What is TCAS and how do alerts work in a flight simulator?

TCAS (Traffic Alert and Collision Avoidance System) is an airborne system that tracks transponder-equipped aircraft, predicts collision risk and warns the crew. In a flight simulator, it converts supported AI, multiplayer or injected traffic into target symbols, Traffic Advisories and, when TCAS II is modelled, vertical Resolution Advisories.

In our Aviation & Real-World Flying section, “TCAS” refers both to the real airborne equipment and to its simulator representation. The distinction matters: a sophisticated airliner can model TCAS II threat logic, while a simpler panel may merely plot nearby traffic under a TCAS label.

What does TCAS mean in aviation?

In aviation, TCAS means Traffic Alert and Collision Avoidance System, an independent airborne safety system intended to reduce the risk of mid-air collision. ACAS, or Airborne Collision Avoidance System, is the broader standards term; TCAS is the name pilots and simulator users encounter most often.

TCAS watches compatible transponder-equipped aircraft around your own aeroplane. It can determine range, bearing and closure, while altitude-reporting transponders provide the vertical information needed for useful threat assessment. It operates independently of an air traffic controller’s radar picture or instructions.

There are two distinctions that matter in a simulator:

• TCAS I provides traffic information and Traffic Advisories, but no avoidance commands.
• TCAS II can also issue vertical Resolution Advisories and coordinate complementary instructions with another suitably equipped TCAS II aircraft.

Many general-aviation traffic displays and basic simulator panels are awareness systems rather than full TCAS II implementations. Seeing aeroplane symbols on a navigation display does not prove that the aircraft can generate RAs.

How does TCAS work?

TCAS works by tracking each contact’s movement and calculating whether the aircraft are likely to lose safe separation, rather than applying a simple warning circle around your aeroplane.

1. Detect traffic. Real TCAS equipment interrogates compatible Mode C and Mode S transponders and listens to their replies.
2. Build a track. It estimates each target’s range, bearing, relative pressure altitude, vertical trend and closure rate.
3. Assess the threat. The logic considers both predicted time to conflict and distance. Sensitivity changes with altitude and operating phase, so there is no single alert distance that applies everywhere.
4. Issue an advisory. A developing threat first becomes a Traffic Advisory where time permits. A more urgent conflict can produce a Resolution Advisory.
5. Coordinate the response. When both aircraft have compatible TCAS II equipment, Mode S messages allow one system to command a climb while the other commands a complementary descent.

This explains why close traffic may produce no warning when it is diverging, while a fast-closing aircraft several miles away can trigger an alert. A contact that appears suddenly or has unstable position data may also progress directly to an RA without a recognisable TA stage.

Does TCAS use radar or ADS-B?

TCAS is not primary radar: it uses radio interrogations and transponder replies, so it cannot detect every physical object in the sky. Compatible systems may use ADS-B data for surveillance or combine ADS-B and TCAS-derived contacts on the same display.

A flight simulator usually skips the radio exchange. The aircraft reads positions and altitudes from the simulator’s AI, multiplayer or injected-traffic system, then applies whatever threat logic the aircraft developer has modelled. Traffic visible outside the cockpit is not necessarily exposed to the aircraft’s TCAS code.

What does the “Traffic, traffic” alarm mean?

The spoken “Traffic, traffic” alarm normally means a Traffic Advisory: another aircraft has become a potential threat and requires immediate attention, but TCAS has not yet commanded an avoidance manoeuvre. Pilots generally call this an alert or advisory rather than a traffic alarm.

A TA means monitor the contact, look outside when conditions permit and prepare for a possible RA. It does not mean climb, descend or turn solely because of the displayed bearing.

RA calls may include commands such as “Climb”, “Descend” or “Adjust vertical speed”. A preventive RA can tell the pilot to avoid a particular vertical speed without requiring an immediate altitude change. TCAS II guidance is vertical; it does not command a left or right turn.

How do you read a TCAS target?

A TCAS target normally shows relative altitude in hundreds of feet, plus an arrow when the contact has a significant vertical trend. Exact colours, label positions and arrow thresholds vary by aircraft.

• +05 normally means the traffic is 500 feet above your aircraft.
• -12 normally means the traffic is 1,200 feet below your aircraft.
• An upward arrow means the target is climbing; a downward arrow means it is descending.

The arrow does not show horizontal direction. The altitude value is also relative pressure altitude, not a precise measurement of visual height above the ground. Multiplayer lag and low traffic-update rates can make the target symbol trail the aeroplane’s apparent position.

Does every TCAS issue climb and descend commands?

No: TCAS I and traffic-awareness systems provide TAs only, while TCAS II can generate Resolution Advisories. Some simulator aircraft display traffic accurately but omit RA logic, vertical-speed guidance, coordination or aural calls.

An aircraft may also be operating in TA-only mode, which deliberately inhibits RAs. Selected commands can be suppressed near the ground or in particular configurations, so a lack of climb or descent guidance is not automatically a fault.

How should you respond to a TCAS alert?

Treat a TA as an attention warning and an RA as vertical avoidance guidance, while following the procedures documented for the simulated aircraft.

