Traffic Alert and Collision Avoidance System and the Human Element
Terry D. Gilfillan
Embry-Riddle Aeronautical University

Abstract
This paper describes the history of the Traffic Alert and Collision Avoidance System (TCAS). It will discuss the types or categories of and the operation of the system of TCAS, to include the components involved, how the system interacts with other aircraft that are equipped with TCAS, and the related warnings and/or notifications of the system. Also discussed is the human element related to the operation and interaction of TCAS. These human elements include the human senses and situational awareness.

Traffic Alert and Collision Avoidance System and the Human Element
Two planes collide in the skies somewhere over the Grand Canyon on 30 June 1956. Because of this collision and many others, the aviation industry came to the realization that some type of system or process had to be created to help with the prevention of these collisions. Several companies began the design of collision avoidance systems, however it wasn’t easy, as they had to overcome two specific obstacles. First, the air carrier would have to install or outfit their fleets with the new hardware for the system and that would prove to be very expensive. The second obstacle; there would still be a lot of planning and development to do before TCAS would be up and running efficiently. That being said, TCAS was still in development on 31 August 1986 when a midair collision between a small Piper aircraft with a family of three on board and an Aero Mexico DC-9 with manifested passengers. This collision occurred over Cerritos, California and the result was 67 people aboard the two aircraft and 15 people on the ground perished. (The MITRE Corporation, 2006). This horrible tragedy resulted in Congress, in 1987, to pass legislation mandating that the Federal Aviation Administration (FAA) implement an airborne collision avoidance system for passenger-carrying aircraft by the end of 1992. The result was that all aircraft operating in United States airspace and were designated as carrier aircraft with 30 or more passenger seats were required to equip these aircraft with TCAS II and aircraft with just 10 – 30 seats were equipped with TCAS I. (The MITRE Corporation, 2006).
There were a number of mishaps, which led to the congress mandating TCAS. The type of aircraft determined exactly what system would be installed on that particular aircraft. Traffic collision and avoidance system and its use today will be discussed in this paper as well as the operation for the system from the human element. TCAS involves many components that work as one to alert the user to other aircraft in the area. TCAS works automatically, but the human element or human factors still play a role. These elements or factors include human senses like seeing and hearing, decision making and situational awareness. The factors are not just limited to the pilot operating the aircraft, but to the air traffic controllers as well. When TCAS was in it’s developmental stages, all of these factors, perhaps more, were taken into account.
As mentioned earlier, there are two types or categories of the TCAS system. The first is TCAS I, which was designed and developed strictly for the “little guys,” regional airlines and general aviation. TCAS I is solely used for traffic advisories or TA. Traffic advisories are auditory alerts given to pilots when traffic is near. These alerts are designed to assist pilots with identify traffic visually and to ensure separation is maintained. This system is required on aircraft that have between 10 – 30 seats. TCAS II is pretty much the same system which the exception of it contains a component that will issue a resolution advisory or RA which is the highest alert level and in turn gives pilots both and aural and visual pitch commands that will assist the pilot in avoiding a mid-air collision. (Heinlin, H.) The requirement for this system is that aircraft configured with at least 30 seats or more and has a maximum takeoff weight exceeding 15,000 kg or 33,000 lbs. must be installed on these of aircraft. (Federal Aviation Administration) However, beginning 1 January 2005, the requirement changed to mandate that aircraft configured with 19 seats or more and have a maximum takeoff weight of 5,700 kg or 12,500 lbs. (Introduction to TCAS II)
The TCAS systems will provide aural or visual alert notifications if an aircraft is in the area and that aircraft must have a mode S transponder installed to communicate with TCAS. If the aircraft is equipped with TCAS II then the alerts will actually allow the pilot to make corrective movements or evasive maneuvers in order maintain enough distance between both airplanes to prevent a catastrophe.
Pilots are notified by aural or visual cues in TCAS that aircraft with installed with transponders that are in the vicinity and which may identify a potential mid-air collision. There are a number of TCAS systems in which TCAS can actually track a number of targets and this is dependent of the type of system/aircraft. The mode S transponder is very complex and relays a ton of information of what an aircraft is doing. These transponders can coordinate with TCAS when two airplanes are on a collision course and are equipped with both the transponder and TCAS; it will relay to each aircraft the best plan of action to prevent a mid-air collision. The two systems, TCAS and the transponder, will coordinate and agree on which action to give the aircraft. It will actually notify each aircraft to either descend, maintain altitude or climb. The components of TCAS are what make up the brains of the system. Components of TCAS are the Mode S/TCAS Control Panel, the Mode S transponder, the TCAS computer, antennas, traffic and resolution advisory (TA and RA respectively) displays and a aural annunciator. (TCAS II) See Figure 1 below.

