TCAS - A milestone in flight safety

Development of a technical solution

It took more than 30 years to develop a Traffic Alert and Collision Avoidance System (TCAS) before it was finally made mandatory by the U.S. Federal Aviation Administration (FAA) in 1987 in the wake of another incident. Every commercial aircraft had to have such a system on board when entering the USA. In Europe and other countries affiliated to the worldwide aviation organization ICAO, the obligation was not introduced until around the 2000s.

Transponder signals as a key

The main problem of a collision warning system is, besides the implementation and design, the reaction time of the pilots. The systems must be compatible among all aircraft and must be able to communicate with each other. To transmit the signals, the existing transponder system is used. The transponder of a commercial aircraft transmits numerous data, such as flight direction, altitude, exact identification and other data packages to the air traffic controller. During this broadcast, the connected TCAS systems also communicate with each other.

Presentation and implementation

During the exchange, the flight paths of both or more participants are evaluated and checked for possible conflicts. This happens within defined limits. Not only the time span until a dangerous approach is made, but also the pure distance between two airplanes is measured. This can be relevant if one aircraft slowly approaches another from behind. All aircraft in the immediate vicinity are displayed by TCAS on the pilots’ navigation displays. One can see position, flight altitude in relation to one’s own altitude, and any climbing or sinking of other aircraft.

Information to the pilots – the Traffic Advisory

To pass on conflicts to the pilot, TCAS triggers two basic types of messages. First there is a “Traffic Advisory TA”. This is merely a highlighting of nearby traffic, from whose flight path a threat could possibly arise. On most displays this warning is shown in yellow, together with a verbal computer voice “TRAFFIC – TRAFFIC”. In this way, pilots can prepare mentally in time for a possible evasive manoeuvre.

Collision imminent – the Resolution Advisory

If the TCAS now calculates a possible collision in a further step, it reacts with a “Resolution Advisory RA”. This “recommendation” is shown in red and gives the pilot direct recommendations for evasive action. They are only made vertically and are coordinated between all TCAS systems concerned. Thus, one aircraft receives the recommendation to descend while the other system instructs the pilot to climb. If several aircraft are affected, the TCAS can also make one aircraft fly straight ahead while the higher one climbs and the lower one descends. This manoeuvre is always flown manually by the pilot. Coupling to the autopilot is not permitted.

2002 Überlingen air crash

collision warning - tower - air traffic controller

Experience in the early years of TCAS shows that there were often conflicts with the instructions of the air traffic controllers. There have already been several cases where the cockpit crew followed the familiar voice of the controller rather than the instructions of TCAS. This can have dramatic consequences if the radar image is incorrectly assessed and the instructions are incorrect.

The Überlingen accident ended particularly tragically on July 1, 2002, when a DHL cargo plane collided with a Russian commercial aircraft. While the cargo plane followed the recommendation of the TCAS, the Russian crew listened to a wrong instruction of the air traffic controller. Soon after, the two planes collided.

TCAS overrules the human being

In response to such incidents in the past, pilots these days are clearly advised that in such a case, only the TCAS must be followed. The air traffic controller merely receives the information “TCAS RA” and lets the aircraft resolve the conflict among themselves. As soon as the aircraft have passed each other and the danger has passed, the TCAS triggers the command “Clear of Conflict”. Both planes then return to their original flight altitude. This manoeuvre becomes second nature to every pilot, as it is trained over and over again in the flight simulator.