Traffic collision avoidance system - TCAS[PDF]


The TCAS or Traffic alert and Collision Avoidance System is a system designed to reduce the incidence of mid-air collisions with other aircraft.
The TCAS is independent of air traffic control and flight navigation instruments.

ICAO regulation

Definition and rules

Airborne collision avoidance system or ACAS is an aircraft system based on secondary surveillance radar (SSR) transponder signals which operate independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft that are equipped with SSR transponders.
The International Civil Aviation Organization prescribes that an Airborne Collision Avoidance System (ACAS) must be installed and operational for all aircraft heavier than 5700 Kg and all aircraft authorized to transport more than 19 passengers.

Types of ACAS

  • ACAS I gives Traffic Advisories (TAs) but does not recommend any manoeuvres. The only implementation of the ACAS I concept is TCAS I. These equipments are limited to interoperability and interference issues with ACAS II.
  • ACAS II gives Traffic Advisories (TAs) and Resolution Advisories (RAs) in the vertical sense (direction). The only implementations of the ACAS II concept are TCAS II Version 7.0 and Version 7.1.
  • ACAS III gives TAs and RAs in vertical and/or horizontal directions. ICAO SARPs for ACAS III have not been developed. Currently, there are no plans to proceed with such a development
Not all TCAS systems can be considered as accepted ACAS.

TCAS I is mandated in the United States for certain smaller aircraft. TCAS II Versions 7.0 and 7.1 are mandated in Europe and elsewhere Collision avoidance systems can be passive, like for example the PCAS or Portable Collision Avoidance System which only monitors the surrounding aircraft without emitting any signal. This portable system is often used within General Aviation.

Some gliders are now equipted with an avoidance system named FLARM.

ACAS II as a standard

Currently, the only commercially available implementations of ICAO standard for ACAS II (Airborne Collision Avoidance System) are TCAS II versions 7.0 and version 7.1 (Traffic alert and Collision Avoidance System).

ACAS II is an aircraft system based on Secondary Surveillance Radar (SSR) transponder signals. ACAS II interrogates the Mode C and Mode S transponders of nearby aircraft (‘intruders’) and from the replies tracks their altitude and range and issues alerts to the pilots, as appropriate. ACAS II works independently of the aircraft navigation, flight management systems, and Air Traffic Control (ATC) ground systems.

Non-transponding aircraft are not detected.

TCAS instrument onboard

The TCAS system can be implemented as:

  • a dedicated instrument
  • a combined instrument with the vertical speed indicator
  • a combined instrument with the Navigation Display or the Electronic Horizontal Situation Indicator
TCAS views.png

Different modes

TCAS can be currently operated in the following modes:

  • Standby: TCAS does not issue any interrogations and the transponder only replies to discrete interrogations. This mode is used on the ground, outside of the runway.
  • Transponder: the transponder replies to all appropriate ground and TCAS interrogations and TCAS remains in stand-by. This is a passive mode which is the minimum mandatory mode to be set by any airborne aircraft (typically in General Aviation)
  • Traffic Advisory (TA): TCAS issues the appropriate interrogations and perform all tracking functions. However, TCAS will only provide traffic advisories (TA) and the resolution advisories (RA) are inhibited
  • Automatic (TA/RA): TCAS provides traffic advisories (TA) and resolution advisories (RA) when appropriate. This mode is mandatory for all Commercial Aviation aircrafts.

Technical description

The TCAS II is a system that:
  • Monitors the airspace around the aircraft and communicates with all traffic equipped with a corresponding active transponder
  • Queries all surrounding aircraft on the frequency 1030 MHz and each aircraft transponder replies on the frequency 1090MHz
  • Warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision (MAC)
TCAS is only able to interact with aircraft that have a correctly operating mode C or mode S transponder.


TCAS II equipment provides two types of advisories to pilots: Resolution Advisories (RAs) and Traffic Advisories (TAs).

These are defined as follows:

  • Resolution advisory (RA) is an indication given to the flight crew recommending:
    • a manoeuvre intended to provide separation from all threats, or
    • a manoeuvre restriction intended to maintain existing separation.
  • Traffic advisory (TA) is an indication given to the flight crew that a certain intruder is a potential threat.

Protection volume

A protected volume of airspace surrounds each ACAS II equipped aircraft. The size of the protected volume depends on :

  • the altitude of the aircraft involved in the encounter.
  • the speed of the aircraft involved in the encounter.
  • the heading of the aircraft involved in the encounter.

The TCAS system builds a three dimensional map (Protection Volume) of the airspace around the aircraft. The map range depends on the TCAS configuration.

Tcas protection volume.png

TCAS symbols

The map is filled with all intruder aircraft depending on:

  • their distance (deduced from the interrogation and response round trip time)
  • their altitude (as reported by the interrogated aircraft)
  • their bearing (by the directional antenna from the response)

Traffic is represented by small geometrical symbols with associated numbers indicating the vertical separation with respect to your airplane, expressed in hundreds of feet.