Figure 1. TCAS II Block Diagram. This diagram illustrates the components that make up the TCAS system. (TCAS II)

Of all these components that make up the TCAS system, the mode S transponder is by far the key component of TCAS.
Transponders are nothing more than a GPS unit of sorts that air traffic controllers can use to maintain location of an aircraft. The controllers will assign a four-digit code that is exclusive to an aircraft at a given time and the pilot will enter that code into the aircrafts transponder. The code is assigned to an aircraft so ATC can identify aircraft on radar. This four-digit code is referred to as a squawk. As the aircraft moves across the ATC’s radar screen he or she can quickly identify what aircraft it is. This is essential on keeping aircraft flying through the controllers’ airspace as he or she can keep proper and safe separation of the aircraft and to avoid mid-air collisions. While transponders are important when the aircraft is in the air, some larger airports require aircraft to leave the transponder on while taxing around the airfield to avoid any ground collisions with other aircraft. Additionally, the transponders when programmed can transmit an aircrafts heading, airspeed, and the climb/descent rate and the aircrafts altitude. In order for the transponder to send and receive signals TCAS uses antennas.
A component of TCAS, antennas, are used with TCAS to assist in the identification of other aircraft that may be in the area. These antennas are used to communicate with other aircraft by transmitting the aircraft’s location, speed, etc. to other aircraft that are TCAS equipped. Aircraft with these antennas normally will have two antennas installed on the belly and to the top of the fuselage. Although there are two antennas, they will not be used at the same time, but as the system dictates. As TCAS was being developed, there were safety aspects put into place that wouldn’t allow transmitting TCAS while the onboard transponder is transmitting. This would cause false readings and could result in a collision. (Callback 321)
The heart of the entire TCAS system is the TCAS computer. This computer “controls the airspace surveillance, intruder and own aircraft tracking, threat detection and resolution and advisory generation.” (Vert. speed/TCAS) This computer also is equipped with a receiver/transmitter that requests information from other aircrafts transponders. That information that is obtained through the respective transponders are processed in conjunction with other data receives like pressure and radio altimeter data which in-turn controls the collision avoidance parameters that establish the protection area around the TCAS equipped aircraft. If a collision threat is determined, the TCAS computer will decide the best avoidance maneuver. If the aircraft that is determined to be the threat aircraft is installed with TCAS, then the avoidance maneuver will be coordinated through the Mode S transponders. (Vert. speed/TCAS)
The TCAS computer transmits interrogation messages at 1030 MHz to ATC transponders of nearby aircraft and receives reply messages at 1090 Mhz. The Mode S transponders will then generate squitter messages at the rate of about one per second. These squitter messages contain the aircrafts unique identification code. When a squitter message is received from a nearby Mode-S transponder, TCAS sends a Mode-S interrogation to the address contained in the message. The reply is used to determine range, altitude and bearing of the intruding aircraft. Thus, the two different transponders will determine position relative to each other based on the information exchanged with one another. (TCAS - How Does It Work?)
The area in which TCAS scans the airspace is up to 40 NM horizontally in front of the aircraft and 9900 ft. vertically above and below the aircraft. The number of aircraft that may be monitored or tracked by TCAS at any one time is up to 30. (Vert. speed/TCAS)
