Traffic alert and collision avoidance system

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A traffic collision avoidance system or traffic alert and collision avoidance system (both abbreviated as TCAS) is an aircraft collision avoidance system designed to reduce the incidence of mid-air collisions between aircraft.

FlightGear (version 2.3.0 and above) provides an instrument emulating the TCAS II Version 7 standard.


TCAS monitors the airspace around an aircraft for other aircraft, independent of air traffic control, and warns pilots of the presence of other aircraft which may present a threat of mid-air collision. TCAS uses aural annunciation of all warnings - similar to the ground proximity warning system.

Rather than using fixed distances, threats are detected on a basis of time to conflict. Any aircraft on a flight path causing a conflict within the next 20-50 seconds triggers a traffic warning. Traffic coming even closer (15-35 seconds) may also trigger a resolution advisory, i.e. advise each pilot of conflicting aircraft to climb or descend to provide optimal vertical separation.

Threat detection sensitivity increases with altitude, assuming that planes should have a much higher separation at high altitudes, while only a minimum separation can be maintained when flying low - usually in the vicinity of the destination or departure airport. TCAS II increases the threat sensitivity in 7 steps: from 20 seconds at 1000ft to 50 seconds above 20000ft for traffic alerts, and from 15 to 35 seconds for resolution advisories.


TCAS display

The FlightGear TCAS has the following features and limitations:

  • Works with AI and multi-player traffic.
  • Aural traffic alerts (TA) are issued, i.e. "Traffic, traffic!" warnings.
  • Aural resolution advisories (RA) are issued, i.e. "Climb, climb!" or "Descend, descend!".
  • Provides traffic data in order to drive a TCAS display (no threat / proximity / traffic alert / resolution alert).
  • Switchable TCAS mode: off/standby/TA-only/TA-auto (to be connected to transponder cockpit switch).

Additional features

  • Advisory reversion: this may be required due to late or incorrect reaction of the pilot, due to an unforeseen change of the threatening aircraft's flight path, or due to a new multi aircraft threat. In such a situation an opposite vertical movement may become necessary to provide better separation. Reversed advisories are announced by "Climb, climb now!" or "Descend, descend now!" warnings.
  • Normally TCAS II avoids altitude crossing advisories, i.e. the aircraft at higher altitude is advised to climb, the aircraft at lower altitude advised to descend. However, there are specific situations where the minimum vertical separation can only be achieved by altitude crossing (e.g. since the lower aircraft is already climbing, or the higher aircraft already descending fast). TCAS uses a specific advisory annunciation for such cases ("Climb, crossing, climb!" and "Descend, crossing, descend!") to alert pilots of this specific situation.
  • Parked/taxiing aircraft are ignored by the FlightGear TCAS (RL pilots switch their TCAS transponders off).
  • Coordination of resolution advisories with AI aircraft. When a TCAS resolution advisory is issued, the conflicting AI plane moves in the opposite vertical direction (Beware: AIpilots always want to be on schedule (no exceptions! :) ), so eventually they may ignore any TCAS threat and try to return to their original flight path).
  • Currently there is no coordination of resolution advisories for MP traffic in FlightGear. So with two TCAS equipped MP aircraft approaching each other, there is no guarantee that the respective resolution advisories are actually in opposite direction. However, due to the symmetry of the resolution algorithm, resolution advisories still will be in opposite direction in almost all cases.
  • MP aircraft ignored in the pilot list are also ignored by the TCAS and TCAS display.


In a threat situation a traffic alert is always annunciated first. A resolution advisory may follow several seconds later when the conflicting aircraft approach even closer.

However, TCAS does not guarantee to always issue a resolution advisory:

  • No advisories are issued when the TCAS is switched to "TA-only" mode (or less). TCAS cockpit switch must be set to mode 3 (named "TA/RA" or "TA-auto") to enable resolution advisories.
  • Below 1000ft only traffic alerts are announced, but no advisories (TCAS II standard).
  • No advisory is issued when the TCAS module is unable to determine a safe resolution based on vertical movement (i.e. no safe option due to multiple aircraft threat, or low altitude).

Description of Aural Alerts

The complete list of TCAS alerts supported by FlightGear is:

Aural annunciation Description
Traffic, traffic! warning, conflicting traffic
Climb, climb! advisory, climb immediately to avoid conflicting traffic
Climb, crossing, climb! advisory, climb immediately, conflicting traffic will descend - crossing altitudes
Climb, climb now! reversed advisory, ignore previous descend advisory, now climb immediately
Descend, descend! advisory, descend immediately
Descend, crossing, descend! advisory, descend immediately, conflicting traffic will climb - crossing altitudes
Descend, descend now! reversed advisory, ignore previous climb advisory, now descend immediately
Maintain vertical speed, maintain! advisory, keep climbing/descending to avoid collision
Adjust vertical speed, adjust! advisory, reduce climb or descent rate to avoid collision
Don't climb! advisory, fly level to avoid collision. Issued at low altitudes instead of a "descend" advisory. Pilots may descend if they can confirm safe terrain clearance.
Clear of conflict. All clear. Follow ATC instructions.


