Acas X Technology and the Next Generation of FlyingResearch Paper

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¶ … Airborne Collision Avoidance Systems (TCAS/ACAS)

The collision avoidance systems TCAS/ACAS used by large craft are essential in avoiding midair collisions. Over time they have been developed in such a way that they can now supply pilots with advisories that, when heeded, can help guide the crafts out of dangerous situations where collisions may occur. The latest generation of system safety guidance based on the philosophy of the FAA has generated the ACAS X, which performs with greater accuracy and precision and reduces the amount of unnecessary warnings and advisories given to pilots. This Next-Gen system not only aids pilots in their piloting of large aircraft and secures the airspace around them, it also conserves energy and helps the entire industry to operate more efficiently and effectively. It has been developed with a number of concerns in mind and demonstrates the latest in collision avoidance systems technology.

Understanding Airborne Collision Avoidance Systems (TCAS/ACAS)


As Ho and Burns (2003) observe, midair collisions, while rare, do occur. Reducing and/or eliminating their risk is the goal of Airborne Collision Avoidance Systems (ACAS) and Traffic Alert and Collision Avoidance Systems (TCAS) (p. 119). Technological factors play a role in the reduction of that risk but so too does the managing of the human factor as the 2002 midair collision of a Russian airliner and a DHL cargo plane over Germany revealed: The Russian airliner received alerts from both the air traffic control (ATC) and the TCAS onboard the plane. The TCAS sent maneuver routes to both aircraft and had both followed these routes, the collision would have been avoided -- but the pilot of the Russian airliner opted to follow the guidance of the ATC, and a midair collision was the result. The incident raised questions about the element of risk involved in the "human factor," especially given the rigor that has gone into developing the TCAS through iteration, heuristic rules, the application of pseduocode, and the complex interactions utilized by TCAS logic to the point where TCAS is now "currently mandated worldwide on all large transport aircraft" (Ho, Burns, 2003, p. 119; Kochenderfer, Chryssanthacopoulos, Weibel, 2012, p. 27).


TCAS/ACAS is designed to prevent the type of midair collisions described above. This technology dates back to the 1970s when following a spate of such accidents the Federal Aviation Administration collaborated with Lincoln Laboratory to develop an onboard collision avoidance system. That system, however, has had to evolve over the years in order to adapt to the ever-changing airspace field. Building on the TCAS yet "rethinking" the concepts and providing a "new approach" to the engineering process, the ACAS X aims to set the bar in international airspace collision avoidance (Kochenderfer, Holland, Chryssanthacopoulos, 2012, p. 17). Part of the way it means to do this is by updating and re-conceptualizing the "logic used to select pilot advisories" deemed "difficult to modify" in the past TCAS system in the light of "new surveillance inputs" (Holland, Kochenderfer, Olson, 2013, p. 275). This new Next-Gen system is to be installed in all airplanes (Jeannin, Ghorbal et al., 2014).

Ten years ago the object of ACAS was simply to give pilots the tools needed for avoiding airborne collisions. With ACAS these tools included "resolution advisories (RAs), which recommend actions (including maneuvers), and traffic advisories (TAs), which are intended to prompt visual acquisition and to act as a precursor to RAs" (Airborne Collision Avoidance System Manual, 2006, p. 13). Then as now, ACAS does not rely on ground-based systems for avoidance and acts as a "back-up collision avoidance service" for air traffic control systems already in place. (This notion of being a "back-up," however, is what contributed to the disaster involving the Russian airliner and the cargo jet). The main purpose of the system was to reduce unnecessary alarms in instances where flight maneuverings are not warranted.

New Developments

Today, ACAS X focuses on upgrading the "design limitations of TCAS" by providing a more cost-efficient, safer, and more streamlined alert system that eliminates unnecessary alerts and mitigates risk in terms of de-escalating the situation through an "iterative tuning process" (Holland, Kochenderfer, Olson, 2013, p. 275). The positive outcomes of ACAS X tests against original TCAS systems reveals that the ACAS X is far more advanced: it "reduces collision risk by 59%, lowers the alert rate by 59%, and issues 28% fewer disruptive alerts" while also utilizing a far simpler alert sequence and providing fewer by half advisories for changes in maneuvering (Holland, Kochenderfer, Olson, 2013, p. 275). Essentially, the ACAS X is more refined, simplified, strategic, intuitive, and helpful to pilots.

