Traffic collision avoidance system

A traffic alert and collision avoidance system, also known as an Airborne Collision Avoidance System, is an aircraft collision avoidance system designed to reduce the incidence of mid-air collision between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of MAC. It is a type of airborne collision avoidance system mandated by the International Civil Aviation Organization to be fitted to all aircraft with a maximum take-off mass of over or authorized to carry more than 19 passengers. In the United States, CFR 14, Ch I, part 135 requires that TCAS I be installed for aircraft with 10–30 passengers and TCAS II for aircraft with more than 30 passengers. ACAS/TCAS is based on secondary surveillance radar transponder signals, but operates independently of ground-based equipment to provide advice to the pilot on potentially conflicting aircraft.
In modern glass cockpit aircraft, the TCAS display may be integrated in the navigation display or electronic horizontal situation indicator.
In older glass cockpit aircraft and those with mechanical instrumentation, an integrated TCAS display including an instantaneous vertical speed indicator may replace the mechanical IVSI, which only indicates the rate at which the aircraft is descending or climbing.

Impetus for a system and history

Research into collision avoidance systems has been ongoing since at least the 1950s, and the airline industry has been working with the Air Transport Association of America since 1955 toward a collision avoidance system. ICAO and aviation authorities such as the Federal Aviation Administration were spurred into action by the 1956 Grand Canyon mid-air collision.
Although ATCRBS airborne transponders were available, it was not until the mid-1970s that research focused on using their signals as the cooperative element for a collision avoidance system. This technical approach enabled an independent collision avoidance capability on the flight deck, separate from the ground system. In 1981, the FAA decided to implement the Traffic Alert and Collision Avoidance System, which was developed based on industry and agency efforts in the field of beacon-based collision avoidance systems and air-to-air discrete address communication techniques that used Mode S airborne transponder message formats.
A short time later, prototypes of TCAS II were installed on two Piedmont Airlines Boeing 727 aircraft, and were flown on regularly scheduled flights. Although the displays were located outside the view of the flight crew and seen only by trained observers, these tests did provide valuable information on the frequency and circumstances of alerts and their potential for interaction with the ATC system. On a follow-on phase II program, a later version of TCAS II was installed on a single Piedmont Airlines Boeing 727, and the system was certified in April 1986, then subsequently approved for operational evaluation in early 1987. Since the equipment was not developed to full standards, the system was only operated in visual meteorological conditions. Although the flight crew operated the system, the evaluation was primarily for the purpose of data collection and its correlation with flight crew and observer observation and response.
Later versions of TCAS II manufactured by Bendix/King Air Transport Avionics Division were installed and approved on United Airlines airplanes in early 1988. Similar units manufactured by Honeywell were installed and approved on Northwest Airlines airplanes in late 1988. This limited installation program operated TCAS II units approved for operation as a full-time system in both visual and instrument meteorological conditions on three different aircraft types. The operational evaluation programs continued through 1988 to validate the operational suitability of the systems.

Incidents

The implementation of TCAS added a safety barrier to help prevent mid-air collisions. However, further study, refinements, training and regulatory measures were still required because the limitations and misuse of the system still resulted in other incidents and fatal accidents, which include:

