Airborne Collision and Avoidance System — Page 82, Lesson 71

I want to walk you through the Airborne Collision and Avoidance System, or ACAS. This is a critical safety system designed for today’s higher traffic densities and greater speed differences between aircraft. The International Civil Aviation Organization, ICAO, named it ACAS, but the principal manufacturer in the United States called its system the Traffic Alert and Collision Avoidance System, abbreviated TCAS, and that name is now a widely accepted alternative. The system is designed to provide an additional margin of safety and keep commercial aircraft clear of conflict, independently of Air Traffic Control. That means it works as a backup or supplement to ATC, not relying on ground controllers to keep you safe. For the system to function, an aircraft must carry a transponder and have the facility to interrogate other aircraft transponders. So your aircraft sends out interrogations, and other aircraft reply with their transponders. There are four systems in use: TCAS I, II, III, and IV, each with increasing levels of protection. All commercial air transport turbine-powered aircraft registered in Europe with a maximum take-off mass, MTOM, greater than 5700 kilograms, or with more than 19 passenger seats, must have at least ACAS II version 7. Let’s look at TCAS I first. TCAS I is a first-generation collision avoidance system. It simply warns the crew of other traffic in the vicinity of their aircraft. It will detect and display range and approximate relative bearing. If the TCAS display aircraft and the intruder are both carrying Mode C, relative altitude will also be displayed. Mode C gives altitude information from the transponder. TCAS I encourages flight crew to look for the conflicting traffic by generating visual and aural warnings called Traffic Advisories, or TAs. The aural warning is the phrase "Traffic, Traffic". Importantly, TCAS I does not give any resolution advisory information — that is, it does not tell you a course of action to follow. It just says "look out". Now TCAS II. TCAS II detects intruders in the TCAS aircraft's vicinity, assesses the collision risk, and presents warnings to the crew in the form of TAs and also Resolution Advisories, or RAs. An example of an RA is "Climb" or "Decrease Descent". So RAs offer manoeuvring advice in the vertical plane to resolve the conflict. If your aircraft and the intruder both have Mode S data link transponders, the system will co-ordinate the RAs to provide complementary vertical avoidance instructions. That means one aircraft gets a "Climb" command and the other gets a "Descend" command, so they don't both manoeuvre in the same direction. The rest of this chapter deals with TCAS II only and discusses both visual and audible TAs and RAs in detail. Now the principle of operation. TCAS II operates on the secondary radar principle, using the normal SSR frequencies of 1030 MHz and 1090 MHz, but in an air-to-air role. Secondary surveillance radar normally works ground-to-air, but here the TCAS aircraft itself acts as the interrogator and the intruder replies. Using this principle, the TCAS system creates two protective three-dimensional bubbles around the TCAS equipped aircraft, as shown in Figure 35.1.