Collision Avoidance Systems in enhancing aircraft safety

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Collision Avoidance Systems are instruments that are have been developed to enhance air traffic safety. The systems have been developed to prevent mid-air collisions by detecting threat aircrafts in air traffic and developing means of maneuvering to avoid collisions. To ensure effective application of collision avoidance systems, human factors have been greatly impacted by the technology.  Introduction of the systems have called for further training of aviation workers to be able to utilize the system effectively and also to fit into the demands of the system. This paper discusses Collision Avoidance Systems and their applications. In addition, the paper discusses the human factors implications of introducing collision avoidance systems in the aviation industry.

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            Collision Avoidance Systems (CAS) is a set of hardware and software that are used together with other systems in the aircraft to enhance safety by providing the pilot with better means of visualizing the air traffic and avoiding collision. The systems are used for avoiding unintended collisions with other aircrafts or even collisions with obstacles in the air or the ground. The introduction of collision avoidance systems in aviation is an effort to increase air safety. Mid-air collision is avoided by the systems providing surveillance measures on any aircraft that may collide with the subject aircraft. From this information, the aircraft crew is able to take appropriate collision avoidance measures usually involving vertical separation. By introducing the CAS, the human components in flights have been compelled to adjust so as to fit into the system (Hawkins, 1993). While the system has been designed to be easily used by aircraft crew, it has demanded the crew to receive relevant training to be able to respond effectively to the demands of the technology.

Development of CAS

After a tragic air accident involving collision of two commercial aircrafts in 1956 in Grand Canyon, the aviation industry was compelled to find ways of preventing air traffic collision in the future (Kuchar & Drumm, 2007). Between 1971 and 1975, there were proposals for development of Collision Avoidance Systems that would be installed on aircrafts to facilitate safety in the air. The Beacon-based Collision Avoidance System was among the first CAS to be fitted in aircrafts. This technology was borrowed from the military aircraft where transponders were already being used. It was however unfortunate that the BCAS had limitations especially in high density air.

            Although it was not mandatory to fit the BCAS into all aircraft, the U.S. Federal Aviation Administration (FAA) saw the need to develop a better Collision Avoidance System. This was the birth of the Traffic Collision Avoidance System (TCAS). The well known collision avoidance system is fitted in most aircrafts that have more than 15,000 kilograms of maximum takeoff weight. When fitted on the aircraft, TCAS equips the pilot with electronic ‘eyes’ with which the pilot can be able to see the traffic that is close to the aircraft. The TCAS can be able to indicate the traffic that is up to 40 miles away and it responds by an alarm. Upon realizing that another aircraft is close to the aircraft, a Resolution Advisory (RA) is issued and the pilot is able to control the aircraft through the separation process hence avoiding collision (MITRE CAASD, 2010).

            Air traffic can be controlled through various means of separation as determined by not only the Air Traffic Control but also by pilots. Since these separation methods can vary with time and the specific circumstances, the TCAS is supposed to be used mainly as the last resort in air traffic conflict. This means that it is a solution when in cases where normal separation is not working. Since the TCAS should come as a last resort, the system should be such that it does not interrupt with normal separation efforts. If it happens that the system is disruptive, then it would not cease to be air safety measure and instead it can lead to more conflict. Disruptions can occur if the system issues false alarms and signals or the frequency of issuing the alarms and signals is not set to one that would allow enough time to maneuver collision (Kuchar & Drumm, 2007). The fact that the Collision Avoidance Systems are supposed to take effect when all other separation means have failed means that the system should not be reliant (or should have the least reliance) on any of the other systems which have already failed.

            As a conflict resolution means, Collision Avoidance Systems should also be made such that they are functional in almost every place whether the place is remote or it is an oceanic place. It implies that a ground support system is not a necessity and rather the system works independently once fitted in the aircraft otherwise it would be dysfunctional in remote areas or in oceanic places. Also remembering that the system is fitted to facilitate maneuvering of aircraft in case of a conflict involving the one or more aircraft, the other aircraft must be able to communicate with the subject aircraft. The Collision Avoidance Systems in each aircraft should be able to complement the maneuvers of the other aircraft thereby avoiding collision. It is also notable that the CAS should be such that once put into use, the likelihood of resolving the traffic conflict should be the highest (Williams, 2004).

