Traffic Alert and Collision Avoidance System (TCAS): Meaning, Operation, Types

Traffic Alert and Collision Avoidance System (TCAS) is an aircraft safety system and a mandated unit of every large commercial aircraft's flight deck avionics package. Operating continuously while the transponder is activated, this subsystem of aircraft avionics uses radio signals for surveillance of nearby aircraft and serves as the primary source of collision avoidance when pilots do not visually see other traffic.

As a last-defense mechanism against mid-air collision, TCAS generates a Traffic Advisory when another aircraft is less than 4 nm horizontally and 1,000 ft vertically away, or simply within sensor range. If the predicted time to closest approach drops below 25 seconds, the system escalates to a Resolution Advisory that commands an immediate avoidance maneuver. By incorporating these alerts into the FAA's safe and efficient air navigation system, TCAS forms a safety layer in aviation.

Expert behind this article

Jim Goodrich

Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.

What is TCAS in aviation?

TCAS is an aircraft collision avoidance system, specifically the traffic alert and collision avoidance system, which is an airborne system designed to increase cockpit awareness of nearby aircraft and reduce the risk of mid-air collisions. TCAS generates collision warnings known as Traffic Advisory (TA) and TCAS I is able to monitor the traffic situation around an aircraft.

ICAO Annex 10 defines Traffic Alert and Collision Avoidance System (TCAS) as a last-resort airborne system that performs air-to-air surveillance. TCAS is a backup to air traffic separation service, to right-of-way rules, and to see-and-avoid doctrine. It is never a substitute for normal separation but becomes active when those layers fail. The system predicts the future path of nearby aircraft, computes closure rates, and calculates time to reach the Closest Point of Approach by dividing range by closure rate. When threat-detection criteria are satisfied, TCAS announces two escalating alerts.

A Traffic Advisory (TA) is a ‘heads-up’ indication that an aircraft is getting close. TCAS I provides this advisory together with details on bearing and altitude of nearby traffic. A Resolution Advisory (RA) is an instruction that must be followed by the pilot. TCAS II issues the RA, instructing climb or descent and providing specific vertical escape paths so that both aircraft are coordinated. After the conflict is resolved, TCAS announces Clear of Conflict when the danger has passed.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What is the purpose of a TCAS antenna in an aircraft?

A TCAS antenna exchanges data with nearby transponders so the computer can build a protected airspace around the aircraft. One antenna is typically mounted on top of the fuselage, and one is mounted on the bottom. The top antenna is directional and scans the area above, while the bottom antenna, which is omnidirectional or directional, scans the area below. The pair gives 360-degree coverage in both horizontal and vertical directions and automatically switches to optimize signal strength and reduce multipath interference.

How does aircraft TCAS work?

Aircraft TCAS works in tandem with the aircraft's transponder. Each second, the TCAS computer interrogates nearby transponders on 1090 MHz. The interrogation-and-response cycle occurs several times per second with high frequency. Replies contain the intruder's altitude, range and bearing, letting TCAS use its algorithm to determine when to issue TAs or RAs. When an intruder enters the closest protection envelope, TCAS elaborates a resolution advisory; the TCAS computer decides when to issue RAs. Resolution advisory parameters are pushed into register 30 of the aircraft SSR transponder, and the transponder sends the resolution advisory in a DF16 message on 1090 MHz. The resolution advisory gives explicit coordinated instructions to avoid a collision. RA displays advise the pilot how to maneuver, whether to climb or descend. Intruder TCAS uses the received resolution advisory to determine a complementary maneuver, establishing vertical separation.

The TCAS aviation antenna is a pair of L-band elements mounted top and bottom of the fuselage, connected to the transponder. These antennas radiate the 1090 MHz interrogations and listen for replies, giving spherical coverage. Because TCAS ties every calculation to altitude, barometric altitude from the air-data computer is fed to the transponder and differential altitude is compared against established thresholds so that only genuine collision paths trigger an advisory. Performance standards require that TCAS detect traffic within 14 nautical miles and provide at least 25 seconds of warning before the closest point of approach, guaranteeing safe separation at any flight level.

