Here is the message that has just been posted:
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Regarding the “why” on max thrust suddenly: if maneuvering around weather in Heading Select, it’s certainly possible to crank the Speed Select knob instead of the heading bug when intending to make a turn, commanding a higher airspeed inadvertently and no change in heading. The sudden thrust could surprise any crew, especially when tired and distracted by weather or ATC commands.
But the message never showed up?
One might notice an increase in engine noise, lack of turn, and max 399 in the speed window?