Speed Trim 737-800?

[RISK AVERSE]

In my wisdom i thought i would try a bit of hand flying yesterday, and noticed that as we accelerated with the A/P disengaged the aircraft was self trimming back during the T/O phase?

this seems a bit bizaare, as it should be trimming forward. Both the Captain and myself questioned this. But the engineers said it was Speed trim.

I suppose we should know why this happened - but sometimes when your so used to engaging the A/P at 1500ft i assumed that the trimming was due to the A/P...

Any thoughts on this and why it trims the aircraft back instead of forward..

Thanks

RA

[FCS Explorer]

on type experienced i can say: YES! automatic trim during initial climb-out, usually in the *wrong* direction. even if you follow the FD commands precisely.

it's not a trick, it's a boeing.

[bushbolox]

In general It trims the stab to required in trim postion. Not the elevator. draw it it makes more sense. Sometimes, but not after a night larnaca

[CaptainSandL]

Speed trim is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust - i.e. on most take-offs. Speed trim is a dual channel system. Sometimes you may notice that the speed trim is trimming in the opposite direction to you, this is because the speed trim is trying to trim the stabilizer in the direction calculated to provide the pilot with positive speed stability characteristics. The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed or a significant amount of push force to increase airspeed. Whereas pilots are typically trying to trim the stick force to zero. Occasionally these may be in opposition.

S&L