TCAS RA in a Terminal Area with Conflicting Instructions
You are climbing in a jet on a SID in a busy terminal area, cleared to FL150, passing 6,500 ft at 2,500 ft/min. The controller has just issued "Climb FL150, expedite." Simultaneously TCAS annunciates a Resolution Advisory: "Climb, Climb" with a red arc above and a green arc below. The traffic advisory that preceded it showed an intruder at your 1 o'clock, 1.5 NM, 400 ft below, climbing. The other pilot is monitoring and has the aircraft in sight on the TCAS display only, not visually. ATC is talking to another aircraft on the same frequency. Your TCAS is Mode S with full RA capability and the intruder is transponder-equipped.
1. State the correct immediate action and explain how it interacts with the ATC clearance.
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Immediate action: follow the RA. Disconnect the autopilot if it is not already off, smoothly apply the climb commanded by the RA, and announce "TCAS RA" to ATC on the frequency.
Interaction with the clearance:
- The RA commands a climb, which happens to be in the same sense as the ATC clearance to FL150. That is convenient but coincidental; the obligation comes from the RA, not from ATC.
- Under standard TCAS doctrine, the RA takes priority over the ATC clearance for the duration of the RA. ATC clearances are predicated on known traffic; TCAS sees traffic ATC may not have resolved, and the RA is generated to guarantee vertical separation.
- The phrase "TCAS RA" tells the controller that you are manoeuvring under TCAS authority, so they will not issue a conflicting instruction and will provide separation to other aircraft around you.
- Do not ask for permission, and do not delay to finish the ATC exchange. The RA is time-critical; a few seconds of hesitation at 2,500 ft/min closure can erase the margin the RA is trying to create.
- Once clear of conflict, announce "Clear of conflict" and return to the ATC clearance (FL150) unless ATC issues a new one.
Note that the RA here is a climb, so there is no conflict with the clearance; had the RA been a descent, you would still descend and tell ATC.
2. Explain why the intruder being "in sight on the TCAS display only" changes nothing about your response.
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TCAS traffic displayed on the screen is not a visual acquisition of the aircraft. It is a symbolic representation derived from transponder replies, subject to:
- Update latency: the symbol shows where the intruder was a few seconds ago, and at closing speeds in a terminal area that lag can be hundreds of feet.
- Altitude quantisation: Mode C/S altitude is reported in 100 ft increments and can be stale between sweeps.
- No attitude or intent information: you cannot see the intruder's flight path, configuration, or whether they are turning.
Therefore the display does not satisfy the see-and-avoid requirement and does not permit you to disregard the RA. The RA is generated from the same surveillance data but with a conflict algorithm that computes a manoeuvre guaranteed to produce vertical separation, and it is the RA, not the symbol, that you act on. Visual acquisition, if and when it occurs, is used to maintain separation after the RA manoeuvre, but it never overrides the RA itself.
3. After clear of conflict, what is the correct sequence of actions and reports?
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- "Clear of conflict" to ATC, so the controller knows you are back under normal clearance authority.
- Return to the last ATC clearance (FL150) unless ATC issues a revised one, and re-engage the autopilot when stable.
- Report the RA to ATC in enough detail to be useful: intruder relative position, whether the RA was climb or descend, and whether you deviated from the clearance. ATC may need to separate other aircraft from the intruder.
- Note the event for the occurrence report: TCAS RAs are reportable in most States, and the data (time, level, intruder altitude, RA sense) is used for airspace safety analysis.
- Consider the traffic picture: an intruder 400 ft below and climbing at your 1 o'clock suggests a possible level bust or a crossing clearance error. Stay vigilant for a second RA and keep the TCAS display monitored.
- Do not treat the event as closed just because you are clear; if the intruder is still climbing and close, a second RA is possible.