Slasher et al,
If you trim accurately yourself, the speed trim will never sense the requirement for trim, and thus will not interfere with your flying. The system needs to sense quite a lot of out-of-trim force to operate.
If the speed trim operates whilst you are hand-flying, this is a sign that you are not trimming accurately yourself - and poor trimming is often to blame for poor accuracy in hand-flying.
The point about 'Is it a trim runaway?' is a valid one. Two points here - first, this is another Boeing 'nasty'; second, make sure you know how to locate and operate the trim cutout switches in an instant - and bear in mind you may have to do this under positive or negative 'G' depending upon the type of runaway and when it occurs.
[This message has been edited by Portly (edited 24 September 2000).]