TCAS: Collision warning system
increases flight safety

"Electronic protection shield" for every Lufthansa aircraft
Threat caused by near misses significantly reduced

The introduction of the collision warning system TCAS (Traffic Alert and Collision Avoidance System) has substantially reduced the threat posed by dangerous encounters of two aircraft in the air, the so-called near misses. The entire Lufthansa fleet has been equipped with the TCAS II in use today; the system is required on flights in European and US airspace.

TCAS provides the pilot with information about air traffic in his immediate air space and operates independently of radar facilities on the ground. The system makes use of two antennae - one mounted on the top and the other on the bottom of the aircraft fuselage - which transmit or receive the signals of the transponder. A transponder can be "interrogated" on a pre-determined frequency - it then transmits information, such as the altitude of the aircraft and its individual identification, on a second frequency. 

An electronic protective shield of 25 kilometers in range

Every aircraft equipped with TCAS is surrounded by an electronic protective shield with a diameter of about 25 kilometers. The system reacts immediately if a second aircraft enters this area, classifying it as an "intruder". Unlike radar, the TCAS computer does not calculate the precise position of the potentially dangerous aircraft; instead, it shows how much time remains to the point at which the two objects would meet (CPA, closest point of approach). This makes good sense because aircraft are travelling at different speeds; some are climbing, others descending; they may be flying straight or turning.

The potential danger posed by two aircraft approaching each other is classified in one of four categories. As soon as an approaching aircraft enters the electronic protective shield, it is displayed on the TCAS screen; different symbols correspond to the danger categories. Category One is straightforward traffic information. When Category Two is reached, the system calls attention to the possible danger. Starting at Category Three, which means an increase in the threat level, the pilot receives acoustical warnings in addition to the display on the screen. When Category Four is reached, the crew is instructed to begin an evasive manoeuvre in order to avoid a collision. Depending on the proximity of the aircraft, the TCAS system provides warning times of between 20 and 45 seconds, virtually excluding the need for abrupt evasive manoeuvres. As a rule, the passengers remain unaware of the changes in the aircraft's flight path prompted by the TCAS system. 

The computer gives clear instructions

If TCAS recognizes a Category Four threat, i.e., an evasive manoeuvre becomes necessary, one of the two aircraft's TCAS computers must make the initial decision, forcing the other computer to take corresponding counter-measures. The responsibility for the initial decision is unambiguously regulated by an identification address which has been fixed for each aircraft. Evasive manoeuvres based on TCAS are always performed only in a vertical direction: if one aircraft climbs, the other must descend.

TCAS is a passive system which does nothing more than provide information to the crew; it does not intervene in the control of the aircraft. The final decision as to how to deal with a dangerous situation is always made by the pilot.