Learn how TCAS traffic alerts, TAs and RAs work in a flight simulator, what the symbols mean, and why alerts may be missing or false.
TCAS (Traffic Alert and Collision Avoidance System) is an airborne safety system that tracks transponder-equipped aircraft and predicts collision risk. In a flight simulator, it turns compatible traffic data into visual targets, Traffic Advisories (TAs) and, where modelled, vertical Resolution Advisories (RAs) telling you to climb, descend or adjust vertical speed.
A sophisticated airliner add-on may reproduce much of the real threat logic. Simpler aircraft may provide only a traffic map, even if the display is labelled TCAS.
How TCAS predicts a conflict
TCAS predicts conflicts from aircraft movement rather than drawing a fixed warning circle around your aeroplane. It considers range, relative altitude, vertical trend and closure rate, with sensitivity changing according to the flight phase and altitude.
In real aircraft, TCAS interrogates compatible transponders and obtains altitude data from them. Our explanation of Airbus transponder and Mode S operation covers the altitude reporting and coordination data involved. Systems may also blend ADS-B traffic into the display.
A flight simulator often skips the radio interrogation itself and reads aircraft positions from its AI, multiplayer or injected traffic system. It can then apply similar conflict calculations. A fast-converging aircraft may trigger an alert while still several miles away, whereas closer traffic moving away from you may produce no warning.
What do TA and RA alerts mean?
A Traffic Advisory draws attention to a possible threat; a Resolution Advisory gives vertical avoidance guidance. Exact colours, symbols and voice calls vary between aircraft, but the usual sequence is:
| Traffic state | Typical indication | Meaning |
|---|---|---|
| Other traffic | Hollow diamond | Detected traffic that is not an immediate concern. |
| Proximate traffic | Filled diamond | Nearby traffic, but no collision is predicted. |
| Traffic Advisory | Amber circle and “Traffic, traffic” | A potential conflict requiring attention and preparation. |
| Resolution Advisory | Red square, aural command and vertical guidance | A predicted conflict requiring a climb, descent or vertical-speed adjustment. |
A TA normally precedes an RA, but traffic that appears suddenly or converges rapidly may advance directly to an RA. TCAS commands vertical avoidance; it does not tell you to turn left or right.
The number beside a target normally shows relative altitude in hundreds of feet. For example, +05 means 500 feet above your aircraft and -12 means 1,200 feet below. An accompanying arrow shows a significant climb or descent trend, not the intruder's horizontal direction.
Does every TCAS issue climb and descend commands?
No. TCAS I provides traffic advisories but not resolution advisories, while TCAS II can produce coordinated RAs. General-aviation traffic systems and basic simulator panels may also offer awareness or TAs only, so seeing traffic symbols does not prove that RAs are implemented.
Responding to a TCAS alert
Treat a TA as a warning to monitor the threat and an RA as vertical avoidance guidance. For realistic simulator procedure:
- Identify the alert type. Confirm whether you have a TA or an RA and locate the relevant target and vertical guidance.
- For a TA, monitor and scan. Look outside when conditions allow, but do not make an evasive manoeuvre solely from the traffic-display bearing.
- For an RA, follow the vertical command promptly. Keep within the indicated safe vertical-speed area and never manoeuvre opposite to the command. Disconnect the autopilot unless the simulated aircraft specifically supports autopilot or flight-director TCAS responses.
- Watch for revised guidance. An RA can strengthen, weaken or reverse as the geometry changes. Return smoothly towards the cleared altitude after the “Clear of conflict” call.
In real operations, a valid RA takes priority over a conflicting ATC instruction unless following it would endanger the aircraft. Simulator ATC may continue issuing the original clearance because it does not receive the TCAS event; our guide to ATC behaviour and traffic limitations in simulators explains why this mismatch occurs.
Which TCAS mode should you use?
Use TA/RA mode during normal airborne operation when the aircraft supports it, while following that aircraft's checklist for ground and departure settings.
- STBY: TCAS surveillance and alerts are disabled.
- TA: Traffic advisories are available, but resolution advisories are inhibited.
- TA/RA: Both traffic and resolution advisories are enabled.
- Automatic modes: Some aircraft switch between ground, TA and TA/RA states according to weight-on-wheels and flight phase.
Options such as ABV, NORM and BLW usually change the altitude band used to display non-threat traffic; they do not restrict the underlying protection to that band. A genuine TA or RA should still be presented. For panel operation, see our practical guide to traffic-display and TCAS controls. A32NX pilots can also use our mode-selection notes for the NAV/TCAS STBY indication.
Why is TCAS not showing traffic?
Most missing TCAS traffic alerts result from the selected mode, unavailable traffic data or limited aircraft modelling. Check these common causes:
- No traffic source: AI, multiplayer or externally injected traffic is disabled or unsupported by the aircraft.
- TCAS is in STBY or TA mode: STBY suppresses all alerts, while TA mode deliberately prevents RAs.
- Altitude reporting is unavailable: An intruder without usable altitude data cannot support reliable vertical conflict guidance.
- The contact is not a threat: Nearby aircraft may be diverging or separated vertically despite appearing close on a two-dimensional display.
- The implementation is display-only: Some default and add-on aircraft show traffic but do not model full TCAS II logic or aural warnings.
- Ground or low-altitude inhibition is active: Real systems suppress selected RA commands near the ground, and simulator implementations may simplify this behaviour.
- Traffic data is duplicated or unstable: Running more than one traffic source can create ghost contacts, while lag or abrupt position updates may cause nuisance alerts.
Simulator TCAS versus the real system
Even a detailed simulator TCAS is only as credible as the traffic data feeding it. Real TCAS II aircraft exchange Mode S coordination messages so that one aircraft can receive a climb RA while the other receives a complementary descent RA.
Many simulator traffic sources do not model that coordination, and an AI or multiplayer aircraft may neither see nor obey your advisory. A display may also combine TCAS, ADS-B and generic simulator traffic under one label. TCAS simulation is useful for learning cockpit indications and response discipline, but it is not a substitute for approved collision-avoidance training.