I just watched a near mid-air collision video clip on Youtube. When AP is on and TCAS gives resolution advisory ("climb climb" or "descend descend"), does AP automatically perform the advisory if the pilot doesn't take over in time?


3 Answers 3


I know that there are certified TCAS/autopilot/FD systems (380 comes to mind), but the normal modality that I am familiar with is where TCAS alerts are provided to the pilot, who then decides what action to take.

As for the rules on the certified systems, I cannot comment, but I am sure that information is obtainable.

Of course, one can imagine one avoidance maneuver triggering more avoidance maneuvers, and a cascade of paperwork at ATC. So the rules and use cases are likely heavily vetted.

To generally answer your question, the most common TCAS implementations do not couple with AP or FD, and rely on pilot decision making to further avoid conflict.

Addendum #1 The A380 does apparently have an integrated AP/FD mode that works with TCAS. I don't know the rules and the certification criteria and goals, but here is an article on it. http://www.airbus.com/newsroom/press-releases/en/2009/08/easa-certifies-new-autopilot-flight-director-tcas-mode-for-a380.html


Short answer is no.

First of all it is resolution advisory as you said. Its purpose, is to make the Pilots aware of an imminent danger/conflict. As with 'Terrain, Terrain', 'Pull up!, Pull up!', it is up to the pilots whether or not they will follow the instruction (normally they always do, but there are exceptions).

However when the pilots hear 'Climb, Climb' or 'Descent, Descent', or another TCAS alert, they should immediately initiate the required action. After all, these days there are top-notch ATC and confliction prevention systems, as well as high-tec TCAS systems on most of the commercial aircraft.

  • 3
    $\begingroup$ Wrong answer. Some aircraft, for example Airbus 350, have the capability to perform TCAS RA with AP on. $\endgroup$
    – Sami
    Jan 2, 2019 at 23:20

On most aircraft: NO.
On some aircraft: YES.

On most aircraft A/P is incapable of executing the maneuver advised by TCAS RA. The evasive maneuver has to be flown manually. Thus if the A/P is engaged, it has to be disengaged.

Taking the A320 as an example, the A320 TCAS procedure & memory items are (excerpt):

AP (if engaged)……………………………………… OFF
BOTH FDs………………………………………………………… OFF
  • PM selects FD’s OFF after request of PF.

  • Respond promptly and smoothly to a RA by adjusting or maintaining the pitch, as required, to reach the green area and/or avoid the red area of the vertical speed scale.

  • Avoid excessive maneuvers while aiming to keep the vertical speed just outside the red area of the VSI , and within the green area. If necessary, use the full speed range between Vαmax and VMAX.

  • Respect stall, GPWS, or windshear warning.

  • Notify ATC.

On some aircraft A/P is capable of and will autonomously execute the TCAS RA maneuver:

Some aircraft are equipped with AP/FD TCAS MODE ENGAGEMENT capability.

  • If an RA is triggered, the AP/FD TCAS mode automatically, and immediately engages.

  • The PF announces the AP/FD TCAS mode engagement, like any other FMA changes.

  • If the FDs are disengaged, they automatically engage. The FD pitch bar does not flash, and the triple-click aural alert does not sound, in order to avoid to disturb the PF during the evasive maneuver.

  • If the A/THR is disconnected, it automatically becomes armed or active.

  • If APs and FDs are OFF when the RA is triggered, HDG automatically engages.

TCAS procedure with AP/FD TCAS:

  • If an RA is triggered:

    • If the AP is engaged:

      • The PF should keep it ON.
      • The AP guides the aircraft in accordance with the RA orders.
    • If the AP is not engaged:

      • The PF smoothly and firmly flies the FD pitch bar.
      • The FD orders guide the aircraft in accordance with the RA orders.
      • The PF can ask the PM to engage the AP.
    • The PM monitors the evasive maneuver. He does not try to see the reported traffic.

  • The flight crew checks that the guidance of AP/FD TCAS mode leads the vertical speed out of the red area of the vertical speed scale, and in the green area if any.

  • If for any reason during an RA, the aircraft vertical speed does not reach the green area of the vertical speed scale, the PF should disconnect the AP, and override the FD orders, in order to lead the aircraft vertical speed out of the red area of the vertical speed scale.

  • If necessary, the PF must use the full speed range between Vαmax and Vmax.

  • If any “CLIMB” aural alert sounds during the final approach, the flight crew should first apply the AP/FD TCAS procedure. When clear of conflict, depending on the situation, they can consider a go-around.

  • The PM notifies the ATC

  • When clear of conflict:

    • The AP/FD TCAS mode automatically reverts to V/S, in order to capture the FCU selected altitude. In some cases, the AP/FD TCAS mode may revert to an altitude acquire mode or an altitude hold mode.
    • The PM notifies the ATC
    • The PF engages an appropriate vertical mode, or adjusts the vertical speed target, in accordance with ATC clearance.
  • The AP/FD TCAS mode is speed protected, i.e. it ensures that the aircraft speed remains between VLS- 5 kt and Vmax. Therefore, in the case the RA is triggered when the aircraft is close to its performance limits, the RA vertical speed target may not be reached with the AP/FD TCAS mode.


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