AirborneCollision andAvoidanceSystem — Page 90, Lesson 73

I want to walk you through the Airborne Collision and Avoidance System — TCAS — starting with the two key types of alerts it produces: Traffic Advisories and Resolution Advisories. First, let's be clear on what TCAS does. The TCAS can monitor a large number of aircraft simultaneously — up to 30 can be displayed — and it can produce resolution advice against multiple conflicts at the same time. It bases its advice on the target aircraft's Closest Point of Approach, which is denoted by the Greek letter tau — that's the letter T — in the calculations. The actual calculations are not required knowledge for us, so they're not included in this book. Now, depending on 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 TCAS-equipped aircraft, TCAS will generate two main types of advisories. The first is a Traffic Advisory, or TA. 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 it as a warning that something is getting close — you have 45 to 35 seconds before a potential collision. TAs appear as solid amber circles on the TCAS display, and they are accompanied by the synthetic voice saying "Traffic, Traffic." This is a potential collision threat — not yet a certain one, but something you need to be aware of. The second type is a Resolution Advisory, or RA. An RA exists when an intruder penetrates the inner bubble warning area and is between 35 and 15 seconds from the collision area. That's a much tighter window — you have between 35 and 15 seconds. RAs appear as solid red rectangles on the TCAS display, accompanied by various synthetic voice warnings. RAs indicate a serious collision threat. This is the point where you need to act. Let me show you the geometry. You can see in Figure 35.4 the collision warning area at the center, then the caution area around it. The RA zone is from 15 to 35 seconds, and the TA zone is from 35 to 45 seconds. So the outer ring is the caution area for TAs, and the inner ring is the warning area for RAs. Now, a very important point about RAs: they must never be disregarded. That's a hard rule. The only exception is that RAs may be disregarded only when the pilot visually identifies the potentially conflicting traffic and decides that no deviation is necessary. So if you can see the other aircraft and you're certain there's no conflict, then — and only then — can you disregard the RA. Otherwise, you follow it. Let me also touch on a related system that appears later in this excerpt — the Engine Indicating and Crew Alerting System, or EICAS. EICAS alert messages are shown on the upper display unit in three forms. Level 'C' are warnings that require immediate corrective action. Level 'A' are cautions that require immediate crew awareness and possible action. Level 'B' are advisories requiring crew awareness. So the severity goes: C is most urgent, then A, then B. And finally, there's a question about an engine electronics system that in normal conditions of flight shows only the primary engine instruments. That system is an EICAS system with EPR, EGT, and N1 shown on the primary instruments. EPR is Engine Pressure Ratio, EGT is Exhaust Gas Temperature, and N1 is the low-pressure compressor speed — those are your primary engine parameters. So to summarise: TCAS gives you TAs as solid amber circles with "Traffic, Traffic" at 45 to 35 seconds, and RAs as solid red rectangles with voice warnings at 35 to 15 seconds. RAs must never be disregarded unless you visually confirm it's safe. And EICAS categorises alerts as Level C (warnings), Level A (cautions), or Level B (advisories).