For example:

  • -012 = 1200 feet below
  • 121 = 12100 feet above
  • -02↓ = 200 feet below and descending
  • +07 = 700 feet above
  • -01↑= 100 feet below and climbing


Finally, by extrapolating the current range and altitude difference to anticipated future values, the system determines whether a potential collision threat exists.

TCAS is also an active system since it reports to the pilot the position of all aircraft inside the protected volume and warns the pilot about any risk of conflict.

Following the identification of potential collisions, the TCAS automatically negotiates a mutual avoidance manoeuvre (currently, manoeuvres are restricted to changes in altitude and modification of climb/sink rates) between the two (or more) conflicting aircraft. These avoidance manoeuvres are communicated to the pilot by a cockpit display and by synthesized voice instructions.

Traffic Advisory (TA)

In the case of a potential collision, a sounding alert is emitted by the “Traffic Advisory” (TA). This system warns the pilot about every intruder aircraft by a “traffic, traffic” vocal announcement. It does not provide any avoidance manoeuvre suggestion.

TAs are nominally generated 20–48 seconds prior to the predicted Closest Point of Approach which would be 10–13 seconds earlier than any RA, although shorter generation times are possible in some geometries – indeed, in certain cases an RA can occur without a preceding TA. The majority of TAs will not be followed by an RA because often the separation between the aircraft does not drop below the alert threshold for an RA. On average, only 1 in 10 TAs will be followed by an RA.

When a TA is issued, the pilot is meant to initiate a visual search for the traffic causing the TA. If the traffic is visually identified, the pilot has to maintain visual separation.

He is also supposed to avoid any horizontal manoeuvre based on the solely information shown on the traffic display. On the contrary, slight adjustments in vertical speed while climbing or descending, or slight adjustments in airspeed while still complying with the ATC clearance are acceptable.

TA alerts shall not be used by the pilot to separate himself with other pilots especially in controlled area.

Resolution Advisory (RA)

Whenever the conflict situation gets worst after a TA alert and the collision seems impending, an audio message and a visual alert are generated by the « Resolution Advisory » (RA). This alert indicates the concerned airplane and suggests an avoidance action to be immediately executed by the pilot.

The system is conceived in such a way that the TCAS of the other airplane suggests another action.

For example when the TCAS provides a climb advisory to one airplane, the TCAS of the other aircraft suggests a descent suggestion, which gives an increased separation between the two aircraft.
Although the system may sometimes cause false alerts, pilots are instructed to strictly consider all TCAS messages as real alerts which require an immediate action. In the case of a difference between the TCAS RA and ATC instructions, the TCAS RA has always priority.
As soon as the alert disappears, the system announces « clear of conflict ».

List of TCAS advisories

The list of TCAS advisories is presented below:

TCAS alerts.png


The TCAS is a function available in our IVAO pilot interface.

This means that it is always available regardless the aircraft model you choose, even for airplanes which are not equipped in real life. It is up to you to make use of it or not depending on the aircraft you fly with and on the degree of realism you wish to reach.

On IVAO, the TCAS function is useful when ATC is not present in order to prevent collision.

TCAS representation

TCAS ivap.png

Use of TCAS

The symbol of your aircraft is placed in the middle of a circle in the TCAS window.

The radius of this circle can be increased or decreased by clicking on « + » or « - ». The range possible values are 3, 5, 10, 15, 20 or 40 NM.

The TCAS activation button is placed on the left bottom corner of the window. The different modes are:
  • OFF: (inactive TCAS) with « TCAS OFF »
  • TA: « Traffic Advisory » mode
  • TA/RA: « Traffic Advisory/Resolution Advisory » mode

The display mode can be changed with the button placed on the right top corner:
  • NORM: the traffic at +/- 2700 ft is displayed
  • ALL: all traffic within the TCAS range is displayed
  • ABV: the traffic within the interval (-2700, +9000) ft is displayed
  • BLW: the traffic within the interval (-9000, +2700) ft is displayed

Finally, the TCAS sound can be deactivated or activated by the right bottom button.

TCAS Alert, TCAS Caution definition:
  • A TCAS “Caution” is issued whenever an airplane is at ±1600 ft and/or at 7 NM from your position. The intruder aircraft is displayed in yellow.
  • A TCAS “Alert" is issued whenever an airplane is at ±900 ft and/or at 3 NM from your position. The intruder aircraft is displayed in red.

When a TCAS alert is issued, the reaction time allowed to you is very short. You must execute all possible manoeuvres to avoid the collision and inform the ATC when available.
In the case of a high load traffic situation, it is extremely important to inform the ATC as soon as possible in order to avoid conflicts with other aircraft.

See also

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  • VID 150259 - Creation
  • VID 450012 - Update


  • 02:16, 14 May 2021


  • This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.


  • The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.