Airborne Collision and Avoidance System — Page 491, Lesson 585

I want to walk you through the Airborne Collision and Avoidance System — ACAS — and specifically the pilot-facing controls and displays. This is the part of the system you actually touch and read in the cockpit, so let's start with the Function Switch, because that's the master control. The Function Switch has five positions, and each one changes what the system is doing. Let's take them in order. First is Standby. In Standby, warm-up power is applied to the system, but it is not operational. So the equipment is powered up and warming, but it's not transmitting or doing any surveillance yet. Second is On. In this position, the transponder only is operational. That means your aircraft will be visible to ACAS-equipped aircraft — other aircraft with TCAS can see you — but you are not yet generating any advisories yourself. You're a target for others, but you're not actively tracking them. Third is TA. Here the transponder and TCAS are now both operational, but only Traffic Advisories are generated. A Traffic Advisory is a warning about nearby traffic. When you select TA, the words "TA ONLY" will be indicated on the TCAS display. Now, TA would only be selected in accordance with specific procedures. For example, after an engine failure, when the aircraft performance is reduced, or if parallel runway operations are in force. The idea is you're in a degraded or sensitive situation where you don't want the system issuing Resolution Advisories — the climb or descend commands — so you restrict it to traffic warnings only. Fourth is RA/TA. Here the transponder and TCAS are operational, and both Resolution Advisories and Traffic Advisories are generated. This is the normal, full operating mode. A Resolution Advisory is the instruction to climb or descend to resolve a conflict — we'll see those on the display shortly. Fifth is TEST. Pressing the centre TEST button on the function switch initiates a full Built-in Test Equipment — BITE — of the system. After completion of a successful test, the synthetic voice will respond with "TCAS SYSTEM TEST OK". If the system test is unsuccessful, the voice response is "TCAS SYSTEM TEST FAIL". So you get an audible confirmation either way. Now, next to the function switch is the TCAS RNG — the range selector. This selects the range of the TCAS display: either 5, 10, or 20 nautical miles. And this is a critical point — it does not alter the range at which aircraft are detected or when warnings are given. It only changes the scale of the display you're looking at. The surveillance and warning logic is independent of what range you've selected. Now let's look at how the advisories appear on the Electronic VSI — the vertical speed indicator. I want you to picture the display. We have Figure 35.10, a Traffic Advisory, and Figure 35.11, an off-scale traffic advisory. Then Figure 35.12 shows preventative Resolution Advisories, and Figure 35.13 shows a corrective Resolution Advisory. And Figure 35.14 shows the test format on the Electronic VSI. Let me explain the difference between preventative and corrective. A preventative RA is one that tells you to avoid certain vertical speeds — it restricts your climb or descent range, but it doesn't command you to change direction. A corrective RA is the one that commands you to climb or descend — it tells you to change your vertical speed to resolve the conflict. Now, there's an important case 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 lower centre of the display, appropriately colour coded. Up to two lines of information can be displayed. Let me decode an example for you. The display reads "TA 2.2- 04" with an up arrow. Here's what that means: the intruder is creating a Traffic Advisory, 2.2 nautical miles away, 400 feet below you — that's the "-04" meaning 400 below — and the up arrow indicates the intruder is climbing at 500 feet per minute or greater. And here's the key point to remember: TCAS's ability to compute a Traffic or Resolution Advisory is not degraded by lack of bearing information. Even without a bearing, the system can still determine range and altitude and issue the advisory. The bearing is just for your situational awareness on the display. So to tie it all together: the Function Switch controls what the system does — from standby, to being visible to others, to traffic-only, to full RA/TA operation, to self-test. The RNG control changes only your display scale. And on the Electronic VSI, you'll see traffic advisories, preventative and corrective resolution advisories, test formats, and the no-bearing format when bearing information is unavailable.