Definition
ACAS is the international term for an onboard system that monitors the airspace around an aircraft for other transponder-equipped aircraft, warns the crew of potential conflicts, and issues advisories to avoid a mid-air collision. It operates independently of air traffic control and ground-based equipment, using transponder signals exchanged directly between aircraft. In the United States, the implementation of ACAS is known as TCAS (Traffic Alert and Collision Avoidance System).
Plain English
A system in the aircraft that watches for other nearby aircraft and tells the pilots if one is getting too close, including what to do to stay clear.
Context Anchor
Seen in aircraft equipment lists, avionics descriptions, traffic-alerting discussions, and procedures for avoiding other aircraft.
Why Pilots Care
Supplies an independent automated layer of protection against mid-air collisions when visual lookout or air traffic control separation is insufficient.
Intuition Check
ACAS is not a replacement for looking outside, following air traffic control instructions, or obeying right-of-way rules. It is an added safety system that helps the pilot detect and avoid a possible collision.
Example Sentence 1
The crew complied with the ACAS resolution advisory and climbed to avoid the converging traffic.
Example Sentence 2
Before operating in RVSM airspace, the crew verified that the ACAS was serviceable and properly configured.