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

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).