            As earlier stated, the Traffic Collision Avoidance System is made such that a pilot is able to visualize track all the aircrafts that are close to the reference aircraft. From this, it becomes possible to make an assessment of how the threat aircraft is moving and thereby determine the appropriate maneuver. In specific, the TCAS able to determine an aircraft that is inappropriately close to own aircraft in about 45 seconds prior to the Closest Point of Approach (CPA). As such, the TCAS issues a Traffic Alert (TA) which makes the flight crew aware of the conflict thereby facilitating the crew to view the aircraft that is conflict with. The issuance of a Resolution Advisory happens in case the possibility of the collision increases which is mainly an approximate 30 seconds prior to Closet Point of Approach. The importance of the Resolution Advisory is to instruct the crew whether to or not to climb or descend. This is why the RA is given on a vertical plane. The importance of having the other aircraft fitted with TCAS is realized at this point since the RA is supposed to be conveyed to the threat aircraft thus facilitating complementary maneuvers. Upon both aircrafts communicating on the potential conflict, the crew act as fast as possible and eventually a vertical separation of about 300-800 feet is achieved (Williams, 2004). The minimum vertical separation distance is mainly recommended to be 400 meters (Kuchar & Drumm, 2007). Further vertical separation that may be required is communicated by the TCAS since it continues evaluating the situation till the threat is no longer in existence. At this point, the Resolution Advisory is removed and a sign to indicate that the conflict is over is issued. This directs the crew to go back to the normal air traffic level.

            It is pertinent to note that there exists two variants of TCAS i.e. TCAS I and TCAS II.  Each version is used in specific aircrafts and has different features with TCAS II having more advanced features. The specifications for which aircraft will use specific TCAS are in accordance with a law passed by the Congress in 1986 (MITRE CAASD, 2010). TCAS 1 is supposed to be fitted in all aircraft that has 10-30 seats. On the other hand, all aircraft having 30 seats and over are supposed to have the TCAS II. TCAS I is has less features than TCAS II. TCAS I have the capability of locating the position and altitude of any aircraft that may be within 10-20 miles. Potential threat is communicated by means of color-coded symbols which are the Traffic Advisory component of the system. TCAS I is limited in that instead of providing the crew with solutions to the threat, the crew is only enabled to visualize the threat aircraft and then left to make appropriate maneuver decisions. On the other hand, TCAS II is capable of providing vertical RA in addition to Traffic Advisory where the pilot is advised whether to climb or to descend with complementary maneuvers facilitated if the other aircraft has the same system (Kuchar & Drumm, 2007).

            There have been improvements on traffic avoidance systems both to minimize chances of inappropriate signaling as well as incorporating features other systems that avoid terrain. The TCAS for instance was characterized by a very high sensitivity to traffic even when the traffic was in controllable vicinity. The system would therefore send inappropriate TA and RA signals which were a nuisance to aircraft crew. Sometimes bridge or ship transponders would trigger the TA thus making the pilots develop a tendency to pay no attention to the advisory sometimes without realizing what the real cause is. Improvements have however been done on the systems and new TCAS versions have emerged with reduced RAs. This has the effect of reducing inappropriate conflicts arising from unnecessary signals (MITRE CAASD, 2010). On another note, terrain avoidance systems have been merged with traffic avoidance systems. This combination of terrain awareness systems with traffic awareness systems offers aircraft crew not only ability to avoid traffic collision but also to avoid terrain collision during separation. This is an enhanced safety feature in the aviation industry (Thales, 2010).

Human factors implications of CAS

            The introduction of Collision Avoidance Systems into the aviation industry has had an impact on factors in aviation. It is first important to understand so some of the human factors involved in aviation. According to Reinhart (2008) human factors can take different definitions including the human-machine interface, the psychological wellbeing of the aircraft crew as well as being able to function even when in extreme situations. Human factors are very important in flight as they determine airworthiness. They are therefore considered as an integral part to flight safety. The Software, Hardware, Environment and Liveware (SHEL) model developed by the International Civil Aviation organization (ICAO) is an important tool for understanding the importance of human factors in aircrafts and how the factors interact. Human factors in the SHEL model are the Liveware which interacts with all other components including human-machine interaction and human-human interactions (Hawkins, 1993).

            Recognizing the introduction of the CAS into the aviation industry as introduction of a new machine helps in determining how human factors are affected. In addition, there are specific human-human interactions that are required with the coming of the CAS. In essence, there is a need to maintain safety in air traffic. Humans interact with hardware, software, the flight environment as well as interaction with other humans. For effective interaction of the human factors with the Traffic Collision Avoidance System to be achieved, adequate quality training on how to operate the CAS has been a strict requirement in the aviation industry. This acts as a means of enhancing controller proficiency. Professional training of all the persons involved in aviation (i.e. ATC and pilots) has been a requirement for smooth and effective application of the collision avoidance systems. For instance, the ability of the pilot to operate the TCAS and to respond to its cues has been an important aspect created by the introduction of CAS (Hawkins, 1993). Equally, it should be noted the ATC team is also a useful component in preventing collisions since they are involved in communications with the pilot on any collision that may be ahead.

            Aircraft crew has had to understand the human-machine interface that exists in TCAS. For instance, the pilot is supposed to understand the Plan Position Display of the TCAS such that it becomes possible to indicate the possible position of the threat aircraft. The Plan Position Display utilizes three color symbols of which a pilot must understands well for appropriate measures to be taken to avoid collision. A white symbol indicates that the aircraft that has led to generation of a TA is not of much interest whereas a red symbol indicates that the aircraft is a threat and a RA symbol is usually issued. A yellow symbol indicates a disappearing threat aircraft. Understanding such symbols among other data provided by the TCAS is pertinent if air safety is to be guaranteed (Williams, 2004). This has implications on training since every pilot must now be trained on these aspects of the modern technology in air safety.

