ACAS

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“ACAS” stands for “Airborne Collision Avoidance System”, an on-board system for preventing midair collisions. It operates independently of ground-based equipment, relying on signals from the secondary surveillance radar (SSR) transponder present on aircraft.

Primary function:

ACAS provides pilots with alerts about potential conflicts with other aircraft equipped with SSR transponders. The system processes the signals received from the transponders to estimate the position and speed of other aircraft, and in case of a collision risk, it provides instructions to the pilots for evasive maneuvers.

Versions:

There are different versions of ACAS, with different capabilities and functionalities. The most widespread versions are:

**ACAS II:** provides resolution advisories (RAs) that instruct the pilot on how to climb or descend to avoid collision.
**ACAS X:** in addition to resolution advisories, it also provides maneuver advisories (MAs) that instruct the pilot on how to turn right or left to avoid collision.

Importance:

ACAS is a fundamental aviation safety system that has significantly contributed to reducing the number of midair collisions. The International Civil Aviation Organization (ICAO) has made it mandatory to install on all airliners and many other types of aircraft.