1996 Charkhi Dadri mid-air collision accident over New Delhi
1999 Lambourne near-collision, involving a Boeing 737-300 and a Gulfstream IV. The airspace above Lambourne is the waiting zone for Heathrow. The event is notable as both planes entered the zone from different directions leading to an imminent head-on collision. The traffic advisory almost immediately turned into a resolution advisory with a projected time for collision of less than 25 seconds.
2001 Japan Airlines mid-air incident, in which the Captain of Japan Airlines Flight 907, 40-year old Makoto Watanabe, chose to descend, ordered by the air traffic controller, when TCAS told the flight crew to climb, nearly colliding with the descending JAL Flight 958 DC-10 en route from Busan to Tokyo's Narita Airport.
2002 Überlingen mid-air collision, between a Boeing 757 and a Tupolev Tu-154, where the Tupolev pilots declined to follow their TCAS resolution advisory, instead following the directions of the air traffic controller, while the Boeing pilots followed their TCAS-RA, having no ATC instruction.
2006 collision between Gol Transportes Aéreos Flight 1907 and an Embraer Legacy 600; the Embraer's transponder had inadvertently been switched off, disabling its own TCAS and rendering the plane invisible to the TCAS on board flight 1907.
2011 Fribourg near-collision, involving Germanwings Airbus A319 Flight 2529 and Hahn-Air-Lines Raytheon Premier I Flight 201. Air traffic control at Geneva allowed flight 2529 to sink to flight level 250 but entered flight level 280 as usual for handover to traffic control at Zürich. Air traffic control at Zürich allowed flight 201 to climb to flight level 270. This triggered a resolution advisory for the Airbus to sink and for the Raytheon to climb which was followed by both aircraft. Nine seconds later Geneva instructed the Raytheon to sink to flight level 260 which they then followed. It led to a situation where both planes passed at minimum distance. Shortly later the Raytheon was lower than the Airbus and TCAS issued a reversal RA for the Airbus to climb and for the Raytheon to sink.
Overview

System description

TCAS involves communication between all aircraft equipped with an appropriate transponder. Each TCAS-equipped aircraft interrogates all other aircraft in a determined range about their position, and all other aircraft reply to other interrogations. This interrogation-and-response cycle may occur several times per second.
The TCAS system builds a three dimensional map of aircraft in the airspace, incorporating their range, altitude, and bearing. Then, by extrapolating current range and altitude difference to anticipated future values, it determines if a potential collision threat exists.
TCAS and its variants are only able to interact with aircraft that have a correctly operating mode C or mode S transponder. A unique 24-bit identifier is assigned to each aircraft that has a mode S transponder.
The next step beyond identifying potential collisions is automatically negotiating a mutual avoidance manoeuver between the two conflicting aircraft. These avoidance manoeuvers are communicated to the flight crew by a cockpit display and by synthesized voice instructions.
A protected volume of airspace surrounds each TCAS equipped aircraft. The size of the protected volume depends on the altitude, speed, and heading of the aircraft involved in the encounter. The illustration below gives an example of a typical TCAS protection volume.

◇	Distant traffic
◆	Traffic within 6 NM horizontally and 1200 feet vertically
●	Traffic close enough to trigger TA, within 40 seconds of potential collision
■	Traffic close enough to trigger RA, within 25 seconds of potential collision

System components

A TCAS installation consists of the following components:
;TCAS computer unit: Performs airspace surveillance, intruder tracking, its own aircraft altitude tracking, threat detection, resolution advisory manoeuvre determination and selection, and generation of advisories. The TCAS processor uses pressure altitude, radar altitude, and discrete aircraft status inputs from its own aircraft to control the collision avoidance logic parameters that determine the protection volume around the TCAS aircraft.
;Antennas: The antennas used by TCAS II include a directional antenna that is mounted on the top of the aircraft and either an omnidirectional or a directional antenna mounted on the bottom of the aircraft. Most installations use the optional directional antenna on the bottom of the aircraft. In addition to the two TCAS antennas, two antennas are also required for the Mode S transponder. One antenna is mounted on the top of the aircraft while the other is mounted on the bottom. These antennas enable the Mode S transponder to receive interrogations at 1030 MHz and reply to the received interrogations at 1090 MHz.
;Cockpit presentation: The TCAS interface with the pilots is provided by two displays: the traffic display and the RA display. These two displays can be implemented in a number of ways including displays that incorporate both displays into a single, physical unit. Regardless of the implementation, the information displayed is identical. The standards for both the traffic display and the RA display are defined in DO-185A.

Operation

The following section describes the TCAS operation based on TCAS II, since this is the version that has been adopted as an international standard by ICAO and aviation authorities worldwide.
FAA guidance notes that multiple ACAS II variants may be permitted in U.S. airspace, including TCAS II versions 6.04A Enhanced, 7.0, and 7.1, as well as ACAS Xa, and describes equipage constraints for certain operations such as RVSM airspace.