            While designing the Collision Avoidance Systems, there has been a need to consider cultural differences for safety to be a guarantee. This forms part of the human interactions with the system software. For instance, an aural component of the human-machine interface requires that pilots receive aural alerts which are supposed to guide the pilot on the action to take to avoid collision. It has been identified that the pitch used to communicate the aural alert may influence the pilot’s action depending on the pilot’s background. For instance, while pilots from Western background will respond appropriately to high pitched voice, pilots from other cultures tend to ignore the female pitch and will act with a male voice which is more authoritative (Williams, 2004). Design of aural prompts to fit specific groups of pilots is therefore an important implication of the CAS.

            Flight physiology being an important component of human factors has been one of the significantly aspects affected by the Collision Avoidance Systems. The visual component of the human physiology has been stressed for a pilot to be component enough to visualize and respond to the warnings of the TCAS. When for instance the TA and RA are issued, the pilot is required to take several actions which mean that he/she must utilize their visual power. Issuance of a TA especially with (TCAS I) requires the pilot to try and visualize the threat aircraft and then decide on the appropriate separation move to take. In case of RA, the pilot has to be well aware that prompt action must be taken within 5 seconds without which conflict is likely to occur. The implication of such precise and detailed instructions is that pilots must be trained enough on how to recognize and respond to the RA and TA signals. In addition, they must undergo refresher training as frequent as possible for them to keep up to date and competent with the TCAS requirements.

            As earlier mentioned, the psychological wellbeing of the pilot must be optimum or otherwise decision making on how to use the available resources appropriately and promptly is impaired. It should be noted that as much as the Collision Avoidance Systems are an automation step of improving air traffic safety, the human component is very crucial and all players must remain alert and sober. If the crew is not sober, the likelihood of failure to effectively respond to the cues of CAS is very high and safety is not guaranteed (Reinhart, 2008). It is however important to note that CAS are an important automation in the aviation industry’s safety which have enhanced the guarantee of an aircraft crew undertaking their task with almost assured prevention of mid-air collision.

                        Human-human interactions have been greatly impacted by the TCAS. This is especially in the communication and coordination of safety during flight. Whereas it was initially the mandate of the Captain to ensure safety in a flight, the introduction of CAS has caused ICAO to change free the Captain from this responsibility (Williams, 2004). This however should be noted that it happens only in case of a collision threat and the aircraft is fitted with the TCAS. The change of this mandate implies that all the crew members must cooperate and work together towards responding promptly to any RA that may be issued. Important to remember is that RA requires as prompt action as possible and therefore there is no provision for evaluating the RAs wisdom.


            Collision Avoidance Systems have been a very useful technology in preventing mid-air collisions in air traffic. With the ability to perform surveillance of any threat aircraft within vicinity and thereby provide the pilot with collision avoidance maneuvers, the systems have enhanced air safety to a large extent. It is no wonder that the U.S. Congress mandated the fitting of Collision Avoidance Systems in all aircrafts flying within U.S. air. The Traffic Collision Avoidance System developed by U.S. FAA is the best known CAS and has served effectively as an air-safety instrument. Improvements in the designs of the CAS are expected to continue enhancing air traffic safety. With introduction of the CAS, human factors involved in safe flight have been affected in different ways. It has been important to offer enough training to all persons involved in aviation especially the aircraft crew. Although the systems have enhanced flight automation, they have also demanded more alertness and sobriety on the part of pilots and aviation traffic controllers. In essence, human interaction with machines (hardware and software), the environment as well as human-human interactions have been restructured to accommodate the technology. Overall, Collision Avoidance Systems are a big welcome to the aviation industry for their ability to enhance air safety and humans must be ready to bear the implications of this worthy technology.


Hawkins, F. H. (1993). Human factors in flight. ISBN: 978-1857421354: Ashgate Publishing.

Kuchar, J. K. and Drumm, A. C. (2007). The traffic alert and collision avoidance system. Lincoln Laboratory Journal, 16(2): 277-296.

MITRE CAASD. (2010). Traffic alert and collision avoidance system. Retrieved 13, April 2010

Reinhart, R. O. (2008). Basic flight physiology. 3rd Edition. ISBN007149488X, 9780071494885: McGraw-Hill Professional.

Thales. (2010). Terrain and traffic collision avoidance system – T2CAS. Retrieved 12, May 2010 from

Williams, E. (2004). Airborne collision avoidance system. 9th Australian Workshop on

Safety Related Programmable Systems (SCS’04), Brisbane. Conferences in Research and Practice in Information Technology, Vol. 47. Tony Cant, Ed.


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