Let's pick this up right where the regulations get practical — Page 249, Lesson 326

Let's pick this up right where the regulations get practical. We're now looking at how you actually use ACAS — the Airborne Collision Avoidance System — and what the rules demand of you as the pilot when it starts talking to you. First, the overarching principle. The whole point of ACAS alerts and the procedures that go with them is to help you, but they do not relieve you of the basic responsibility for the safety of the aeroplane. In every case, you are expected to exercise your best judgment and your full authority in choosing the best course of action. So ACAS is a tool, not a replacement for your command decision. Now, let's break down the alerts themselves. ACAS generates two distinct types of alerts: Traffic Advisories, or TAs, and Resolution Advisories, or RAs. A Traffic Advisory is intended to alert you to the possibility of an RA being generated. Think of it as a heads-up — the system is telling you "watch out, something might develop here." And here's the critical rule: pilots are not to manoeuvre the aircraft in response to a TA only. You do not turn or climb or descend just because you get a TA. It's a warning, not a command. Now, if an RA is generated — that's the Resolution Advisory — that's the actual command. The pilot is to respond immediately, unless responding would jeopardize the safety of the aeroplane. So there's a built-in override: if following the RA would put you in danger, you don't follow it. But the default is immediate compliance. And there's a priority hierarchy you need to remember. Stall warning, wind shear, and GPWS alerts will have priority over ACAS. So if you get a stall warning, or a wind shear alert, or a Ground Proximity Warning System alert, those come first. ACAS is important, but it sits below those three in the priority list. One more subtle but crucial point about visual perception. You must be aware that visually acquired traffic may not be the cause of an RA. Just because you see an aircraft out the window doesn't mean that's the one the system is responding to. And visual perception of an encounter can be misleading, especially at night. So don't assume what you see is what the RA is about. Now, what happens if there's a conflict of Resolution Advisories between aircraft? That means both aircraft are getting RAs that, if followed, would put them into each other. In that case, pilots must follow the RA, and the equipment should offer a Reversal Advisory — that's the system telling one aircraft to reverse its manoeuvre to resolve the conflict. Then there's the ATC coordination side. As soon as the flight deck crew's workload permits, ATC should be notified of the RA and the direction of any deviation from the current flight plan. So you tell ATC what the RA told you to do and which way you're deviating. Once the conflict is resolved, the aircraft should be promptly returned to the requirements of the current flight plan, and again, ATC informed. Now here's a tricky scenario. Under some circumstances, it's possible that ATC issues instructions that are unknowingly contrary to ACAS RAs. ATC doesn't always see what the system sees. It is vitally important that ATC is notified when an ATC instruction or clearance is not being followed because it conflicts with an ACAS RA. So if you have to deviate from what ATC told you because the RA demands it, you must tell them why. Finally, the question of when you can disregard RAs. This is tightly restricted. RAs may be disregarded only when the pilot visually identifies the potentially conflicting traffic and decides no deviation from the current flight path is needed. So you see the traffic, you judge that you don't need to move, and only then can you ignore the RA. But even in that case, ATC must be informed of the circumstances. So the whole picture is: TAs warn, RAs command, stall/wind shear/GPWS outrank ACAS, ATC gets notified of deviations and conflicts, and disregarding an RA is a narrow, visually-justified exception that still requires an ATC call. That's the regulatory framework for ACAS use.