Airborne Collision and Avoidance System — Page 484, Lesson 580

I want to walk you through the heart of the TCAS — the Traffic Alert and Collision Avoidance System — and specifically the two levels of advisory it produces. This is where the system actually starts talking to you. First, let's set the stage. The TCAS can monitor a large number of aircraft simultaneously — up to 30 can be displayed at once — and it can produce resolution advice against multiple conflicts at the same time. So it's not a one-track system; it's tracking a whole sky full of traffic and can give you guidance on more than one threat. Now, the key concept here is the Closest Point of Approach, abbreviated CPA. This is the point in space where the intruder aircraft will come closest to you. In the calculations, this is denoted by the Greek letter tau — that's the letter T in the Greek alphabet, written as τ. So when you see tau in TCAS discussions, it's referring to time to the Closest Point of Approach. I should tell you straight away — the actual calculations are not required knowledge for your exams, and they're not included in this book. What matters is understanding what the system does with that timing. Depending on three things — the setting of the TCAS function switch on the control panel, the equipment level of the intruder aircraft, and the phase of flight of your own TCAS aircraft — the system will generate one of two types of advisory. Let me take them in order. First, Traffic Advisories, abbreviated TAs. A TA exists when an intruder penetrates the outer bubble caution area and is between 45 and 35 seconds from the collision area. So think of an invisible bubble around your aircraft — the outer bubble is the caution zone. When another aircraft crosses into that bubble and is 45 to 35 seconds away from the point of collision, you get a TA. On the TCAS display, TAs appear as solid amber circles — solid, not hollow — and they're accompanied by a synthetic voice saying "Traffic, Traffic." This is a potential collision threat. Note the word "potential" — it's a caution, not yet a command. Second, Resolution Advisories, abbreviated RAs. An RA exists when an intruder penetrates the inner bubble warning area and is between 35 and 15 seconds from the collision area. So this is the inner, tighter bubble — closer in time. RAs appear as solid red rectangles on the TCAS display, and they're accompanied by various synthetic voice warnings — different voice messages depending on the situation. RAs indicate a serious collision threat. This is the level where you need to act. Let me show you this visually. Now, an important point about what ACAS is not for. It is not the job of the ACAS system to maintain standard Air Traffic separation minima. That's the controller's job. ACAS is purely a collision avoidance tool. If RAs are responded to smoothly and immediately, then the intended vertical separation varies from 300 to 600 feet — and it's 700 feet above FL410 — depending on the Sensitivity Level, abbreviated SL, which decreases with altitude. So the higher you fly, the lower the sensitivity level, and the separation figure changes accordingly. Let me be precise: 300 to 600 feet normally, but 700 feet above flight level 410. And the Sensitivity Level is what drives that number. Now, Resolution Advisories come in two forms, and this is a critical distinction. The first is a Preventative Advisory. This is a situation where no collision risk exists unless a change of level is initiated by either aircraft. In other words, if everyone just holds their current altitude, you're fine — the danger only appears if someone starts climbing or descending. The synthetic voice advisory for this is "Monitor Vertical Speed." So it's telling you to watch your vertical speed indicator and not change level. The second is a Corrective Advisory. This is a situation where a collision risk does exist, and a manoeuvre is necessary to avert it. Here the synthetic voice produces the appropriate command — it tells you what to do, like "Climb" or "Descend." Let me show you what these look like on the vertical speed tape of the primary flight display. So to pull it all together: TAs are the early warning — amber circles, "Traffic, Traffic," 45 to 35 seconds out, a potential threat. RAs are the action command — red rectangles, voice commands, 35 to 15 seconds out, a serious threat. And within RAs, preventative means "don't change level, monitor your vertical speed," while corrective means "you must manoeuvre now."