ACAS/TCAS: Traffic Advisories, Resolution Advisories, and Inhibitions

022-12-10Pablo Asensio Martínez2026-03-272 min

ACAS (Airborne Collision Avoidance System) is the ICAO standard for systems designed to prevent mid-air collisions. TCAS II (Traffic Alert and Collision Avoidance System) is the equipment that meets this standard.

  • Principle: Operates independently of ground-based ATC. Uses transponder interrogations (Mode C or S) to detect nearby aircraft.
  • Requirement: Mandatory for turbine-powered aircraft > 5,700 kg or > 19 passengers.

Equipment and Operation

TCAS II interrogates the transponders of other aircraft to determine:

  1. Range: From the time delay of the reply.
  2. Bearing: Using directional antennas.
  3. Altitude: From the Mode C/S reply (pressure altitude).
  • Vertical Guidance Only: TCAS II provides resolution advisories (RAs) only in the vertical plane (climb/descend). It does not issue turn commands.
  • Coordination: If both aircraft are TCAS II equipped, they coordinate via Mode S data link to ensure complementary maneuvers (e.g., one climbs, the other descends).

Display Symbology

Traffic is displayed on the Navigation Display (ND) or a dedicated IVSI (Instantaneous Vertical Speed Indicator).

Symbol Description Meaning
Hollow Cyan/White Diamond Other Traffic Traffic detected but not currently a threat (> 6 NM or > 1200 ft vertical).
Solid Cyan/White Diamond Proximate Traffic Traffic nearby (< 6 NM and < 1200 ft vertical) but not yet a threat.
Solid Amber Circle Traffic Advisory (TA) Potential threat (~40 seconds to CPA). "Traffic, Traffic". Visual search required.
Solid Red Square Resolution Advisory (RA) Collision threat (~25 seconds to CPA). Immediate vertical maneuver required.
  • Data Tag: Shows relative altitude in hundreds of feet (e.g., +10 is 1000 ft above) and a trend arrow if climbing/descending > 500 fpm.

Alerts and Pilot Actions

  1. Traffic Advisory (TA): "TRAFFIC, TRAFFIC".
    • Action: Do not maneuver based solely on a TA. Visually search for the intruder. Prepare for a possible RA.
  2. Resolution Advisory (RA): e.g., "CLIMB, CLIMB", "DESCEND, DESCEND", "MONITOR VERTICAL SPEED".
    • Action:
      • Autopilot: Disconnect immediately.
      • Fly: Follow the green arc / Avoid the red arc on the VSI.
      • ATC: Notify as soon as possible ("TCAS RA").
      • Clear of Conflict: Return to clearance only when "CLEAR OF CONFLICT" is heard.

Inhibitions and Limitations

  • No Altitude Data: If an intruder has a Mode A transponder (no altitude reporting), TCAS cannot generate an RA (vertical separation unknown), only a TA (bearing/range known).
  • Low Altitude Inhibitions:
    • < 1000 ft (approx): "TA ONLY" mode usually active (descent RAs inhibited to prevent flying into terrain).
    • < 1450 ft: "INCREASE DESCENT" inhibited.
    • < 400 ft: All alerts inhibited.
  • Performance: "CLIMB" RAs may be inhibited at high altitudes (performance limit) or with landing gear/flaps extended (configuration limit).