What is ACAS in Aviation? (Airborne Collision Avoidance System)

As the number of airplanes in the sky increases, so does the risk of mid-air collisions. One of the most critical safety systems used to detect and avoid airborne collisions is the Airborne Collision Avoidance System (ACAS). This system is responsible for preventing collisions between aircraft in mid-air and is an essential tool for pilots in ensuring the safety and security of every flight.

What is Airborne Collision Avoidance System (ACAS)?

The ACAS is an advanced technology-based system designed to enhance the safety of airspace by providing early warning to pilots and avoiding collisions with other aircraft. The system is based on two avionics devices – the Traffic Alert and Collision Avoidance System (TCAS) and the Ground Proximity Warning System (GPWS).

The TCAS is responsible for detecting and tracking the nearby aircraft that could pose a potential collision threat. It provides a visual and audio alert to the pilot, indicating the direction and altitude of the intruder aircraft. On the other hand, the GPWS is designed to prevent accidents caused by flying too close to the ground. Together, these two systems provide a comprehensive solution for preventing collisions in the air.

Additionally, the ACAS system includes a Mode S transponder that allows the aircraft to communicate its position to air traffic control (ATC). The transponder uses a unique code that identifies the aircraft and provides radar information to other aircraft equipped with TCAS. It helps the pilots to achieve the necessary altitude and speed to maintain safe separation from other aircraft.

Importance of Airborne Collision Avoidance System (ACAS)

The ACAS is a critical and mandatory safety system for commercial airliners. The International Civil Aviation Organization (ICAO) requires all aircraft with a seating capacity of more than 19 passengers or a maximum takeoff weight of over 15,000 pounds to be equipped with ACAS II, which is the current version of the system.

The ACAS has undoubtedly contributed to improving air safety and preventing mid-air collisions. According to the FAA, the system has prevented over 300 potential collisions since its inception. Notably, the system was instrumental in avoiding the Charkhi Dadri mid-air collision between Saudi Arabian Airlines Flight 763 and Kazakhstan Airlines Flight 1907 on 12 November 1996 – one of the deadliest aviation accidents in history.

The ACAS system is not foolproof, and pilots must rely on their training and experience to make the right decisions when receiving an alert. However, the system provides valuable information to pilots that can help them to avoid risks and make informed decisions to avoid potential collisions.


The Airborne Collision Avoidance System (ACAS) is an essential technology-based system that significantly improves air safety by providing early warning to pilots and avoiding collisions with other aircraft. It is a critical system that every commercial airline must-have by law. The system has already prevented several mid-air collisions and continues to be a life-saving technology for aviation. Pilots should undergo regular training on how to use the system properly and make informed decisions to prevent potential airborne collisions. 

Investing in technology that reduces risk and improves air travel safety is critical, and the ACAS is one such innovation. As an aviation enthusiast, it’s essential to stay informed on the latest developments in aviation safety technology and the importance of these innovations for the aviation industry.

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