공중 충돌 방지 시스템 (TCAS) (Traffic Collision Avoidance System)

What Is TCAS?

The Traffic Collision Avoidance System (TCAS) — standardized by ICAO as ACAS (Airborne Collision Avoidance System) — is an independent, airborne safety net that monitors the airspace around an aircraft and, when a collision risk is detected, issues advisories to the flight crew. Critically, TCAS operates independently of ATC and ground radar — it is the last automated barrier between two aircraft and a mid-air collision.

How It Works

TCAS II (the current standard) actively interrogates nearby Mode C and Mode S transponders, building a picture of surrounding traffic. When it detects a potential conflict — called an intruder — it issues two levels of alert:

• Traffic Advisory (TA): "Traffic, traffic" — a visual and audio alert indicating a nearby aircraft that warrants visual acquisition. No maneuver is mandated; pilots search for the traffic.
• Resolution Advisory (RA): A specific vertical maneuver command ("Climb, climb!" or "Descend, descend!") displayed on the VSI/TCAS display. TCAS II coordinates RAs between conflicting aircraft via Mode S datalink — one aircraft climbs while the other descends, ensuring complementary maneuvers.

TCAS integrates with the glass cockpit navigation display to show intruder traffic symbols (white for non-threat, yellow for TA, red for RA). It cross-references ADS-B data where available to improve traffic awareness.

Evolution and Modern Systems

TCAS I (traffic advisories only) was mandated for US airlines in 1989. TCAS II with resolution advisories became mandatory for US air carriers in 1993. A major impetus was the 1978 Pacific Southwest Airlines collision over San Diego. TCAS II Version 7.1 (current) improved RA logic to reduce nuisance advisories and added a "level off" RA. The Überlingen midair collision (2002, 71 fatalities) occurred partly because crew followed ATC instructions instead of the TCAS RA — reinforcing the rule that TCAS RAs take priority over ATC instructions.

Regulatory Requirements

FAA FAR 121.356 mandates TCAS II for all US air carrier aircraft with more than 30 passenger seats. EASA mandates ACAS II (equivalent to TCAS II v7.1) for turbine-powered aircraft with more than 19 seats or above 5,700 kg MTOW. ICAO Annex 6 requires ACAS II globally for large commercial aircraft. A cardinal rule in all jurisdictions: when a TCAS RA contradicts ATC instructions, pilots must follow the RA and notify ATC afterward.