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 how it decides to warn you. This is where the system earns its keep, so let's build it up carefully. First, the big picture. 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-target system; it's tracking a whole sky full of traffic and can give you guidance on more than one threat. Now, here's the key concept that drives everything: the system bases its advice on the target aircraft's Closest Point of Approach, which is denoted by the Greek letter tau (τ) in the calculations. The Closest Point of Approach is exactly what it sounds like — the point in space where the intruder will come closest to you if both of you continue on your current tracks. Tau is the time to that point. The actual mathematics aren't required knowledge for your exam, so we won't go into the formulas — but you must understand that tau is the timer the system uses to decide how urgent a threat is. 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's take the first type: Traffic Advisories, or 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 two invisible bubbles around your aircraft. The outer one is the caution area. When an intruder crosses into it, and tau is between 45 and 35 seconds, you get a TA. On the display, TAs appear as solid amber circles, and they're accompanied by the synthetic voice saying "Traffic, Traffic." This is a potential collision threat — the word "potential" is important. It's a heads-up, not a command. Now the second type: Resolution Advisories, or 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 the inner bubble is the warning area, and the time window is tighter — 35 down to 15 seconds. RAs appear as solid red rectangles on the TCAS display, and they're accompanied by various synthetic voice warnings. RAs indicate a serious collision threat. This is the big one — this is the system telling you to act now. Let me show you this visually. Now, before we go further, I need to correct a common misconception. It is not the job of the ACAS system — that's the Airborne Collision Avoidance System, the formal name for TCAS — to maintain standard Air Traffic separation minima. That's the air traffic controller's job. ACAS is purely a collision-avoidance backstop. Here's the important part: 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, or SL, which decreases with altitude. So the higher you are, the lower the sensitivity level, and the separation the system aims for changes accordingly. Note the phrase "smoothly and immediately" — that's how you're trained to respond to an RA. Now, Resolution Advisories come in two forms, and you must know the difference cold. First, Preventative Advisories. These are situations 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 climbs or descends. The synthetic voice advisory for this is "Monitor Vertical Speed." That's the command — you're not being told to manoeuvre, you're being told to watch your vertical speed and not change it. Second, Corrective Advisories. These are situations where a collision risk does exist, and a manoeuvre is necessary to avert it. Here the synthetic voice produces the appropriate command — something like "Climb" or "Descend." This is the system telling you to change your vertical path now. Let me show you what these look like on the vertical speed tape of the primary flight display. So to tie it all together: the TCAS watches up to 30 targets, measures time to the Closest Point of Approach using tau, and based on how close that time gets, it steps you through two levels of warning — the amber TA at 45 to 35 seconds, and the red RA at 35 to 15 seconds. The RA then splits into preventative — "Monitor Vertical Speed" — and corrective — an actual climb or descend command. And remember, the separation it's aiming for is 300 to 600 feet, or 700 above FL410, depending on sensitivity level. That's the whole decision chain.