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Autothrottle disconnection after pressing the LOC switch

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Hello together,


in my today's flight, the following happened during the descent phase:


After I've pressed the LOC switch to arm the LOC lateral mode, the autothrottle was disconnected (immediately upon the switch actuation).

I had the AUTOTHROTTLE DISC message and the acoustic warning.


Has anybody an idea why the autothrottle was disconnected upon the arming of the LOC mode?


I use a Saitek throttle quadrant button for disconnecting the autothrottle. However, this button seems to be ok.


Thanks for your thoughts.



... I just want to inform you, that I've clicked on the LOC button with my mouse.

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There are a couple of conditions in which the autothrottle will disconnect automatically, there are described in the FCOM v2 p 4.20.9.
However in your case, only one would be plausible: a fault detected in the active autothrottle mode (failure).


_ When this occurs, in which position are the A/T arm switches?

_ What is displayed on the FMA before and after the alert is displayed?

_ Check you have no failure active.

_ Do you use a hardware for the throttles and how is it configured?

_ How is set the A/T manual override option?


Otherwise, one of your hardware must send a command to disconnect the autothrottle anyhow.


By the way, one rule in that forum is to sign your posts with full names.

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Hello Romain,

thank you for your answer. I've read 4.20.9 before posting:

A/T manual override option was set to IN HOLD MODE ONLY   -> thrust levers were not manually overwritten

- reverse thrust levers were in the idle position

- A/T arm switches were in the ARM posision

- failures were disabled


Unfortunately I do not know anymore, what has been displayed in the FMA before and after the alert.


My hardware button for disconnecting the autothrottle is 100% reliable (or was 100% reliable up to now...).


The thing that could indicate that there might be an issue with the software is that the autothrottle disconnection occured in exactly that moment when I pressed the LOC switch.


Kind regards

Jens Müller-Junker

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Try to reproduce it and log as much info as you can regarding your aircraft condition (FMA display, A/T mode, FMC config, speed/speed setting, flaps...).

It may be a software issue but also (no offence) a user issue.

If the problem occurs everytime you can also submit a ticket on the support website.

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