The Traffic Alert and Collision Avoidance System, or TCAS, is an instrument integrated into other systems in an aircraft cockpit. It consists of hardware and software that together provide a set of electronic eyes so the pilot can "see" the traffic situation in the vicinity of the aircraft. Part of the TCAS capability is a display showing the pilot the relative positions of aircraft up to 40 miles away. The instrument sounds an alarm and issues a Resolution Advisory when it determines that another aircraft will pass too closely to the subject aircraft. TCAS provides a backup to the air traffic control system¿s regular separation processes.
The MITRE Corporation conducted early research into collision avoidance technologies under the sponsorship of the Federal Aviation Administration (FAA). TCAS is a direct descendant of technologies invented at MITRE and elsewhere.
Since the early 1960s, MITRE's Center for Advanced Aviation System Development (CAASD) has provided the FAA with Air Traffic Control (ATC) system engineering support. As part of this longstanding partnership, CAASD helped the FAA implement a collision avoidance system for aircraft. The resulting Traffic Alert and Collision Avoidance System, or TCAS, has become a standard for safety in the United States and abroad. Its value is clear: no airline mid-air collisions have occurred in the United States since 1990, when the airlines began equipping their planes with TCAS.
From its inception, TCAS has dramatically improved pilots' chances of successfully averting the threat of a mid-air collision. Pilots have come to rely on TCAS to give them the crucial data to avoid collisions. As their last line of defense, TCAS gives pilots the edge needed to ensure that their crew and passengers have the safest flight possible.
The project benefited from the cooperative efforts of the FAA, airlines, and several other companies. CAASD designed and developed the collision avoidance logic at the heart of the system. The Massachusetts Institute of Technology's Lincoln Laboratory developed air-to-air surveillance. The FAA Technical Center and a team of contractors, including The Analytical Sciences Corporation, Coleman Research Corporation, and Rannoch Corporation, were responsible for software verification and validation. The FAA Technical Center and ARINC Research handled operational evaluations.
On June 30, 1956, two planes collided over the Grand Canyon. In the wake of this and other such airborne disasters, the industry realized they needed a system that could help prevent similar incidents. Companies soon began designing collision avoidance systems, but two problems hampered their efforts. First, adoption of the proposed systems would require the airlines to equip their fleets with expensive new hardware. Second, there was still a lot of development left to do before an adequate system would be ready.
In 1974, MITRE proposed an alternative. Using the transponders already installed in many aircraft for communication with the FAA's ground-based Air Traffic Control Radar Beacon System (ATCRBS), developers took advantage of existing technologies to significantly hasten the design and implementation process. The Beacon-Based Collision Avoidance System (BCAS) was the predecessor of today's TCAS. This system sent interrogation signals to nearby aircraft similar to the FAA's radar system. The transponders then sent back response signals. The system interpreted these signals to determine the location, speed, and course of each plane and used the data to avoid a potential collision.
BCAS test results were promising. On the ground, MITRE equipped a trailer to receive transponder signals as if it were an aircraft. BCAS lived up to expectations, prompting the FAA Technical Center to test the system on one of its aircraft. On the basis of these two tests, the FAA moved forward with further development of BCAS.
A Collision Avoidance System Is Born
In 1981, the FAA chose to pursue the onboard design approach used in BCAS rather than a ground-based collision avoidance system which was also under consideration. At that point, BCAS was renamed TCAS.
There are two different versions of TCAS, for use on different classes of aircraft. The first, TCAS I, indicates the bearing and relative altitude of all aircraft within a selected range (generally 10 to 20 miles). With color-coded symbols, the display indicates which aircraft pose potential threats. This constitutes the Traffic Advisory (TA) portion of the system. When pilots receive a TA, they must visually identify the intruding aircraft and may alter their plane's altitude by up to 300 feet. TCAS I does not offer solutions, but does supply pilots with important data so that they can determine the best course of action. An illustration of TCAS range and altitude criteria shows the horizontal and vertical distances to monitor traffic and issue advisories to maintain safe separation of aircraft.
In addition to a traffic display, the more comprehensive TCAS II also provides pilots with resolution advisories (RA¿s) when needed. The system determines the course of each aircraft; climbing, descending, or flying straight and level. TCAS II then issues an RA advising the pilots to execute an evasive maneuver necessary to avoid the other aircraft, such as "Climb" or "Descend." If both planes are equipped with TCAS II, then the two computers offer compatible RA¿s. In other words, the pilots are protected from maneuvers advice that would effectively cancel each other out, resulting in a continued threat.
TCAS queries other aircraft, receives information, displays traffic and reacts by
warning pilots when there is a potential threat.
MITRE's key contribution to the development of TCAS was its work on the collision avoidance logic for TCAS II. The software uses the collected data on the flight patterns of other aircraft and determines if there is a potential collision threat. The system doesn't just show the other planes on a display like a radar screen, but offers warnings and solutions in the form of traffic advisories (TA¿s) and resolution advisories (RA¿s).
