737NG Speed Trim?

I think you are confusing the effects of high speed flight, with the effect of an increase in speed on the trimmed condition of an aircraft. Most will start to pitch up as they accelerate from a trimmed level-flight condition. This is not Mach tuck, but is the effect countered by the speed trim. Best regards,Robin.

That's exactly the point. The confusion begun when I thought that Kristoff was asking about speed trim (normal manual flying condition) so I tried to explain. You introduced the Mach Tuck Effect into your first answer, so my explanation seemed to be related to that. In fact, I wasn't aware of the meaning of the word "tuck". In Brazil, we study this effect as "TUCK UNDER" and I associated the word "UNDER" with this effect, completely ignoring the "TUCK". Anyways, I'm always keen to learn, so I must say thank you to you guys who helped to elucidate. Sorry for confusing those 2 concepts!