Traffic Alert & Collision Avoidance System (TCAS)

TCAS, short for Traffic Alert and Collision Avoidance System, is a system equipped on an aircraft that identifies the location and tracks the progress of aircraft equipped with transponders or transmitter-responder devices (Wikipedia, 2009b 2). The goal of TCAS is to prevent mid-air collisions between aircraft operating within the same airspace by warning pilots of transponder-equipped aircraft that may present a collision threat.

TCAS operates independently of Air Traffic Control (ATC) by communicating with other transponder-equipped aircraft to build a 3-dimensional map of aircraft in the same airspace. By extrapolating the current range and altitude difference to anticipated future values, TCAS determines the potential of a collision threat. The existence of a collision threat results in subsequent communication of avoidance manoeuvres to flight crew by cockpit display or voice instructions, depending on the TCAS version installed (Wikipedia, 2009 1). Many countries have mandated the carriage of TCAS II, and ICAO has proposed a worldwide mandate of TCAS II Version 7 by 2003 (FAA. pp.40, 2000).

TCAS Versions


  • Display traffic similar to TCAS but have a shorter range (generally less than 7 nautical miles)
  • Rely on transponder replies triggered by ground and airborne systems that query nearby transponders for altitude information
  • Monitored by third-party systems for traffic information (Wikipedia, 2009 1)


  • Less expensive and less capable than TCAS II and TCAS III
  • Designed primarily for general aviation use
  • Displays the approximate bearing and relative altitude of all transponder-equipped aircraft within a range of approximately 40 miles. Figure 1 shows a TCAS display (Traffic Advisory).
  • Generates collision warnings via a Traffic Advisory (TA) that warns the pilot of nearby aircraft. When the flight crew receive a TA, it is up to the Pilot in Command to decide what actions to take in response to the TA. The pilot may notify air traffic control for assistance in resolving the conflict (Wikipedia, 2009 1)
Figure 1 (image embedded from Wikipedia on 05 Aug 2009)


  • More comprehensive than TCAS I
  • Used in the majority of commercial aviation aircraft
  • Operates the same as TCAS I but has greater range and bearing accuracies than TCAS I and a Resolution Advisory (RA) function
  • In the event of a potential collision, TCAS II will issue a Resolution Advisory (RA) to each pilot that consists of direct, vocalised corrective or preventive vertical manoeuvring commands to avoid a collision
  • TCAS II systems coordinate resolution advisories to maximise aircraft separation so that if one aircraft is instructed to climb, the other aircraft will be instructed to descend (Wikipedia, 2009 1)


  • Whereas TCAS II only offers vertical RAs and TAs, TCAS III offers both vertical and horizontal RAs and TAs. Horizontal directives commanding left or right manoeuvres are beneficial for two conflicting aircraft close to the ground with minimal vertical manoeuvring space.
  • TCAS III attempted to use its directional antenna in order to assign a bearing and assign a horizontal maneuver action (Wikipedia, 2012). However, the directional antenna was deemed to have limited accuracy (Wikipedia, 2012). Further testing and analysis determined that the concept was unworkable using available surveillance technology (Wikipedia, 2012).
  • Hence, the project was suspended and there are no plans for implementation of TCAS III (Wikipedia, 2009 1)


  • Due to the limited accuracy in using directional antenna for TCAS III, TCAS IV uses information encoded by the aircraft in the Mode S transponder reply (Wikipedia, 2012). It will require a reliable source of position (e.g. INS, GPS) in order for the aircraft to be encoded.
  • This project is also being abandoned since the appearance of new trends of data link - ADSB (Wikipedia, 2012).

Safety Concerns

  • TCAS may induce midair collisions if the threat aircraft's reported altitude is inaccurate or if the threat aircraft makes an abrupt manoeuvre that defeats the TCAS resolution advisory
  • Recommended avoidance manoeuvres produced by TCAS may direct the pilot to descend the aircraft toward the ground below a safe altitude
  • Pilot uncertainty regarding how to act when a request to climb is initiated while flying at maximum altitude (Wikipedia, 2009 1)

Current TCAS version being used

  • TCAS II version 7.0 is currently being used by most general aviation. TCAS II version 7.0 offers TA and RA but does not provide reverse RA if one of the conflicting aircraft does not follow TCAS RA while TCAS II version 7.1 offers reverse RA (Eurocontrol, 2012).
  • TCAS II version 7.1 added function to produce reverse RA can help to prevent incident such as the notable Überlingen Mid-Air Collision accident in 2002.
  • FAA and EASA published the technical standard order for TCAS II version 7.1. (Eurocontrol, 2010) and that version 7.1 will fully replace version 7.0 by year 2014 (Wikipedia, 2012).

