Airborne Collision and Avoidance System — Page 491, Lesson 585

Let’s pick up right where the system’s controls leave off. We’ve got the Function Switch, and I want to walk you through each position, because each one changes exactly what the system is doing for you. First, Standby. In Standby, warm-up power is applied to the system, but it is not operational. Think of it as the system warming up its electronics, ready to go, but not yet transmitting or tracking anything. Next, On. In the On position, only the transponder is operational. That means your aircraft will be visible to other ACAS-equipped aircraft, but you are not yet receiving any traffic or resolution advisories yourself. You’re a target for others, but you’re not actively looking. Then we have TA. Here, the transponder and TCAS are both operational, but only Traffic Advisories are generated. The display will show “TA ONLY” on the TCAS display. TA would only be selected in accordance with specific procedures. For example, after an engine failure, when aircraft performance is reduced, or if parallel runway operations are in force. So this is a degraded mode—you still see traffic, but you won’t get the climb or descend commands. Then RA/TA. This is the full operational mode. The transponder and TCAS are operational, and both Resolution Advisories and Traffic Advisories are generated. This is the normal, full-capability setting. Finally, TEST. Pressing the centre TEST button on the function switch initiates a full Built-in Test Equipment, or BITE, of the system. After a successful test, the synthetic voice will respond with “TCAS SYSTEM TEST OK.” If the test is unsuccessful, the voice response is “TCAS SYSTEM TEST FAIL.” So you get an audible confirmation of system health. Now, let’s look at the TCAS RNG, which stands for Range. This selects the range of the TCAS display—either 5, 10, or 20 nautical miles. It’s important to understand that this does not alter the range at which aircraft are detected or when warnings are given. It only changes the scale of what you see on the display. Detection and warning logic stay the same regardless of the range you select. Now, let’s talk about how advisories appear on the Electronic VSI. We have Figure 35.10 showing a Traffic Advisory, and Figure 35.11 showing an Off Scale Traffic Advisory. These are the visual representations of traffic that TCAS has detected. Then we have Preventative Resolution Advisories in Figure 35.12. A preventative RA is one that tells you to maintain your current altitude—it’s a “don’t climb, don’t descend” type of advisory, preventing you from entering a conflict. Then Figure 35.13 shows a Corrective Resolution Advisory. This is the more urgent one—it tells you to actually change your vertical speed, like “climb” or “descend,” to resolve the conflict. And Figure 35.14 shows the TCAS Test Format on the Electronic VSI—what the display looks like when you run that BITE test we talked about. Now, there’s a very important concept called No Bearing Advisories. If TCAS is unable to track the bearing of an intruder—possibly due to antenna screening—the RA or TA will appear in the lower centre of the display, appropriately colour coded. Up to two lines of information can be displayed. Let me decode that example for you. “TA 2.2- 04” means the intruder is creating a Traffic Advisory, 2.2 nautical miles away, 400 feet below you. And the up arrow indicates the intruder is climbing at 500 feet per minute or greater. Here’s the critical point: TCAS’s ability to compute a Traffic or Resolution Advisory is not degraded by lack of bearing information. Even without knowing the direction, TCAS can still tell you there’s a threat and give you the vertical guidance you need. That’s a key safety feature—you don’t need bearing to get a valid advisory. So, to tie it all together: the Function Switch controls what the system does, the RNG controls your display scale, and the advisories—whether Traffic, Preventative, or Corrective—are what the system gives you to act on. And even when bearing is lost, the system still protects you.