Product Focus: TCAS

Obviously encouraged by the fact that no midair accidents involving commercial aircraft have taken place since traffic alert and collision avoidance systems (TCAS) came into widespread use, regulatory agencies are expanding the requirement to install these units.

In the United States, the Federal Aviation Administration (FAA) has issued a notice of proposed rulemaking (NPRM) that would require cargo airline carriers to equip their fleets with TCAS II by Dec. 31, 2002. Despite the cargo carriers’ and the Air Cargo Association’s firm belief that automatic dependent surveillance-broadcast (ADS-B) is a relatively more advanced technology that can do a better job to prevent midair collisions (see March 2002, page 21), the proposed rule probably will become law.

This will place cargo carriers in tandem with passenger-carrying airlines operating in the United States. Since 1993, FAA has required that airliners carrying more than 30 passenger seats be equipped with TCAS II and, since 1995, that air transport aircraft with 10 to 30 seats have TCAS I on board.

Playing Catch-Up

However, the United States is playing catch-up. Eurocontrol required the installation of the airborne collision avoidance system (ACAS II) by March 31, 2001, in all commercial aircraft that carry more than 30 passengers or are heavier than 33,075 pounds (15,000 kg). And the agency plans to extend that requirement to aircraft that have more than 19 seats or are heavier than 12,570 pounds (5,700 kg), effective Jan. 1, 2005. Other countries, including India, Australia and Japan, also have TCAS mandates.

ACAS II is the most capable system, having the most advanced collision avoidance software, called Change 7. Replacing the Change 6.04 software (introduced in 1994) in TCAS II, Change 7 incorporates 225 system-logic updates. According to a white paper, Change 7 covers "surveillance logic, collision avoidance logic, and aural/visual annunciations to address system performance issues, such as phantom resolution advisories and unclear annunciations."

Verbal resolution advisories (RAs) issued by TCAS have been modified by Change 7. For example, the command "adjust vertical adjust" is given rather than "reduce climb" or "reduce descent." At the same time, the TCAS vertical speed indicator shows a "fly-to" green arc, instructing the pilot to level off after a climb or descent RA.

Change 7, designed to significantly reduce nuisance alerts, has had FAA approval since 1999. But the U.S. agency hasn’t mandated Change 7’s use. Nevertheless, all new factory-installed TCAS II systems incorporate the newest software.

Coming a Long Way

TCAS has a fairly long history. As far back as the 1950s, Bendix Avionics developed the computer algorithm that defines the rate of closure between approaching aircraft. The quest for a collision avoidance system came to a head on June 30, 1956, when two airplanes collided over the Grand Canyon. In 1974, the Mitre Center for Advanced Aviation System Development (CAASD) invented the surveillance function for TCAS, according to Andrew Zeitlin, Mitre CAASD’s principal engineer for ATM avionics. The company proposed the use of the transponders that are already installed on aircraft for communication with FAA’s ATC radar beacon system. The proposed system sent interrogation signals to nearby aircraft, similar to the ground radar system.

FAA had been weighing the option of a ground-based or airborne collision avoidance system, but in 1981, the agency opted for the airborne design. At about the same time, Mitre CAASD turned the surveillance technology over to the Massachusetts Institute of Technology’s Lincoln Lab, so it could concentrate on the collision avoidance logic and on safety studies for TCAS. In an FAA program to develop TCAS, Mitre CAASD and Lincoln Lab worked with a team that included ARINC Research (operational evaluation) and Coleman Research and Rannoch Corp. (software verification). TCAS test and evaluation was conducted at the FAA Technical Center–"We flew a lot airplanes at each other," says Zeitlin. Meanwhile, RTCA drew up the minimum operational performance standards (MOPS). FAA certified TCAS in the late 1980s.

There are two versions of TCAS. TCAS I systems indicate the bearing and relative altitude of aircraft within a selected range. They provide a display of surrounding air traffic and sound an alert, or traffic advisory (TA) of a potential conflict. TCAS II takes the extra step of also providing the pilot with RAs, i.e., the best action to take to avoid a conflict.

Further advancements have been made to TCAS, such as self-limiting interrogation. This has been developed to protect against possible interference with ground radar. Both air traffic control radar and TCAS use the 1030- and 1090-MHz frequency channel and, because TCAS normally interrogates other aircraft as often as every second, a high number of TCAS aircraft in an area could impact radar performance. With the self-limiting feature during TCAS surveillance, the units in an area "count one another, and when the density gets too high, they automatically reduce their power or interrogation frequency," says Zeitlin.

Although manufacturers are continually improving their TCAS products, FAA currently feels no urgency to plan another software upgrade program like Change 7, according to Zeitlin. The technology "is not perfect," he adds, "but TCAS with Change 7 is very successful."

Any plans to increase situational awareness or detect traffic at longer ranges probably will involve the use of ADS-B, Zeitlin concludes. Adopting ADS-B, which employs GPS navigation and a data link, also would help reduce the airborne use of the 1030- and 1090-MHz frequency channels.

TCAS for Rotorcraft

So far, TCAS manufacturers have targeted largely the operators of corporate, regional and large air transport aircraft. But, Honeywell recently announced the introduction of its KTA 970 TCAS I for helicopters and small fixed-wing aircraft. It has been certified to TSO (technical standard order) C118 minimum operational performance standards (MOPS). The new TCAS can track up to 60 aircraft and display up to 30 aircraft at ranges up to 40 nautical miles.