공중 충돌 방지 시스템 (TCAS)

Overview

The Traffic Collision Avoidance System (TCAS II) is an independent airborne safety net that protects commercial aircraft against mid-air collision. Unlike ATC radar, which provides separation from the ground, TCAS operates autonomously aboard the aircraft, requiring no ground infrastructure. It interrogates the Mode C and Mode S transponders of nearby aircraft, builds a 3D traffic picture, and — when a collision threat is detected — issues a Resolution Advisory (RA) instructing the crew to climb, descend, or maintain their current vertical speed. Compliance with RAs is mandatory under ICAO SARPS and FAA/EASA regulations.

TCAS development was driven by several high-profile mid-air collisions. The 1978 San Diego collision between a Pacific Southwest Airlines 727 and a Cessna, killing 144 people, and the 1986 Cerritos collision between an Aeromexico DC-9 and a small aircraft, killing 82, galvanized congressional action. The FAA mandated TCAS II for all US air carriers in 1993. ICAO mandated it globally for aircraft above 5,700 kg from 2003. The current standard is TCAS II Version 7.1, which added coordinated "Adjust Vertical Speed, Adjust" advisories that smooth the vertical rate rather than requiring abrupt reversals.

How It Works

TCAS uses the aircraft's Mode S transponder to actively interrogate surrounding aircraft on the 1030 MHz frequency. Responding transponders reply on 1090 MHz with their altitude (Mode C or Mode S squawk). By measuring the reply's bearing (using a directional antenna array) and range (via reply time), TCAS computes each intruder's position and vertical trend. Aircraft are categorized as no threat (white/open diamond), proximate traffic (solid white diamond), Traffic Advisory (TA — amber circle, within about 35–48 seconds of closest approach), or Resolution Advisory (RA — red square, within about 15–35 seconds).

When two TCAS-equipped aircraft converge, their TCAS computers coordinate via Mode S data link to issue complementary RAs — one aircraft climbs, the other descends. This coordination is invisible to ATC. The RA instructs the crew via voice annunciation ("CLIMB, CLIMB" or "DESCEND, DESCEND") and a vertical speed guidance display on the VSI/IVSI, showing the green band (correct response range) and red band (avoid range). Post-maneuver, TCAS issues a "CLEAR OF CONFLICT" annunciation and a "Return to ATC clearance" reminder.

Key Components

• TCAS Computer: The central processing unit that interrogates transponders, tracks intruder aircraft states, computes collision geometry, and selects Resolution Advisories using the Collision Avoidance System (CAS) logic.
• Directional Antenna Array: Top and bottom antennas on the fuselage provide 360-degree bearing coverage and determine which quadrant an intruder is in (bearing accuracy is sufficient to distinguish general direction, not precise azimuth).
• Mode S Transponder Integration: TCAS leverages the aircraft's existing Mode S transponder for both interrogation transmission and peer-to-peer coordination with other TCAS-equipped aircraft.
• Cockpit Display — Traffic Display (TD): A dedicated TCAS display or ND overlay showing intruder aircraft as diamonds or squares with relative altitude and trend arrows.
• VSI/IVSI with RA Guidance: The vertical speed indicator shows colored bands: green (fly to), red (avoid), enabling precise RA compliance without memorizing the specific commanded rate.

Aircraft Applications

• Boeing 737-800: Honeywell CAS-100 or ACAS 9000 TCAS II v7.1, displaying on the ND in traffic mode with simultaneous weather radar overlay.
• Airbus A320-200: Rockwell Collins TCAS 4000 or Honeywell TCAS II, integrated with the ECAM alerting system for RA audio and visual annunciations.
• Boeing 777-300ER: Honeywell TCAS II with ADS-B In hybrid surveillance, reducing active interrogation of ADS-B-equipped aircraft and extending situational awareness range beyond the TCAS interrogation limit.
• Boeing 787-9: Honeywell TCAS II integrated with Common Core System, displaying on any ND with full RA guidance symbology and ACAS X transition roadmap for future upgrades.

Advantages and Limitations

TCAS II has been credited with saving thousands of lives since its mandatory introduction. Studies by EUROCONTROL and the FAA estimate a 5–7 fold reduction in mid-air collision risk for equipped aircraft. The system operates independently of ATC, providing a final layer of defense when separation has already been compromised. Coordinated RAs between TCAS-equipped aircraft ensure complementary vertical maneuvers, eliminating the risk of both aircraft maneuvering in the same direction.

Known limitations include the visual-only nature of bearing information — TCAS cannot precisely determine an intruder's azimuth to within a few degrees, so the traffic display shows only general direction. Non-transponder-equipped aircraft (ultralights, some gliders, military aircraft in restricted airspace) are invisible to TCAS. The system can issue nuisance RAs in high-density airspace when ATC is providing close but legal separation; version 7.1 reduced but did not eliminate this issue. Crew non-compliance with RAs remains a documented safety concern; ICAO and aviation authorities conduct ongoing campaigns to reinforce the mandatory compliance culture.