Airborne Collision and Avoidance System — Page 491, Lesson 582

I want to walk you through the Airborne Collision and Avoidance System — TCAS — and we're starting with how it classifies the traffic around you and how that traffic is drawn on your display. First, the two categories of non-threatening traffic. Proximate Traffic appears as a solid cyan diamond. It represents transponder-equipped aircraft within 6 nautical miles and within plus or minus 1200 feet of your relative height. TCAS does not consider this traffic a threat — it displays it purely to improve your situational awareness. Then Other Traffic appears as hollow cyan diamonds. These are transponder-equipped aircraft within range of the display and within plus or minus 2700 feet relative height. But here's the twist: that window shifts depending on your climb or descent mode. In the climb it's plus 8700 to minus 2700 feet; in the descent it's plus 2700 to minus 8700 feet. That's a function of the "Above/Below" switch on your control panel. Again, this is displayed for situational awareness only. The key point: the predicted flight paths of both Proximate and Other Traffic do not penetrate the Collision Area of your aircraft. That's what keeps them out of the threat category. Now the Data Tag. If the intruding aircraft is transmitting Mode C, its traffic symbol carries an altitude tag showing relative altitude in hundreds of feet. A plus sign means the intruder is above you; a minus sign means below. A trend arrow — up or down — appears alongside the symbol when the intruder's vertical rate is 500 feet per minute or greater. If the intruder is non-altitude-reporting, no altitude number and no trend arrow appear at all. And if TCAS II's direction-finding techniques fail to locate the azimuth of another aircraft, a "no bearing" message appears on the screen. There's also the Off Scale Traffic Advisory. When TCAS is tracking an intruder that's outside your selected display range but has entered the caution or warning areas, one-half of the appropriate symbol appears at the appropriate bearing at the edge of the display area. The symbol shows in its proper colour, and its data tag is displayed providing there's room. Now, where does all this appear? TCAS range and bearing information can show on a variety of displays. First, a Dedicated Plan Position Indicator — the PPI. On that display you'll see the relative attitude intruder symbol, a 2 NM range ring, an off-scale message, a no-bearing intruder, a "TA only" message, your own aircraft symbol, a "TD fail" flag that could show "TCAS OFF", a vertical direction arrow showing rate greater than 500 feet per minute, and the selected range display. Second, the Electronic Vertical Speed Indicator — the electronic VSI. Third, TCAS can be superimposed on the Navigation Display of EFIS-equipped aircraft, in MAP mode. Finally, the Combined TCAS and SSR Control Panel. It's produced in various forms, but all perform the same functions. The TCAS controls on it are what you use to select range, set the Above/Below switch, and manage the system's operating modes. So to tie it together: TCAS classifies traffic by proximity and threat, draws it with specific symbols and data tags, and presents it on whichever display your aircraft is equipped with — a dedicated PPI, an electronic VSI, or the EFIS navigation display — all controlled from the combined TCAS/SSR panel.