As CAASD's Dr. Andrew Zeitlin points out, "Because of the pilots' normal workload, we don't expect them to spend all of their time looking at the screen. It's there when needed, but more important, it speaks up and advises them as they need to make a maneuver to avoid a collision."
Aside from the logic design, much of MITRE's work on TCAS involved creating and running computer simulations to test the system. "Because it's expensive to fly test encounters," says Dr. Zeitlin, "we have developed some very powerful tools where we can generate millions of encounters on the computer and evaluate the logic exhaustively. We can also play back radar data from ordinary traffic and get a feel for how the system works and how much disruption you get day to day or at different locations with ordinary traffic." On occasion, MITRE has also assisted the FAA and other organizations in evaluating special encounters. "For example, if somebody has a near-miss and they want to know what TCAS's role was or what would TCAS have done in the encounter, we can simulate the encounter and give advice," says Zeitlin.
Taking to the Skies: The Congressional Mandate
On August 31, 1986, while TCAS was still in development, a collision occurred over Cerritos, California, involving an Aeromexico DC-9 and a small Piper aircraft carrying a family of three. The DC-9 was descending toward Los Angeles International Airport in clear skies, flying at 6,500 feet. The Piper hit the DC-9's tail, causing both aircraft to plummet from the sky.
The accident resulted in the deaths of all 67 people aboard the two planes, as well as 15 people on the ground. In the aftermath of this accident, Congress passed a law requiring the FAA to mandate the use of TCAS. By 1993, all carrier aircraft operating within U.S. airspace with more than 30 passenger seats were equipped with TCAS II. Aircraft with 10 to 30 seats were required to employ TCAS I.
Evolving to Meet Safety Needs
When the airlines were using the more advanced version 6.01 of the TCAS logic, ti became clear that some improvements still needed to be made. The system was issuing RA¿s in some situations, such as final approach, when traffic may be close but is safely under control. Many pilots saw these RA¿s as a nuisance. The system was basically too sensitive, with unnecessary TA¿s and RA¿s even being triggered by transponders on bridges and ships.
According to Dr. Zeitlin, "There was a growing tendency among pilots to ignore the advisory, even when they didn't necessarily have full knowledge of the situation. Everyone was concerned that one day they would ignore one that was necessary."
In 1992, CAASD developed logic version 6.04 to alleviate these problems. Delta Airlines, the first carrier to voluntarily use the new logic, reported an 80 percent reduction in RA¿s. The following year, CAASD developed an additional improvement to the logic, version 6.04A. Airlines began equipping their fleets with this version in 1994.
The Final Generation
In 1997, CAASD finished work on one final major change to the TCAS logic, version 7. It was approved by the RTCA standards committee and the FAA, and is the version installed on all new aircraft. It has also been adopted by the International Civil Aviation Organization (ICAO) as the international standard. Version 7 is required for aircraft throughout European and most other countries. American carriers who fly to these countries have had to upgrade from 6.04A to 7 on their international planes, and can voluntarily upgrade the equipment already on their U.S. fleets. Version 7 also will be required in the U.S. for operation in Reduced Vertical Separation Minima (RVSM) airspace beginning in 2005.
Version 7 logic yields at least a 20 percent reduction in RA¿s over the previous version. CAASD ran simulations using radar data from Europe, where they encounter more high-altitude en route conflicts. The 7.0 software resulted in a 40 percent reduction in unnecessary RA¿s. The new logic also significantly improves TCAS performance in several other important areas.
CAASD personnel conducted safety studies to evaluate the performance of each successive version of the TCAS logic. In a 1997 report on version 7, CAASD's Dr. Michael McLaughlin examined the reduced risk of collision in aircraft equipped with TCAS II versus the risk in aircraft without TCAS. Based on the likelihood of incursions into a protected zone around aircraft with a radius of 500 feet and a height of 200 feet--defined as Critical Near Mid-Air Collisions (NMACs)--McLaughlin concluded that "TCAS should reduce NMAC probability by at least 90 to 98 percent," depending on whether one or both aircraft in an encounter are equipped with TCAS.
Though NMACs, especially those involving commercial, passenger aircraft are already extremely rare, McLaughlin notes that "TCAS is intended to reduce their probability even further."
Although the FAA has said that version 7 will be the final logic for TCAS, the monitoring of Version 7 usage in Europe has illuminated several technical and operational issues. CAASD is participating in a new RTCA effort to evaluate a change proposal. Other work within RTCA and ICAO is considering issues regarding the interoperability of TCAS with other technologies, such as ADS-B. CAASD's expertise undoubtedly will ensure and ongoing role in the evolution of the airborne collision avoidance function.
April 6, 2008