Originally Posted by
silverstrata
And regards the speed-trim system, it ALWAYS trims in the wrong direction. Whenever it operates, you just trim it back the other way. Most absurd system ever invented.
Not that absurd. (And not unique to B737 either...)
Part 25 regs require that there be a minimum stick force "per knot" (actually, per 6 knots, to be pedantic) to change speed from the trim condition (as a demonstration of static longitudinal stability). Achieving this under all conditions can be tricky. So a design solution applied is to have an automatic system act to increase the required force - to introduce a degree of "artificial stability" in effect. One way to do this is to have the aircraft automatically "trim the wrong way". that way, when hand flying (and not trimming) you find you have to pull more to slow down (part of the pull is the natural pull the aircraft requires, part is to fight the opposite auto-trim), and similarly push more to accelerate.
Unfortunately these systems are often called things like "auto trim" or have "trim" in their name, and that leads people to sometimes think that their intention is to HELP the pilot trim the a/c, whereas they are designed to do the exact opposite.