022 Instrumentation topic guide
TCAS: Traffic and Resolution Advisories
TCAS does not simply measure how far away another aircraft is. It interrogates nearby transponders, usually via Mode S, and from the replies builds range, altitude difference, and closure rate over successive interrogations, then projects that data forward to estimate the time remaining to the closest point of approach. That projected time, not raw distance, is what actually drives when an alert is issued.
The exam tests this system in three linked ways: the difference between what a Traffic Advisory and a Resolution Advisory each mean, why an RA only ever commands a vertical manoeuvre, and the rule that a pilot must follow that vertical command even if it seems to conflict with what air traffic control is saying at the same moment.
What TCAS is actually computing
Because the system works from time to the closest point of approach rather than distance alone, two aircraft can be quite close together without triggering anything, if their flight paths mean that distance is stable or opening, while two aircraft further apart but closing fast can trigger an alert sooner. This projection is also what lets TCAS give a meaningfully earlier warning for a fast head-on closure than for a slow overtake.
Traffic Advisory and Resolution Advisory
A Traffic Advisory tells the crew where to look; it carries no manoeuvre instruction and is typically issued with something in the order of 40 seconds remaining to the projected closest point of approach. A Resolution Advisory goes further and commands a specific vertical manoeuvre, typically with something in the order of 25 seconds remaining, because by that point a positive, immediate action is what actually restores separation.
Resolution Advisories are vertical only on TCAS II. Vertical movement is far more predictable and controllable at the closure speeds involved than a lateral turn would be, and it lets two TCAS-equipped aircraft coordinate directly with each other through the Mode S data link, so that one is commanded to climb while the other is commanded to descend, rather than both independently choosing the same sense by chance.
Following the RA, and when TCAS steps back
Because an RA is generated from a direct, continuously updated picture of the conflicting traffic, the standard procedure is to follow it promptly, even where it appears to contradict an instruction just given by air traffic control, and to advise control of the manoeuvre once workload allows rather than querying it first. Delaying to seek clarification defeats the purpose of a system built specifically for last-resort, time-critical conflicts.
Close to the ground, TCAS steps back in stages: Resolution Advisories are inhibited below a defined radio height, and Traffic Advisories are inhibited even lower still, because there is too little room and too little time left for a vertical manoeuvre to be the right tool, and other systems, along with the crew's own lookout and standard procedures, take over that protection instead.
Worked example
Worked example: a Resolution Advisory against an ATC instruction
An aircraft receives a Resolution Advisory commanding a descent at the same moment that air traffic control instructs the crew to climb for separation from other traffic. What should the crew do?
- AComply with the ATC climb instruction immediately, since ATC retains overall separation responsibility.
- BQuery ATC before manoeuvring, since the two instructions conflict.
- CFollow the RA and manoeuvre as commanded, advising ATC of the deviation as soon as workload allows.
- DDisregard both instructions and hold the current flight path until the conflict resolves itself.
Show the answer and walkthrough
Correct answer: C
- A. This reverses the priority that applies once an RA has been issued; the RA is based on the more immediate picture of the conflicting traffic and takes precedence.
- B. Seeking clarification first spends the seconds an RA is specifically designed to use for an immediate manoeuvre instead.
- C. Correct: standard procedure is to comply with the RA promptly and inform control afterwards, not to negotiate the conflict in real time.
- D. Taking no action ignores the entire reason the RA was issued and removes the one immediate protection actually available.
Step by step
- Recognise that the RA is generated from TCAS's own, directly sensed picture of the conflicting aircraft, independent of the picture ATC is working from.
- Apply the standard rule: an RA is followed even where it conflicts with a simultaneous ATC instruction.
- Sequence the response correctly: manoeuvre first, then notify control, rather than querying control before acting.
- Confirm this fits the system's purpose: RAs exist for last-resort, time-critical separation, which only works if compliance is prompt rather than negotiated.
Common mistakes
Prioritising a simultaneous ATC instruction over the RA
This is the single most examined TCAS trap. The RA reflects a faster, more direct picture of the actual conflict, and the standard rule is to follow it regardless of what control is saying at that moment.
Treating a Traffic Advisory as if it demands a manoeuvre
A TA is purely informational, telling the crew where to look. Manoeuvring on a TA alone, before any RA is issued, is not the trained response and the exam distinguishes the two deliberately.
Expecting RA protection during the final stages of an approach
Resolution Advisories, and then Traffic Advisories, are inhibited below defined radio heights close to the ground, so a stem set low on approach is testing whether you know TCAS has already stepped back at that point.
Related topic guides
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Last reviewed July 2026