The current safety status and philosophy in dealing with this system is found in Federal Aviation Administration's (FAA) embrace of the ACAS X technology, developing a plan for it to be manufactured as part of all large aircraft: it is to serve as a guidance system for pilots, designed as it is under a set of geometric configurations tested by researchers within a hybrid systems theorem framework. The FAA has determined that the system is the best next-generation collision avoidance alert system. The FAA takes system safety principles seriously and applies management and engineering principles to the overall framework for the optimization of safety. This, of course, requires a detailed plan that integrates engineering with training, experience, and application.

FAA System Safety Philosophy

The FAA defines system safety requirements within a program that promotes a balance of performance safety and cost. The ACAS X provides that unique balance of giving pilots the primary guidance needed to avoid collisions at a cost that is affordable for airlines. At the same time, no system safety principle can wholly eradicate risk and so there is the acceptance of limited risk in the area, which fall under hazard control implementation strategy. This strategy must be quantified, which can be difficult to do prior to actually having to act out the strategy, and with only hypothetical knowledge of an accident's severity, hazard control calculations are purely theoretical and hardly practical.

Essentially the safety system, in this case the ACAS X, acts as the guard against causes emanating from contributory hazards. Between the potential hazard and the potential harm, the ACAS X predicts a maneuvering strategy for both planes involved in sequence. Under this safety status, there are four categories of risk occurrence: probable, remote, extremely remote and extremely improbable. Of the four, probable is the most likely to occur. Its qualitative state exists in that it has been anticipated by developers of the safety system to take place at least once during the product's lifetime. Quantitatively, the probability of likelihood is expected per hour at a rate of 1 x 10 to the -5th power (or a 0.00001 rate of occurrence per hour). This rate decreases with the remote categorization to 0.0000001 per hour, and qualitatively is assessed to have an unlikely occurrence in the life of the product. Extremely remote is classified as having the possibility of occurring within an entire fleet once or twice, per se, and the extremely improbable is classified as having no likelihood of occurring. These ratings are then cross-indexed with severity ratings, ranging from catastrophic to negligible to produce a chart of consequence classifications that can guide the system safety assessment.

The top priority in the safety order of precedence is to design the system so that there is the very minimum of risk allowed. The second priority is to have safety devices incorporated into the strategy, along with warning devices (third priority) and procedurals/training (fourth priority -- the human element of the strategy).

The strategy can be effectively seen in simple ways such as the design of hardware to use low voltage (and if none is available then to use interlocks). Also a "5M System of Engineering" is utilized in the application: the M's consist of: Mission (aim of the safety system), Man (the human factor), Machine (the hardware/software combination), Media (the environment), and Management (the regulations, procedurals and policies). All of these contribute to the effectiveness of the ACAS X, which, as can be seen is merely one "M" of the 5 (FAA System Safety Handbook, 2000, p. 16).

Advanced Tech and Guidance Abilities

ACAS X works by detecting and tracking large aircraft utilizing sensors within surveillance systems on the plane giving velocity and placement locations of other planes. Algorithms coded into the ACAS allow the system to give high-probability readings of other planes in the vicinity "by representing relative positions and velocities as a probabilistic state distribution" based on the logic of the system's code (Airborne Collision Avoidance System X, 2015, p. 1; Wiolland, 2006).

The ACAS X improves and builds on the previous TCAS model in that its logic is able to be tailored according to various parameters or needs within the flying community of Next Generation unmanned travel, jets, airliners, drones -- all equipped with the ACAS system to ensure continuity and cohesion in the airspace. Not only does it provide vertical advisories (something TCAS did), but it also gives… [END OF PREVIEW]

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