Introduction
ACAS stands for an airborne collision avoidance system. It is essentially a specific type of equipment that warns pilots of aircraft in the vicinity that may pose a threat of collision. It is a general term that encompasses a number of practical implementations of short-range collision avoidance systems such as Airborne Radar, a Ground Proximity Warning System, FLARM, Traffic Collision Avoidance System (TCAS), and other systems. TCAS is, therefore, is one of the practical implementations of ACAS.
Main body
TCAS scans the surrounding airspace in order to detect the presence of other aircraft that present collision danger. For TCAS to function, an aircraft has to have an active transponder installed. Such transponders are required by the International Civil Aviation Organization. The system can work in an automatic mode connecting with TCAS installed in the other aircraft to prevent a collision. Separate instructions are given to two vehicles by computers while pilots control their execution.
The system continuously requests the position of other aircraft with an installed transponder sending a radio signal on the 1.03 GHz frequency. Other aircraft use 1.09 GHz radio frequency to respond. The request-response cycle is completed within milliseconds. The position parameter is three-dimensional and includes other aircrafts’ altitude, proximity, and bearing. Once these data are received, the system calculates altitude and proximity including their predicted values, and generates a verdict on the danger of collision. TCAS consists of a computer unit for calculations, antennas for signal reception and transmission, and a user interface with traffic and resolution advisory displays (they are usually imbued into a single display unit).
On a display called cockpit display of traffic information (CDTI), the pilot is able to see several signs. An open blue or cyan diamond means that the detected aircraft poses no danger and is distant enough. A filled blue or cyan diamond means that an aircraft poses no danger but is in close proximity (>6 nautical miles of range, and +-1200 feet altitude). A yellow or orange-filled circle means that traffic is detected in close proximity and an alert is issued. A red-filled square means the ultimate level of danger and prompt evasion maneuvers are in order. Depending on the position of the detected aircraft, the system may issue the following alerts: traffic advisory or TA, Resolution advisory or (RA), and Clear of conflict.
The additional usage of the TCAS system is to prevent terrorist attacks that are aimed at destroying passenger vehicles by a metal-to-metal collision. TCAS detects a threat and lets the pilot avoid it by changing the aircraft’s position. The data gathered by TCAS on the incidence of various traffic and resolution advisory alerts can be used for research and enhancement of the system to make flights safer.
Conclusion
Airbus A320 is equipped with TCAS II version 7.1. The difference between TCAS I and II that is installed in A320 is that it offers vocalized instructions for pilots. TCAS II uses coded text announcements such as “descend, descend” which means that the pilots need to descend at 1500–2000 ft/min. Once the instructions are fulfilled, the vocalized assistant will announce “clear of conflict” which means that the danger is successfully avoided and the previous altitude needs to be restored. The visual cockpit representation of TCAS in Airbus A320 is the same as described above. The A320 TCAS system may work in standby, transponder, traffic advisory-only, and automatic modes.