Current TCAS Limitations

  • TCAS is not fitted to many smaller aircraft due to the absence of any legal requirement to do so and / or the high costs involved in installing the system
  • Current implementations do not support horizontal separation advisories
  • Does not take into account terrain/ground clearance or obstacle awareness
  • Fails to facilitate automatic reporting of issued RAs to ATC
  • TCAS displays do not provide information about RAs issued to other conflicting aircraft and hence if conflicting aircraft are actually complying with RAs
  • Fails to take into account aircraft performance differences or current aircraft configuration during the creation of resolution advisories. As a result, RAs may demand action outside the normal or safe flight envelope during a particular phase of flight. Similarly, RAs may not be optimal for the aircraft type involved in the conflict.
  • TCAS is primarily range based however in some situations a time based representation may be of more assistance
  • TCAS uses extrapolations to predict future flight paths based on flight path history rather than utilising actual aircraft routing (flight plan information) and relevant ATC instructions (Wikipedia, 2009 1)
  • TCAS performs badly in a 3-way conflicts since it does not gives instructions to manuever horizontally (Wikipedia, 2012).

Pilot's action

The pilot should trust and follow TCAS instruction as RA inherently possess a more current and comprehensive picture of the situation than the controllers whose radar/ transponder usually updates at a much slower rate than TCAS interrogations (Wikipedia, 2012). Whenever possible, pilot when receive both RA and controller's instruction, pilot should inform the controller of RA's instruction and controller are no longer responsible in providing instructions until the aircraft is clear of conflict (Wikipedia, 2012). Only then the controller resumes responsibility of separation of aircraft (Wikipedia, 2012).

Currently, RA downlink research is undertaken in the SESAR projects 4.8.3 and 15.4.3 (Eurocontrol, 2011). If proved to be feasible, RA downlink will provide a faster and an automatic notification of RA which will remain on the controller screen until Clear of Conflict (Eurocontrol, 2011).

TCAS in Action

The following video demonstrates a TCAS Traffic Advisory and Resolution Advisory during approach

(Video embedded from YouTube on 04 Aug 2009)

Incidents involving TCAS

  • 1996 Charkhi Dadri mid-air collision involving Saudi Arabian Airlines Flight 763 and Kazakhstan Airlines Flight 1907.
  • Due to the failure of the Kazakhstan pilot to follow ATC instructions to maintain altitude.
  • This accident made it mandatory for aircraft flying into and out of India to be equipped with TCAS which could have informed both sets of pilots that they were both on a collision course and hence prevented the accident.
  • For a more comprehensive analysis of the incident, please refer to Head On Collision
(Image embedded from Shiselvan on 10 Oct 2009)
  • 2001 Japan Airlines Mid-Air Incident involving Japan Airlines Flight 907 and Japan Airlines Flight 958.
  • The two aircraft were given conflicting instructions from their TCAS and ATC whilst they were on a collision course with each other at the same altitude.
  • Due to confusion amongst ATC, the pilot of Japan Airlines Flight 958, based on visual judgment elected to dive to avoid the other aircraft.
  • The two aircraft missed each other by less than 100 metres.
  • For a more comprehensive analysis of the incident, please refer to 2001 Japan Airlines mid-air incident
(Image embedded from Wikipedia on 10 Oct 2009)
  • 2002 Überlingen Mid-Air Collision involving Bashkirian Airlines Flight 2937 and DHL Flight 611.
  • The two aircraft were given conflicting instructions from their TCAS and ATC whilst they were on a collision course with each other at the same altitude.
  • The Bashkiran pilot elected to follow ATC whereas the DHL pilot followed TCAS, hence causing both aircraft to collide.
  • What was significant from this accident was the failure to learn from the previous near miss by the two Japan Airlines aircraft as mentioned above with regard to resolution of conflicting instructions by ATC and TCAS as ICAO did not further investigate the Japan accident till after this collision.
  • For a more comprehensive analysis of the incident, please refer to Mid Air Collision
(Image embedded from Wikipedia on 10 Oct 2009)
1. Wikipedia (2009). Traffic Collision Avoidance System. Retrieved August 03, 2009 from the World Wide Web:
2. Wikipedia (2009b). Transponder (aviation). Retrieved August 04, 2009 from the World Wide Web:

3. Eurocontrol. (December 2010). TCAS Version 7.1. Retrieved on September 14, 2012.

4. Eurocontrol. (January 2012). TCAS II Version 7.1. Retrieved on September 14, 2012.

5. Eurocontrol. (2011). RA Downlink. Retrieved on September 14, 2012.

6. FAA. (2000). Introduction to TCAS II Version 7. Retrieved on September 14, 2012.

Want to know more?

Wikipedia - TCAS
This website provides further information about TCAS including the worldwide regulatory situation

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