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I know you can press the AT disconnect button on the sides of the throttle.

You can press the A/THR button on the FCU.

When does it disconnect automatically? For example, on landing.

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The autothrust disconnect is described in the Flight Crew Operating Manual:

A/THR DISCONNECT

When the A/THR is disconnected, it is neither armed nor active. The A/THR can be disconnected in two ways:

  • Standard disconnection
    • The pilot pushes the instinctive disconnect pushbutton on the thrust levers (which immediately sets the thrust corresponding to the lever positions) or
    • The pilot sets both thrust lever to IDLE detent.
  • Non-standard disconnection
    • The pilot pushes the A/THR pushbutton on the FCU while A/THR is armed/active, or
    • The system loses one of the arming conditions.

(Airbus A320 FCOM - Autoflight - Flight Guidance)

On landing, the autothrust is actually not automatically disconnected. The Airbus will remind the pilots to retard the thrust levers to idle with the "RETARD, RETARD" callout when passing 20 ft (see e.g. this answer). Moving the thrust levers to IDLE will then disconnect the autothrust system.

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  • $\begingroup$ Bringing the throttle back to idle on landing and A/THR disconnecting - is there an RA requirement for that to happen or will that happen at altitude also. $\endgroup$
    – dashman
    Commented Nov 13, 2021 at 16:13
  • $\begingroup$ @dashman You start to smoothly retard the thrust levers at the "RETARD, RETARD" callout (20 ft RA) and ideally they should reach the IDLE detent in the moment of touchdown. $\endgroup$
    – Bianfable
    Commented Nov 13, 2021 at 16:15
  • $\begingroup$ I don't think I was clear. If I bring the thrust levers back to idle at 1000 AGL - will the A/THR disconnect or only below a certain RA. $\endgroup$
    – dashman
    Commented Nov 13, 2021 at 16:41
  • $\begingroup$ @dashman Ah, sorry. Yes, the A/THR will always disconnect when both thrust levers are in IDLE, regardless of altitude. $\endgroup$
    – Bianfable
    Commented Nov 13, 2021 at 16:54
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    $\begingroup$ This somehow links with TAM accident mentioned in this question, and you also answered. I guess the spoilers didn't deploy because the A/T was still active. $\endgroup$
    – mins
    Commented Nov 13, 2021 at 18:48

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