Although the sky is so big and aircraft is so small, there were few incidents of midair collision during the early years of aviation. By the 1950s, air travel had become commonplace, and the skies became more crowded.

A collision between aircraft is one of the most sudden and catastrophic transportation accidents imaginable. The first recorded collision between aircraft occurred at the “Milano Circuito Aereo Internazionale” meeting held between 24 September and 3 October 1910 in Milan, Italy.

The first fatal collision occurred in-Douai, France, on 19 June 1912. Captain Marcel Dubois and Lieutenant Albert Peignan, both of the French Army, crashed into one another, killing both pilots. And who can forget the midair collision over the Grand Canyon in 1956 resulting in 128 fatalities.

At that time, that was the worst commercial disaster in history. This collision later eventually led to the establishment of the Federation Aviation Act (FAA) in 1958.

The establishment of the FAA led to major improvements in both airspace design and air traffic control. The air space was designed to keep aircraft separated. The enhancements to airspace design and air traffic control significantly improved the safety of the airspace. However, there were still midair collisions.

A mid-air collision involving a commercial airliner over San Diego, California, in 1978 resulted in 144 fatalities. This collision spurred the development of TCAS, or the Traffic Alert and Collision Avoidance System.


What is TCAS?

TCAS is a system equipped on an aircraft that identifies the location and tracks the progress of aircraft equipped with transponders to prevent mid-air collisions between aircraft operating within the same airspace by warning pilots of transponder-equipped aircraft.

The TCAS is independent of air traffic control and flight navigation instrument. It was based on the fundamental concepts of BCAS (Beacon Collision Avoidance System designed to operate in low-density airspace), but there were enhancements that enabled its use in high-density airspace.

How TCAS Works?


TCAS processes are organized into several elements, as shown in Fig 2. First, surveillance sensors collect state information about the intruder aircraft (e.g., its relative position and velocity) and pass the information to as set of algorithms to determine whether a collision threat exists. If a threat is identified, a second set of threat-resolution algorithms determines an appropriate response.

If the intruder aircraft also has TCAS, the response is coordinated through a data link to ensure that each aircraft maneuver in compatible direction. Collision avoidance maneuvers generated and displaced by TCAS are treated as advisories to flight crews, who then take manual control of the aircraft and maneuver accordingly. Pilots are trained to follow TCAS advisories unless doing so would jeopardize safety.

The TACS system builds a three-dimensional map (Protection Volume) of the airspace around the aircraft. The map range depends on the TCAS configuration.



Challenges for TCAS

TCAS has been very successful in preventing mid-air collisions over the years, but the way in which the logic was designed limits its robustness. Fundamental to TCAS design is the use of a deterministic model. However, recorded radar data show that pilots don not always behave as assumed by the logic.

Not anticipating the spectrum of responses limits TCAS’s robustness, as demonstrated by the collision of two aircraft in 2002 over Überlingen, Germany. TCAS instructed one aircraft to climb, but one pilot descended in accordance with the air traffic controller’s instructions (illustrated in fig 4), leading to a collision with another aircraft whose pilot was following TCAS.

If TCAS recognized the noncompliance of one of the aircraft and reserved the advisory of the complaint aircraft from a descent to a climb, the collision would have been prevented. A modification was later developed to address this specific situation, but improving the overall robustness of the logic requires a fundamental design change.

As the Überlingen accident shows, however, safety cannot be taken for granted, and areas of improvement will always exist in systems that rely on integrating humans and automation for information processing and decision making.


Just as the airspace has evolved since the 1950s, it will continue to evolve over the next decade. Significant change will occur with the introduction of the next generation air traffic management system, which will be based on satellite navigation. This improved surveillance will allow aircraft to fly closer together to support traffic growth.

Unfortunately, the current version of TCAS cannot support the safety and operational requirements of this new airspace. With aircraft flying closer together, TCAS will alert pilots too frequently to be useful. To meet these requirements, a major overhaul of the TCAS logic and surveillance system is needed. TCAS is currently limited to large aircraft capable of supporting its hardware and power requirements.

The aircraft must also have sufficient performance to achieve the required vertical rates of climb or descent that the advisories currently demand. Although a collision avoidance system for small aircraft might help improve safety within general aviation, TCAS cannot be adapted for small aircraft without a costly redesign.

The real challenge lies in integrating new collision avoidance technologies with the existing systems and procedures.

Article Credit: Nishesh Maharjan
Institute of Engineering, Tribhuvan University
Photo Credit: Shawn from Airdrie, Canada


Add Comment

Click here to post a comment

Your email address will not be published. Required fields are marked *