In my view the amazing feature of TCAS is its capacity to handle advisories between contrasting aircraft. This cooperative system prevents both aircraft from creating the similar elusive action.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

How does TCAS detect other aircraft?

TCAS monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder. TCAS does not detect aircraft that are not transponder equipped. TCAS works by sending interrogations to other aircraft's transponders. The device’s surveillance function operates by issuing interrogations at 1030 MHz that transponders on nearby aircraft respond to at 1090 MHz. These signals interrogate the transponders of other aircraft nearby. An onboard system sends its own interrogations and detects those replies. TCAS employs a receiver to detect replies to interrogations from transponders on nearby aircraft. The distance to the other aircraft is calculated from the time difference between the interrogation and the reply. Using repeated interrogations, TCAS can track the other aircraft's direction and closing speed.

TCAS continuously predicts the future path of nearby aircraft relative to your own. When TCAS pings the area, any aircraft within range that has its transponder on will answer with its presence and altitude. A transponder responds with identification and altitude data when it receives a radar or TCAS signal. TCAS offers information on the approximate bearing and altitude of other aircraft. After receiving transponder signals from nearby aircraft, the TCAS system constructs a 3-D map of the airspace. TCAS tracks up to dozens of aircraft simultaneously.

TCAS detects other aircraft by listening for their transponder replies. Each jet announces location and height, so the instrument receives the signals of close aircraft and builds an extensive photograph of the traffic scene. Because the system does not depend on radiolocation, it must differentiate a close bogey on a possible conflict track from a far airplane at good height. I deem this system extremely useful as it allows aircraft to maintain safe distances.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What are the types of TCAS in aviation?

The types of TCAS in aviation are given in the table below.

The Federal Aviation Administration has identified three classes of TCAS equipment: TCAS I, TCAS II and TCAS III.

TCAS I is the cheaper, lighter system intended for smaller aircraft. It provides traffic advisories up to forty-eight seconds before the closest point of approach, yet it offers no maneuver or resolution advisories. The pilot must decide any evasive action after receiving the traffic display.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

TCAS II is the most comprehensive and widely adopted version. Installed on most modern airliners, it adds coordinated resolution advisories to the traffic advisory function, instructing pilots to climb or descend safely when a conflict is detected.

TCAS III was a proposed next-generation version meant to provide horizontal as well as vertical resolution advisories but has not been widely deployed.

Do GA aircraft have TCAS?

GA aircraft are not required to have TCAS. Some turbine-powered general aviation aircraft are equipped with TCAS II, yet TCAS II cost significantly higher than GA aircraft typical, so GA operators with TCAS II will likely be flying business jets. For lighter airplanes, TCAS I can be installed on general aviation fixed-wing aircraft and helicopters, but most rely on alternatives. General aviation uses PCAS, FLARM, ADS-B IN or see and avoid. PCAS provides traffic alerts but no resolution advisories, while FLARM is a passive collision avoidance system and ADS-B IN is used by many private pilots for collision avoidance. GATCAS, a general aviation TCAS avionics, incorporates a low-power solid-state transmitter, a 16-bit microprocessor and a microprogrammed sequencer, offering a lower-cost option.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

Can a plane fly without TCAS?

Aircraft fly without operational TCAS on some occasions, yet aircraft operating without operational TCAS have limited or no protection depending on the equipment of the intruder aircraft. Operators voluntarily decide not to dispatch an aircraft without TCAS in certain airspace, while some departure or destination airports do not accept aircraft without TCAS. Germany allows aircraft to operate without TCAS II for up to 3 days, and EASA rules allow an aircraft to operate with TCAS II out of service for up to 10 days. A monitoring study in core European airspace reports 1.4% of TCAS-equipped aircraft operated with TCAS out of service.

