TOGA disengage or disappear

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Hi to everybody, I've a problem with the TOGA during the take off.

Model 747.000 and ER version

After entering in the runway, I activate the autothrottle and then I push the throttle to 50%. Then I push the TOGA button and the system push the throttle to the power established by the fmc.

After a while I hear a sound like a FMC message (but nothing appears on the fmc and on both EICAS) and after a second the throttle come back to the last position I left them before pushing the TOGA button, and the throttle is free to move even if the autothrottle level is ON.

I happened with the quantas livery (and presume configuration). Tried with default 747.400 (pmdg livery) directly on runway and everything on, same for engines. Set on fmc fantasy route, altitude and set V1, Vr,V2. TOGA and everything ok!

Reloaded on runway with everything on (as before). Turned off engines, re-set everything on fmc. Turned on engines. TOGA and as said before. It disconnect or disappear. I don't know also what word to use. No message appears.

I don't know if there's a problem in the procedure and so is my fault, or I've a software problem.

Any ideas?

Thanks for the help

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If you check the FMA during the takeoff roll, you will notice that when you press the TOGA button, THR REF is announced on the left which means that A/T sets the thrust to the thrust ref. Then passing 80kt, the FMA display changes to HOLD which means that the A/T no longers commands the thrust levers and you are free to move them. Passing 400ft, the mode will revert back to THR REF and the A/T will take authority again over the thrust levers and thrust control. That is perfectly normal.

What probably happens is that your throttle sends spikes which are ignored by the A/T (provided you set the "A/T MANUAL OVERRIDE" to "IN HOLD MODE ONLY") during the initial roll below 80kt but then when the mode reverts to HOLD, the signal from your throttle brings the thrust levers back to where you left them when you pushed TOGA (50%).

To avoid that, you can either set the option "A/T MANUAL OVERRIDE" to "NEVER" (in which case whatever the A/T mode is as long as A/T is engage the signal from your throttle will be ignored).

Or you can check the config of your throttle to identify why it sends spikes (it can be bad USB connection for example).

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