Annotated TCAS display

The TCAS display shows any aircraft with enabled transponder (transponders should be switched off while taxiing or when the aircraft is parked).

The following symbols and colors are used by a TCAS display:

Color Symbol Description
blue or cyan open diamond no threat, distant traffic
blue or cyan filled diamond no threat, but proximity traffic (less than 6 nm in range and within +/-1200ft)
yellow or orange filled circle threat: traffic alert
red filled square threat: resolution advisory was issued to evade this aircraft

Furtheremore, an upward or downward arrow next to any TCAS symbol shows climbing or descending traffic.

Also, the relative altitude is shown in steps of 100ft: +12 above a TCAS symbols means traffic 1200ft above, a -24 below a TCAS symbol means traffic is 2400ft below.


Pilots should always follow a TCAS resolution advisory - even in case it contradicts an ATC instruction (see ACAS II bulletin No 1).


  • Remember to immediately report any TCAS advisory to ATC.
    • Phraseology: "<callsign> TCAS RA" (pronounced "TEE-CAS-AR-AY")
  • When an ATC instruction contradicts an active TCAS resolution advisory, keep following the TCAS advisory and report to ATC.
    • Phraseology: "<callsign> Unable, TCAS RA."
  • Pilots must report to ATC and return to their assigned altitude as soon as the conflict is cleared.
    • Phraseology: "<callsign> Clear of conflict, returning to <assigned clearance>"

A detailed description of TCAS-related ATC procedures is available in the ACAS II bulletin No 10: When ATC meets TCAS II

Add to an airplane

In many countries the installation of a TCAS instrument is mandatory for civil transport aircraft (Europe/EASA: mandatory for aircraft capable of carrying at least 19 passengers, USA/FAA: any aircraft capable of carrying at least 30 passengers).

TCAS instrument

Instrument List

Add the device to your plane by adding it to the sim/instrumentation section (usually in a separate instrumentation.xml file).



Usually planes only have a single TCAS installed - so using multiple instances wouldn't be too common.

<Plane>-set.xml file

Configuration parameters are stored in a different section. Add the following XML template to the instrumentation section of the plane's ...-set.xml file. Do not mix up this section with the one above (section above was within the <sim> tag, this section is outside the <sim> tag).

XML Template:

  <!-- here: other data from ...-set.xml -->
    <tcas>                               <!-- -->
                                          <!-- TCAS ENABLE -->
          <mode type="int">3</mode>       <!-- 0=off, 1=standby, 2=TA-only, 3=auto(TA/RA) -->

          <file-prefix type="string">Aircraft/MyAircraft/Sounds/tcas/</file-prefix>
                                          <!-- custom sound path and/or file prefix -->
      <speaker>                           <!-- Speaker Configuration -->
        <max-dist> 2 </max-dist>          <!-- Max. distance where speaker is heard -->
        <reference-dist> 1 </reference-dist> <!-- Distance to pilot -->
        <volume> 1.0 </volume>            <!-- Volume at reference distance -->


  • You can connect the instrumentation/tcas/inputs/mode property to a cockpit switch (transponder mode), so the pilot can manually switch the TCAS mode.
  • The instrumentation/tcas/serviceable property may be used to simulate a device failure.

TCAS display

A TCAS display is implemented by a separate device which reuses information provided by the TCAS instrument. Currently, only the wxradar instrument supports displaying traffic according to TCAS data.

The following example shows additional settings required for wxradar in order to enable the TCAS display mode:

                <tcas type="bool">true</tcas>
                    <color n="0"> <!-- distant targets -->
                        <red type="float">0</red>
                        <green type="float">1</green>
                        <blue type="float">1</blue>
                    <color n="1"> <!-- proximity targets -->
                    <color n="2"> <!-- TA threat targets -->
                        <red type="float">1</red>
                        <green type="float">0.5</green>
                        <blue type="float">0</blue>
                    <color n="3"> <!-- RA threat targets -->
                        <green type="float">0</green>
            <echo-texture-path type="string">Aircraft/..../wxecho.png</echo-texture-path>


  • Remember to reference a specific "wxecho.png" image providing TCAS-style symbols (see 777-200ER).
  • The TCAS display mode requires the installation of a TCAS instrument.