Airborne Collision and Avoidance System — Page 484, Lesson 573

I want to walk you through the Airborne Collision Avoidance System, and I want to start by setting the scene. Today, with higher traffic densities and greater speed differences between aircraft, we simply need a system that keeps us clear of conflict independently of Air Traffic Control. That's the whole point of this equipment — it's an extra margin of safety that works on board the aircraft itself, not from the ground. Now, the official international name, from ICAO, is ACAS — Airborne Collision Avoidance System. But the principal manufacturer in the United States called its system TCAS, which stands for Traffic alert and Collision Avoidance System, and that name has become so widely accepted that we use it interchangeably. So when you hear ACAS and TCAS, they're referring to the same family of systems. For this system to work, your aircraft must carry a transponder, and it must have the facility to interrogate other aircraft's transponders. That's the fundamental requirement — you're actively asking other aircraft who they are and where they are. There are four systems in use today: I, II, III, and IV, and each one gives increasing levels of protection. Now, here's a regulatory point you need to remember. All commercial air transport turbine-powered aircraft registered in Europe, with a maximum take-off mass greater than 5700 kilograms, or with more than 19 passenger seats, must carry at least ACAS II, version 7. That's a hard legal requirement. Let's look at TCAS I first. It's the first-generation system, and it does something quite simple — it warns the crew of other traffic in the vicinity. It detects and displays range and approximate relative bearing. If both your aircraft and the intruder are carrying Mode C transponders, then relative altitude is also displayed. But here's the key limitation: TCAS I only generates what we call Traffic Advisories, or TAs. These are visual and aural warnings that encourage the flight crew to look for the conflicting traffic. The aural warning is the phrase "Traffic, Traffic." Crucially, TCAS I gives no resolution advisory information — it doesn't tell you what to do, no course of action. Now TCAS II is a step up. It detects intruders in the vicinity, assesses the collision risk, and presents warnings in two forms: Traffic Advisories, the TAs, and Resolution Advisories, the RAs. The RAs are the important ones — they give actual manoeuvring advice in the vertical plane to resolve the conflict. The aural commands include "Climb," "Increase Climb," "Descend," "Increase Descent," "Monitor Vertical Speed," "Decrease Climb," and "Decrease Descent." So you can see, the advice is always in the vertical plane — up or down. There's a clever co-ordination feature here. If your aircraft and the intruder both have Mode S data link transponders, the system will co-ordinate the RAs between the two aircraft, so that you get complementary vertical avoidance instructions. In other words, if one aircraft is told to climb, the other is told to descend — they don't both try to climb into each other. For the rest of this chapter, we deal with TCAS II only, and we'll look at both the visual and audible TAs and RAs in detail. Now, the principle of operation. TCAS II works on the secondary radar principle, using the normal SSR frequencies — 1030 megahertz and 1090 megahertz — but in an air-to-air role. That's the key difference: normally secondary radar is ground-to-air, but here it's aircraft-to-aircraft. Using this principle, the TCAS system creates two protective three-dimensional bubbles around the TCAS-equipped aircraft. Think of two invisible spheres surrounding your aircraft, and the system is constantly assessing whether any intruder is penetrating those bubbles. That's the foundation we'll build on as we go deeper into how the system determines bearing and range.