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This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
TCAS is an airborne safety system that independently monitors the airspace around an aircraft for other transponder-equipped traffic and issues alerts to help flight crew avoid mid-air collisions. TCAS operates by interrogating the transponders of nearby aircraft and analysing the responses to calculate each intruder’s range, bearing, altitude, and rate of closure. Using this data, TCAS computes a Collision Avoidance Volume (CAV) around the protected aircraft and determines whether any traffic is projected to penetrate it within a defined time threshold. TCAS II, the current standard required on commercial transport aircraft worldwide, issues two levels of advisory. A Traffic Advisory (TA) alerts the crew to nearby traffic at 35-48 seconds before projected closest approach, prompting visual search and increased awareness. A Resolution Advisory (RA) commands a specific vertical manoeuvre - “Climb, Climb” or “Descend, Descend” - at 15-35 seconds before closest approach, and the crew must follow the RA immediately, overriding ATC instructions if necessary. TCAS systems on conflicting aircraft coordinate their RAs via a datalink to ensure complementary manoeuvres. TCAS cannot issue horizontal avoidance advisories and does not detect non-transponder traffic.

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In Virtual Airline Operations

On VATSIM and IVAO, TCAS functions in aircraft add-ons that implement it, and TAs and RAs can occur during busy online sessions. Virtual pilots flying VACA routes should follow RA commands immediately if issued, just as in real operations, and notify virtual ATC after the manoeuvre is complete. TCAS adds a layer of realism and safety awareness to online flying and is an important system to understand when operating in shared virtual airspace.
  • TA - Traffic Advisory
  • ATC - Air Traffic Control
  • TEM - Threat and Error Management