The TCAS 2 mandate is being met as the FAA pushes the TCAS 1.


For the past year, all civil airliners with more than 30 seats operating in or into the USA have been equipped with the traffic-alert and collision-avoidance system (TCAS).

Only the BAC/Aerospatiale Concorde is, for technical reasons, exempt. Now, all those TCAS 2 installations feature improved software which, the US Federal Aviation Administration says, has dramatically reduced the false-alarm rate.

The TCAS is a family of airborne devices functioning independently of the ground-based air-traffic-control (ATC) system and providing collision-avoidance protection for a broad spectrum of aircraft types. It is mandatory in the USA, but, if it is to achieve global acceptance, the success of the new software is crucial.

FAA officials and the TCAS manufacturers - which include AlliedSignal Commercial Avionics Systems, Rockwell International's Collins Air Transport division and the Honeywell Air Transport Systems division - believe that the airborne safety device will eventually become an international standard.

Commuter operators in the USA are scrambling to fit the TCAS 1, since the FAA moved forward the expected compliance date (Flight International, 11-17 January, P9). Meanwhile, work on the next-generation TCAS 4 is expected to culminate in a flight evaluation in late 1997.



In 1986, an AeroMexico airliner collided with a small aircraft over Cerritos, California, killing 82 people. The following year, the US Congress ordered TCAS 2 equipment to be installed on passenger aircraft with more than 30 seats. The FAA requires aircraft with ten to 30 passenger seats to have the TCAS 1 installed by 31 December, 1995, two years earlier than first planned.

The TCAS 1 provides proximity warning only, to assist the pilot in seeing intruder aircraft. The TCAS 2 offers both traffic advisories (TAs) and resolution advisories (RAs) - recommended escape manoeuvres in the vertical plane - to avoid conflicting traffic. Displayed TAs and RAs are supplemented by synthetic-voice advisories generated by the TCAS computer.

The TCAS 4, which has replaced the terminated TCAS 3 programme, would provide RAs in the horizontal, as well as the vertical, plane. Although ordering development of the TCAS 3, the US lawmakers did not require the system to be fitted.

The TCAS makes use of the radar-beacon transponders routinely carried by aircraft for ground-ATC purposes. The level of protection provided depends on the type of transponder which the target aircraft is carrying. Mode A offers TAs, while Mode C provides both TAs and vertical RAs. When both aircraft have TCAS 2 equipment, a Mode S data interchange allows a co-ordinated resolution.

Mode S has a selective-address feature. The TCAS listens for the spontaneous transmissions (squitters) generated once a second by Mode S transponders. The squitter contains the individual Mode S address of the sender.

Following receipt of a squitter, the TCAS sends a Mode S interrogation to that address and, from the reply, determines range, bearing and (if available) altitude of the target aircraft. The TCAS tracks each Mode S target, using conflict-avoidance logic for advisory detection and display.

New software, known as Version 6.04A, was developed when it was ascertained that earlier versions yielded unnecessary altitude-crossing RAs in encounters involving climbing and/or descending aircraft levelling off with 1,000ft (300m) of altitude separation.

By 31 December, 1994, all TCAS 2 units were updated with Version 6.04A, the software change being considered a major step in fine-tuning the TCAS 2 to be more acceptable to both pilots and air-traffic controllers. The FAA's TCAS programme manager, Thomas Williamson, says: "Indications are that there has been a reduction in unnecessary alerts, ranging between 50% and 70%."

Williamson says that Version 7 is due to be introduced within the next two to three years, to incorporate operational enhancements and technical improvements proposed by the aviation community and features required by proposed International Civil Aviation Organisation (ICAO) standards and recommended practices (SARPS) for hardware, software and procedures for an airborne collision-avoidance system (ACAS). The project head says that Version 7 might involve some minor hardware changes, but that the bulk of the changes will involve the software.

He says that draft ACAS SARPS were approved in November 1994 and are being evaluated by ICAO member states. He estimates that ratification should occur in late 1995. The SARPS would set an international standard on which individual nations would then base their own TCAS 2 implementation.

The international aviation community is monitoring the US TCAS programme closely, as many countries consider whether to adopt the system. Williamson says: "We have not met a domestic or foreign air carrier which opposes TCAS. It is a question of timing and money. I don't think any airline can afford not to put TCAS in their aircraft. Lawyers will have a field day if there is a mid-air collision and TCAS was not installed."

The UK, for example, is evaluating an ACAS for its airspace, but it is unclear whether the UK Civil Aviation Authority, or Eurocontrol, will back required use of the TCAS in European airspace. France, Germany and Switzerland are also said to favour TCAS implementation.

