Airborne Collision and Avoidance System — Page 491, Lesson 582

I want to walk you through the TCAS display symbology now — the symbols you’ll actually see on screen and what each one is telling you. This is the part where situational awareness becomes a concrete picture. Let’s start with the two traffic categories. The first is Proximate Traffic. It appears as a solid cyan diamond. That symbol represents a transponder-equipped aircraft within 6 NM and within ±1200 feet relative height. Now, TCAS does not consider this traffic a threat — it’s displayed purely to improve your situational awareness. So a solid cyan diamond means: close enough to matter, but not a collision risk. The second category is Other Traffic. These appear as hollow cyan diamonds — note the contrast: solid for Proximate, hollow for Other. Other Traffic represents transponder-equipped aircraft within range of the display and within ±2700 feet relative height. But here’s the nuance: that window changes with your climb or descent. In the climb it’s +8700 / −2700 feet; in the descent it’s +2700 / −8700 feet. This is a function of the “Above/Below” switch — the switch tells TCAS which vertical band to look at. Again, this is displayed for situational awareness, not as a threat. Now, the key distinction between these two and an actual threat: the predicted flight paths of Proximate and Other Traffic do not penetrate the Collision Area of your aircraft. That’s the defining line — if a flight path would penetrate the Collision Area, TCAS would treat it as a threat and give you a Resolution Advisory. These two categories are below that threshold. Next, the Data Tag. If the intruding aircraft is transmitting Mode C — that’s altitude-reporting — the traffic symbol will have an associated altitude tag showing relative altitude in hundreds of feet. So a tag reading “+12” means 1200 feet above you. To indicate whether the intruder is climbing, level, or descending: a + sign means the intruder is above, a − sign means below. Then there’s the trend arrow — an up arrow ↑ or down arrow ↓ appears alongside the symbol when the intruder’s vertical rate is 500 feet per minute or greater. Below that rate, no arrow. Two important failure cases here. First, if the intruder is non-altitude reporting — no Mode C — then no altitude number and no trend arrow will appear beside it at all. Second, if TCAS II direction-finding techniques fail to locate the azimuth — the bearing — of another aircraft, a “no bearing” message appears on the screen. Now let’s talk about the Off Scale Traffic Advisory. When TCAS is tracking an intruder that is outside the selected display range, but that intruder has entered the caution or warning areas, then one-half of the appropriate symbol appears at the appropriate bearing at the edge of the display area. So you see half a symbol clipped at the screen edge, pointing in the direction of the threat. The symbol appears in its proper colour, and its data tag is displayed — providing there’s room on the screen. Let me show you what this all looks like on the actual displays. TCAS range and bearing information can appear on a variety of displays. The first is the Dedicated Plan Position Indicator — the PPI. On that display you’ll see the own aircraft symbol at the centre, the intruder symbol, a 2 NM range ring, the relative attitude readout, the off scale message, the no bearing intruder, a TA ONLY message, and a TD FAIL flag — which could also show “TCAS OFF”. There’s also the vertical direction arrow, which shows the rate is greater than 500 feet per minute, and the selected range display. That figure shows the Electronic Vertical Speed Indicator — the EVSI — which is the second display type. TCAS information can be superimposed on it. The third display type is the Navigation Display of EFIS-equipped aircraft — TCAS superimposed in MAP mode. Finally, there’s the Combined TCAS and SSR Control Panel. The control panel comes in various forms, but all perform the same functions. The TCAS controls are what you’ll use to select range, the Above/Below mode, and the TA/RA operating modes. So to tie it together: solid cyan diamond is Proximate Traffic within 6 NM and ±1200 feet; hollow cyan diamond is Other Traffic within ±2700 feet, with the climb/descent asymmetry driven by the Above/Below switch. Neither penetrates the Collision Area. The data tag gives you relative altitude in hundreds of feet, the +/− sign gives above/below, and the trend arrow appears at 500 feet per minute or greater. No Mode C means no tag. No bearing means a message. And off-scale traffic shows as half a symbol at the display edge. That’s the full picture of what TCAS is painting for you.