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 for why this system exists at all. Today’s higher traffic densities and greater speed differences have generated a need for an Airborne Collision Avoidance System. That’s the core reason — more aircraft in the sky, moving at very different speeds, means we need an extra layer of protection that works independently of Air Traffic Control. Now, ICAO named this system ACAS, but the principal manufacturer in the US called its system the Traffic alert and Collision Avoidance System, or TCAS, and that name has become the widely accepted alternative. So when you hear ACAS and TCAS, they’re referring to the same family of systems. The system is designed to provide an additional margin of safety and keep commercial aircraft clear of conflict, independently of Air Traffic Control. That independence is a key point — it’s a self-contained safety net on board the aircraft, not something that relies on a ground controller. For it to work, an aircraft must carry a transponder and have the facility to interrogate other aircraft transponders. So it’s actively asking other aircraft’s transponders questions, and listening for their replies. Presently there are four systems in use — I, II, III and IV — each with increasing levels of protection. So the Roman numerals mark the generations, and each one adds more capability. And here’s a regulatory requirement you need to know: all commercial air transport turbine powered aircraft registered in Europe greater than 5700 kg MTOM, or with more than 19 passenger seats, must have at least ACAS II, version 7. MTOM is Maximum Take-Off Mass. So that’s the legal baseline for European commercial operations. Let’s look at TCAS I first. TCAS I is a first generation collision avoidance system, and it simply warns the crew of other traffic in the vicinity of their aircraft. It will detect and display range and approximate relative bearing. So you see how far away the traffic is, and roughly what direction it’s in. If the TCAS display aircraft and the intruder are both carrying Mode C, then relative altitude will also be displayed. Mode C is the transponder mode that reports altitude, so if both sides have it, you get the vertical picture too. TCAS I encourages the flight crew to look for the conflicting traffic by generating visual and aural warnings — these are called Traffic Advisories, or TAs, and the aural warning is the phrase “Traffic, Traffic”. But here’s the critical limitation: TCAS I does not give any resolution advisory information, meaning it doesn’t tell you a course of action to follow. It just says “look out, there’s traffic”. Now TCAS II is where the real protection comes in. 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 Resolution Advisories, or RAs. The RAs are the manoeuvring commands, and they come as phrases like “Climb”, “Increase Climb”, “Descend”, “Increase Descent”, “Monitor Vertical Speed”, “Decrease Climb”, “Decrease Descent”. So the RAs offer manoeuvring advice in the vertical plane to resolve the conflict — it’s all about vertical manoeuvres, not horizontal ones. And there’s a clever co-ordination feature. 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. Mode S is the selective addressing transponder mode, and the data link lets the two aircraft talk to each other so that one climbs while the other descends — complementary instructions, not conflicting ones. That prevents both aircraft from manoeuvring into each other. The rest of this chapter deals with TCAS II only, and discusses both visual and audible TAs and RAs in detail. So we’ll be focusing on TCAS II from here on. Now let’s look at 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. SSR is Secondary Surveillance Radar. 1030 MHz is the interrogation frequency — that’s what TCAS transmits on to ask the question — and 1090 MHz is the reply frequency, what the other aircraft’s transponder responds on. The key phrase is “air-to-air role” — normally secondary radar is ground-based, interrogating aircraft from the ground, but here the aircraft itself is doing the interrogating, aircraft to aircraft. Using this principle, the TCAS system creates two protective three-dimensional bubbles around the TCAS equipped aircraft. Two bubbles — that’s the key structure. One is the outer advisory bubble, which generates the Traffic Advisories, and the inner one is the resolution bubble, which generates the Resolution Advisories. I’ll show you that in Figure 35.1. So to summarise what we’ve covered: ACAS is the ICAO name, TCAS is the manufacturer’s name that stuck. TCAS I gives you traffic advisories only — range, bearing, and relative altitude if both have Mode C. TCAS II adds resolution advisories — vertical manoeuvre commands — and co-ordinates them via Mode S data link if both aircraft have it. And the whole thing works on secondary radar principles at 1030 and 1090 MHz, creating two protective bubbles around your aircraft.