.
“I see a little bit of a misperception between the two acts, but I think at the end of the day we’re going to pick the best pieces of those two acts and come up with a viable path to safer airspace.”
During a recent event, the Air Line Pilots Association (ALPA) urged action on ADS-B In. “The Jan. 29, 2025, crash near Washington National Airport, which killed 67 people, exposed a critical vulnerability: not every aircraft operating near busy airports is required to share the same real-time safety information,” the association noted.
“To be clear, this is not a matter of ROTOR versus ALERT. ALPA supports incorporating the strongest components of both pieces of legislation. The goal must be a final law that gives pilots the full picture when seconds count, strengthens real-time aircraft tracking, requires meaningful ADS-B In capabilities, and limits exemptions that could leave dangerous gaps in crowded airspace.
“As Congress works to reconcile the two bills, it must not settle for a partial fix.”
Fabrice Kunzi, president of avionics manufacturer Avidyne, summarized the ADS-B In situation by comparing the mandate for ADS-B Out to the advent of cellphones. “I don’t understand why we made the decision of splitting ADS-B into Out and In,” he said. “It’s kind of like, let’s build a cellphone that only broadcasts my voice. I have no idea what the other person’s saying, but I just keep broadcasting. Why do we split that?
“I was around when we did the ADS-B Out mandate, and there was a large amount of discussion and study groups and economic analysis and all kinds of stuff that was focused on whether the mandate should be ADS-B Out-only or cover ADS-B In. The way the chips fell in the end was that the benefit that ADS-B In provides is so massive that an operator would be foolish not to equip with ADS-B In right now. You can’t say the same thing about ADS-B Out, and that’s why that was mandated, because the market wouldn’t take care of that on its own.
“But they said once everybody’s equipped with ADS-B Out, obviously everybody is going to equip with ADS-B In. Now the problem is, ADS-B has this unique…Achilles heel, where my benefit from ADS-B In is directly proportional to the amount of equipage of everybody else. If I'm American Airlines, I'm going to get a lot of benefit once United and Delta and Southwest equip, but if they don’t, then I don't get as much benefit. You end up in this weird prisoner’s dilemma where you're like, ‘Okay, I’m not going to be the first, but once everybody else equips, then I’ll equip.’ And so everybody’s just kind of waiting and saying, ‘Are you going to go first? You’re going to go first, right?’ That’s what happened for the last 10 or 15 years. People just didn’t choose to equip.”

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