How is ACAS Developing in Business Aviation?

Having offered an outline of Airborne Collision Avoidance Systems (ACAS) and an overview of Traffic Collision Avoidance Systems (TCAS) operations in business aircraft, Ken Elliott looks at hybrid surveillance TCAS, and ACAS X beyond TCAS II…

Ken Elliott  |  26th April 2023
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    Ken Elliott
    Ken Elliott

    Ken Elliott is a veteran with 52 years of aviation experience, focussed on avionics in General and Business...

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    Some manufacturers of TCAS II are offering hybrid surveillance TCAS that uses data provided by Mode S transponders, modified and retrofitted for ADS-B. The ADS-B passively monitors and provides position and trend information on aircraft within range.

    Hybrid surveillance TCAS II can sense and display targets replying to active Mode S transponder interrogations and passive ADS-B In data from other aircraft. Indeed, hybrid surveillance TCAS may increase the maximum range of target aircraft from 100nm to 150nm.

    Changes TCAS II v7.0 to 7.1 TCAS v7.0 could present conflicting advisories and cause pilot misunderstanding, such as confusion over the response to an “adjust vertical speed adjust” aural advisory, or two opposing aircraft commanding the same visual IVSI/ADI climb or descend command.

    TCAS II v7.1 resolved these potential conflicts by coordinating the data between two aircraft to avoid similar commands and changing an aural alert (see Figure 3 below).

    A typical avoidance alert sequence would be:

    1.    Initial “climb, climb” or “descend, descend”.
    2.    Changes the next alert by replacing “adjust vertical speed adjust” with “level off, level off”.
    3.    “Clear of conflict”.

    Comparisons of TCAS II v7.0 with v7.1
    Figure 3a: 'Adjust Vertical Speed, Adjust' vs 'Level Off, Level Off' - Aural Alerts

    Figure 3b: RA Reversal Issued After a Non-Compliance Threat is Detected

    ACAS X (beyond TCAS II)

    While TCAS II v7.1 is an effective traffic advisory and avoidance system, its older technology has been surpassed by advancements in processing. There is also a need for an ACAS that can deal with closer aircraft proximity due to a higher traffic density, and different modes of flight via Urban Air Mobility (UAM), including eVTOL and Remotely Piloted Aircraft Systems (RPAS).

    Keeping in mind that TCAS II is an ACAS X system, its evolution as ACAS Xa will implement newer technology.

    The ICAO ACAS X standard spans multiple aircraft types and user groups, requiring new processing methods in collision avoidance. Evolving aircraft platforms such as eVTOL and RPAS are smaller than current TCAS II equipped aircraft, and this demands constraints of Size, Weight and Power, plus cost (SWAPc), in the development of future products.

    Aside from the ACAS X (implemented as TCAS I and II), there are currently five planned standards within the ACAS X collision advisory family, including:

    • ACAS Xa: An active surveillance version to replace TCAS II, as a seamless transition for the flight crew.
    • ACAS Xo: A future and enhanced version of Xa that will function for close operations, such as parallel runways, reducing nuisance advisories.
    • ACAS Xp: Similar to a TCAS I, this future General Aviation version of ACAS X will rely solely on passive ADS-B In to monitor intruders.
    • ACAS Xr: Designed to support rotorcraft, eVTOL, and associated UAM operations.
    • ACAS Xu (including ACAS sXu): A version of Detect and Avoid (DAA) collision avoidance, specifically designed and developed for RPAS, including small drones (sXu).

    Space Based ADS-B, when fully available and operating at extremely low signal latency, will play a role in enroute passive surveillance and collision prevention, too.


    ACAS is reliant on information provided by other aircraft systems. Figure 4 (below) demonstrates this connectivity. Note that TCAS II is reliant on the radar altimeter, currently undergoing resolution for potential 5G interference concerns.

    Figure 4: Typical TCAS II System Block Diagram

    This system provides ‘Height Above the Ground’ that is essential for the TCAS II RA solution. The TCAS II will provide a very different RA when the aircraft maneuvers close to terrain than when enroute at higher barometric altitudes.

    Because other targets may either be above or below the interrogating aircraft, directional antennas must be installed on top of, and beneath the aircraft fuselage for both TCAS and Mode S transponder systems.

    Typically, most aircraft will use the same controller for selection of both TCAS, and Mode S transponders’ functions. Because aircraft manufacturers design cockpits around a suite of integrated avionics, the display of TCAS can vary by each make and model of aircraft. 

    At a minimum there must be an IVSI display for an RA and 360-degree map display for aircraft within range.

    Is ACAS/TCAS compulsory for Business Aviation operations in your part of the world? Continue reading this article in the AvBuyer April digital edition, or online by clicking the Page 3 button below to find out…

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