What is TCAS and how does it operate?

• A. Traffic Collision Avoidance System; Ground/ATC based system; Last line of defense
• B. Traffic Collision Avoidance System; Aircraft-based system; Last line of defense
• C. Terrain Collision Awareness System; Ground-based
• D. Traffic Control Advisory System; Onboard

Answer: (B)

TCAS is an onboard system that uses interrogations of nearby aircraft transponders to detect potential mid-air conflicts and to provide avoidance guidance. It operates independently of ground-based air traffic control, making it an aircraft-based safety net that can function even if ATC separation isn’t achieved.

When a potential threat is identified, TCAS issues a Traffic Advisory to alert the crew that another aircraft is nearby. If the threat continues, it then may issue a Resolution Advisory with specific vertical guidance (for example, climb or descend) to avoid collision. The pilots are expected to follow these advisories, even if they conflict with ATC instructions, because TCAS is designed to prevent mid-air collisions.

This combination—detection through transponder signals, onboard processing, and autonomous avoidance guidance—is why TCAS is described as a last line of defense. It is not terrain-related, and it is not a ground-based system or a Traffic Control Advisory system; its correct name is Traffic Collision Avoidance System, and it is onboard.