Airborne Collision and Avoidance System — Page 491, Lesson 587

I want to walk you through the actions a flight crew must take when the Airborne Collision and Avoidance System — TCAS — gives you a Traffic Advisory, or TA, and a Resolution Advisory, or RA. This is the part of the syllabus where we turn the theory of how TCAS works into the actual drill you will fly. Let’s start with the Traffic Advisory, the TA. A TA is an alert that tells you there is a potential threat out there, and it is warning you that a Resolution Advisory — the RA — may follow. And that RA, if it comes, could require you to change your flight path. So the moment you get a TA, your job is to assimilate the information — take in what the system is telling you about where the traffic is — and then commence a visual search of that part of the sky. You are also to prepare to respond to an RA if the situation worsens. Now here is a critical point: if you cannot see the potential threat, and it continues to give you cause for concern, you should seek advice from ATC. But note this hard rule — you do not manoeuvre on the basis of a TA alone. A TA is a heads-up, not a command to turn. Now the Resolution Advisory, the RA. This is the command. When you receive an RA, pilots are to initiate the required manoeuvre immediately, adjusting flight path, aircraft power, and trim accordingly. So it is not just a turn — you are adjusting the flight path, you are adjusting power, and you are adjusting trim, all to comply with the advisory. While the pilot flying is doing that, the crew members not involved in executing the manoeuvre should confirm that the sky ahead is clear of other aircraft, and continue the visual search for the established threat. And you are to inform ATC as soon as possible of any deviation from an ATC clearance. That is a legal and operational obligation — you have deviated from your clearance, so ATC must know, as soon as possible. Now the most important part — disregarding RAs. The rule is that manoeuvres should never be made in a direction opposite to that given in an RA. Why? Because the sense — that is, the direction of the manoeuvre — may have been determined following an exchange of data with the established threat. In other words, TCAS has talked to the other aircraft’s transponder and worked out which way you both need to go to separate. If you reverse it, you could fly straight into the conflict. So when can an RA be disregarded? Only when pilots visually identify the potentially conflicting traffic and decide that no deviation from the current flight path is needed. That is the single condition — visual identification, and a decision that no deviation is necessary. And there is a second rule for a conflict between ATC and TCAS: if you receive simultaneously an instruction to manoeuvre from ATC and an RA, and both conflict, the advice given by TCAS should be followed. TCAS wins. Now, I want to show you one of the practice questions from this section, because it tests exactly this rule. The question asks when RAs may be disregarded. One option says they may be disregarded only when the pilot visually identifies the conflicting traffic and decides no deviation is necessary — that matches the rule we just covered. Another option adds the extra condition that you must also have advised ATC of the other aircraft’s proximity — that is not part of the rule. Another says RAs must never be disregarded — that is wrong, because there is that one permitted case. And another says the same as the correct one but without the visual identification requirement — that is also wrong, because visual identification is essential. So the key is: visual identification, plus a decision that no deviation is needed. That is the only circumstance. Let me leave you with the core of this lesson. A TA is information — assimilate it, look, prepare, but do not manoeuvre. An RA is a command — act immediately, adjust flight path, power, and trim, clear the sky ahead, and tell ATC as soon as possible. And never manoeuvre opposite to an RA, because the sense was determined by data exchange with the threat. If ATC and TCAS conflict, TCAS wins. And an RA may be disregarded only when you visually identify the traffic and decide no deviation is needed.