ATC procedures are designed to assure flight safety without reliance on TCAS II, but TCAS will not provide warnings if aircraft are flying without transponders activated. Some mid-air collisions occurred where TCAS did not provide warnings because aircraft were flying without transponders activated. Military aircraft operate with their transponders off based on mission requirements, further reducing TCAS effectiveness. Looking ahead, ACAS Xa will be a direct replacement for TCAS II, and ACAS Xo will be collision avoidance tuned to work in some currently difficult operational situations like closely spaced parallel approaches.

I would be hesitant in boarding an aircraft that lacked a Traffic Alert and Collision Avoidance System, for its absence would remove a layer from the defense-in-depth plan of action that makes aviation journeys so secure. TCAS constitutes an important substitute for the see-and-avoid duty, giving the cockpit primary commands when a possible collision looms.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What are the TCAS aviation requirements?

The TCAS aviation requirements are listed below.

• The FAA requires the Traffic Collision Avoidance System (TCAS) or TCAS II in Part 121 aircraft.
• TCAS II has a requirement to provide reliable surveillance out to a range of 14 nm and in traffic densities of up to 0.3 aircraft per square nautical mile.
• The European Aviation Safety Agency (EASA) requires ACAS II (effectively TCAS II, version 7.1) for all fixed-wing turbine-powered aircraft that have a maximum takeoff weight greater than 5,700 kg or have more than 19 passenger seats.
• The FAA requires TCAS I for turbine-powered passenger-carrying aircraft having more than 10 and less than 31 seats.
• TCAS II is required for large turbine aircraft weighing over 33,000 pounds or over 30 passenger seats.
• TCAS I requires a transponder that can respond to Mode A, Mode C, and Mode S interrogations.
• The FAA's TSO-C-119b (Version 7.0) is required for TCAS II in RVSM airspace.
• Modern TCAS II systems require aircraft to have a Mode S transponder.
• TCAS I must be version 6 (TSO C-118) or later.
• TCAS I must provide a 30-second warning at up to a 1200-knot closure rate.
• TCAS II requires a 2 Hz interrogation rate.
• TCAS I does not require full Mode S capability.

What is the history of TCAS in aviation?The history of TCAS in aviation begins with a mid-air collision in 1956 between a United DC-7 and a TWA Super Constellation over the Grand Canyon that killed 128 occupants and prompted public awareness of mid-air collisions. A program sponsored by the FAA extended over more than a decade, entering full-scale nationwide implementation after the 1978 PSA Flight 182 collision with a Cessna 172 near San Diego Airport that killed 144, which increased FAA efforts to complete an effective collision avoidance system.

TCAS development began in 1975 based on industry and agency efforts in the field of beacon-based collision avoidance systems. Researchers developed the Traffic Collision Avoidance System by 1981, and the FAA decided to implement TCAS in 1981. Piedmont Airlines flew the first operational TCAS on a regularly scheduled flight in 1981 using two Boeing 727s with specially trained observers to validate the system. Initial operational evaluation of TCAS was conducted by Piedmont Airlines in 1982.

TCAS II was introduced in 1989 as the current generation of instrument warning TCAS. TCAS II versions 6.04a (1993) and 7.0 (1999) were developed, tested, certified, and installed. Development of TCAS II version 7.1 was initiated by EUROCONTROL after discovery of two safety issues with the current TCAS II version, with the final version agreed in March 2008. Today's versions compute vertical escape paths and include coordinated collision resolution advisories like ‘descend, descend!’ and ‘level off, level off!’. The evolution continues with ACAS-X, a family of Airborne Collision Avoidance Systems being developed by airlines, manufacturers, and regulators, each with a suffix to identify the type of aircraft or operation for which it is intended and optimised. ICAO Annex 6 Part I requires aeroplanes to be equipped with Airborne Collision Avoidance System (ACAS II).

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air