1. Identify the advisory. Confirm whether the alert is a TA or RA and locate the highlighted target and vertical-speed guidance.
2. For a TA, monitor and scan. Maintain awareness, look for the traffic when practical and be ready for an RA. Do not make an abrupt manoeuvre solely from a TA display bearing.
3. For an RA, follow the vertical guidance promptly. Keep the vertical-speed indication out of the prohibited red area and follow any green fly-to indication. Never manoeuvre opposite to the command.
4. Use the correct automation procedure. Disconnect the autopilot unless the simulated aircraft specifically supports an autopilot or flight-director TCAS response. Aircraft implementations differ, so the add-on manual takes precedence.
5. Expect revised guidance. An RA can strengthen, weaken or reverse as the geometry changes. Continue following it until “Clear of conflict”.
6. Recover smoothly. Return towards the cleared altitude or flight path after the conflict is clear rather than making an abrupt correction.

In real operations, a valid RA takes priority over a conflicting ATC instruction unless following it would endanger the aircraft. The crew advises ATC as workload permits. Simulator ATC often continues repeating the original clearance because it has no knowledge that the aircraft add-on generated an RA.

This procedure summary is useful for realistic simulation, but it is not a substitute for an aircraft operating manual or approved collision-avoidance training.

Which TCAS mode should you use?

Use TA/RA mode for normal airborne operation when the simulated aircraft supports it, while following the aircraft checklist for ground, departure and abnormal settings.

A mistake we see constantly is treating the traffic-overlay switch as the TCAS master control. Selecting traffic on a navigation display may reveal targets without arming TA/RA logic. Likewise, the transponder and TCAS controls may share one panel but perform different jobs: altitude reporting lets other systems track you, while the TCAS mode determines which alerts your aircraft can issue.

For the common airliner control layout, our Boeing 737 simulator TCAS setup and alert guide covers mode selection, display controls and practical response.

Do ABV, NORM, BLW or display range change TCAS protection?

No, these controls normally filter non-threat traffic for display rather than changing the underlying collision protection. A genuine TA or RA should still appear on a properly modelled system even when the target falls outside the selected altitude filter or display range.

ABV emphasises traffic above, BLW emphasises traffic below and NORM uses the normal altitude band. The precise limits are aircraft-specific. Display range changes how much of the surrounding area you can see; it is not the TCAS detection range.

Why is TCAS not showing traffic or giving alerts?

When simulator TCAS shows nothing, the usual cause is missing traffic data, an incorrect operating mode or an aircraft that does not implement full TCAS logic.

1. Confirm a supported traffic source exists. AI, live, multiplayer and external injector traffic can use different interfaces. Our explanation of how simulator traffic reaches cockpit systems helps identify which source the aircraft can detect.
2. Check platform traffic settings. For Microsoft Flight Simulator, use the relevant instructions for supplying MSFS 2020 with live or injected traffic or configuring AI and injector traffic in MSFS 2024.
3. Verify the aircraft’s capability. Look for explicit support for TAs and RAs. A display described as traffic, TIS, ADS-B or traffic radar may not implement TCAS II.
4. Check power and mode selection. Make sure the relevant display is powered, the transponder is reporting altitude as required and TCAS is not in STBY or TA ONLY.
5. Remove display filters while testing. Select a useful map range and normal altitude filter. These should not suppress real threats, but they can hide ordinary contacts and make a working system appear empty.
6. Use plausible conflict geometry. Nearby traffic does not automatically qualify as a threat. It must have a sufficiently close predicted path and closure rate, with usable altitude data.
7. Avoid competing traffic sources. Running live traffic, multiplayer and an injector together can create duplicate or unstable tracks. Ghost contacts and abrupt position updates can cause nuisance alerts or targets that vanish.
8. Account for ground inhibition. Testing while parked, immediately after take-off or close to the ground may suppress some advisories and RA commands.

Why can I see an aircraft but get no TCAS alert?

You can see an aircraft without receiving an alert because visual proximity alone does not satisfy the TCAS threat criteria. The traffic may be diverging, safely separated vertically, outside the aircraft’s traffic-data feed or missing usable altitude information.

The reverse can also happen: a contact that is difficult to see may produce a TA because its projected path and closure rate create a genuine threat. TCAS calculations are based on movement and predicted separation, not apparent size through the windscreen.

How realistic is TCAS in a flight simulator?

A detailed simulator TCAS can reproduce target displays, alert progression and much of the pilot response flow, but its realism is limited by the aircraft model and the quality of the traffic data feeding it.

Real TCAS II aircraft exchange coordination messages so that their RAs complement one another. Many AI and multiplayer aircraft neither receive nor obey your simulator-generated advisory, and they may continue on the conflicting path. Some add-ons simulate a coordinated-looking RA without any corresponding action from the other aircraft.

Simulator traffic can also update slowly, teleport after network corrections or omit transponder state and reliable pressure altitude. Those limitations explain many late alerts, false alerts and disappearing contacts. TCAS simulation is valuable for learning cockpit indications and disciplined responses, but it cannot validate real-world collision-avoidance performance.