The computer will then get the avionics that is installed on the aircraft involved by transmitting signals to the avionics. The avionics then transmits the information to the flight deck which in-turn is displayed on the TCAS display. Depending on what aircraft type and what system version is installed on an aircraft will determine the colors and shapes used within the system to identify aircraft and altitude. Some examples for traffic display would be; a white aircraft symbol would mean “own aircraft” and is displayed continuously. A range ring, which is white, means a two-mile radius range around the aircraft. If a solid red square is displayed, that means there is a threat-level intruder and if a solid yellow circle is displayed, then there is a potential threat-level intruder. Finally, if there is a displayed cyan colored solid diamond, there is proximate traffic within ±1200 ft. and 6 nm of own aircraft and if a cyan colored open diamond is displayed, then this means there is other traffic beyond 6 nm and/or greater than ± 1200 ft. of own aircraft. (Table 3C-109. TVI-920 Transponder/TCAS Display Symbols)
Warnings in TCAS are displayed many different ways. There are warnings and notifications that may be an aural warning or a visual warning. Aural warnings that are initiated by TCAS will be dependent on the pressing issue or scenario. Warnings that are given by TCAS are done so by a automated voice. These warnings can range from traffic alerts like “Traffic, Traffic!” to pilot instructions. The pilot may hear climb or descend if that is what is required to avoid a mid-air collision. Other aural warnings that are associated with TCAS are; Climb, Climb; Descend, Descend; Climb, Climb NOW, Climb, Climb NOW, Climb; Adjust Vertical Speed, Adjust; and Monitor Vertical Speed just to name a few. (SKYbrary - TCAS Occurrence (OGHFA SE))
The human element of TCAS is certainly a factor that has to be taken into consideration when using this system. If the front-end crew flying the aircraft does not see or hear the aural or visual warnings, then the human element in this has played a role. The first human element is the human senses. Pilots need to be aware to the goings on around them. If they are not listening they may not hear “Traffic, Traffic!” thus not take the appropriate measures to avoid the traffic. Also, if the pilot is not monitoring or scanning then TCAS computer display screen, then there’s a chance of a mid-air collision.
Situational awareness plays a huge role in the successful use of TCAS. Good situational awareness gives the pilots a heads-up of aircraft around them. They will know their heading, altitude and location. Pilots scan out of their windscreen for traffic and don’t just relay on TCAS.
Other elements with the human element arena are the physical aspects of operating the airplane. Things such as filing correct flight plan, physically flying the airplane and ensuring that the flaps are set correctly for both takeoff and landing and that the landing gear is down for landing. All of these elements to the human element play a huge role in the operation of an aircraft safely. The human element can never be taken out of the equation.
TCAS is just one component of a multi-layered defense against mid-air collisions. It has come a long way from nothing to preventing a number of mid-air collisions. TCAS is a very simplified system, but at the same time a very safe and technological innovation within the realm of aviation safety. The components involved within the TCAS system all play a vital role in the effective operation of the system. The heart of the TCAS system, the TCAS computer, is a well-advanced piece of technology that is very precise and accurate that enables the system to function flawlessly in preventing mid-air catastrophes. The human element can never be taken out of anything in aviation, no matter how advanced the technologies get. Sometimes the human element is a good thing, especially in aviation safety. TCAS has certainly increased the human element by leaps and bounds when it comes to situational awareness. Pilot’s using TCAS are forced in a sense, to collaborate with TCAS by way of seeing and listening/hearing and as a result must react to the alerts given by TCAS. TCAS has been a savior when it comes to preventing mid-air collisions and like anything there is room for improvement. TCAS has been here for a while, is well established in the accomplishment of safety of flight now and well into the future.

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