Based on discussions with UK aviation officials, US industry sources believe that the CAA will attempt this year to mandate the TCAS 2 for aircraft with 19 or more passenger seats. They say that the UK wants to require TCAS 2 installations before the end of the decade, but they must still overcome obstacles in getting Eurocontrol to go along with the concept and set a timetable.

British Airways is not waiting. It is buying the TCAS for fleet-wide installation and an official requirement for at least 300 units is pending. BA aircraft now being flown in US airspace carry systems made by AlliedSignal and Rockwell International.



The TCAS 3 was terminated because directional-antenna bearing-error evaluations showed that accurate horizontal-resolution advisories could not be achieved. The problem was caused by disturbances in antenna patterns from aircraft structures and from other antennae located close to the TCAS antenna. Resolution advisories could not be translated from the vertical to horizontal plane.

The FAA believes that the TCAS 4 can be achieved by using data from the global-positioning system (GPS). Research to date indicates that horizontal RAs are attainable by using the Mode S transponder employed with the TCAS 2. The concept is to use Mode S to transmit each aircraft's GPS position and velocity vector to other TCAS-equipped aircraft.

Williamson is "very optimistic" that the GPS will make the TCAS 4 a reality. He says that a prototype squitter has been demonstrated successfully at the MIT/Lincoln Laboratory. "We know we can squitter the GPS position. We know we can monitor the GPS squit and react accordingly," he adds.

"We expect to have a unit we can fly in demonstration in late 1997 or early 1998. A six-to-eight-month flight-evaluation will take place, using a revenue-passenger aircraft," he adds. Williamson expects to complete the research-and-development (R&D) needed for TCAS 4 certification in 1999-2000. Air carriers would then have the option to adopt the system. He says that it is unclear whether the TCAS 2 can be upgraded to the TCAS 4 configuration, although retrofit would be the ideal situation.

Tom Mullinix, AlliedSignal's TCAS programme manager, says that the company is using internal funds for TCAS 4 work. He hopes to win an FAA contract to finish the research. Based on the early R&D, Mullinix believes that his firm's TCAS 2 can be modified to the TCAS 4 configuration.



All TCAS 2 units have the capability to downlink RAs to Mode S ground stations and the FAA has experimented with implementation of the TCAS RA-downlink feature. Controllers could thus be notified automatically of RAs displayed in aircraft under their control.

"There might be some benefit in bringing the controller into the loop. Overall ATC system-management may be improved by letting controllers immediately see the RAs generated in the aircraft cockpit, rather than having the controller wait to be notified by a pilot after the fact," says Williamson.

Up to now, no provision has been made to transmit the RAs to ATC sites from Mode S ground stations for display.

In 1994, the FAA conducted a test at Baltimore/Washington International Airport, in which ten controllers participated in a simulation experiment designed to define the human-factors interface requirements for the displayed RA information.

Initial indications are that RA downlink is useful to controllers in some RA situations and the FAA will demonstrate the RA downlink in a six-month field evaluation at Boston's Logan International Airport, beginning late this year, says Williamson.

While many pilots and the airlines say that the TCAS is a protective bubble which prevents close encounters of the wrong kind, some controllers do not believe in its effectiveness. They are particularly upset about situations where pilots respond to TCAS warnings without first notifying ATC. Some controllers want the TCAS disabled while others prefer to see it operated without RAs.

President of the 15,000-strong US National Air Traffic Controllers Association, Barry Krasner, says that his concern stems from "the system's proven faulty logic, inadequate training for TCAS users and premature implementation of the system". The group says that TCAS RAs "...add a chaotic element to an already strained system". It is unclear whether immediate notification of RAs will mollify the controller.

Based on test results to date, the TCAS may be used routinely in the future for in-trail climbs. In 1994, the FAA - in conjunction with United Airlines and Rockwell International and Delta Air Lines and AlliedSignal - conducted successful separate operational in-trail climb tests over the Pacific Ocean. In the flight evaluation, one aircraft climbed over another, using the TCAS to maintain safe separation. They believe that the method is a safe and efficient means of preventing one aircraft becoming "trapped" beneath another in oceanic airspace and that it produces major fuel savings.

The International Federation of Air Line Pilots Associations (IFALPA) has spoken out against the concept, but Mullinix hopes that IFALPA will embrace the method once it sees the results of the FAA evaluation, which have yet to be published. Williamson says that the test data continues to be analysed and that a final decision on the concept will be made later this year.

Source: Flight International