Originally Posted by
Tomaski
The simplest way to explain it is that the harder one has to push or pull on the control column, the harder it will be to use the manual trim wheel. Thus larger elevator deflections, higher speeds, and (I believe) lower altitudes will increase the dynamic pressure on the elevator which then places more pressure on the jackscrew mechanisms. That said, if the stab is kept reasonably close to a neutral trim state, there is little problem in using the manual trim wheel throughout the flight envelope.
Yes, I wasn't clear enough, I recall there was mention in the now closed threads, that once you got to full AND or ANU trim, there might be areas, even inside the normal W/B, Altitude and IAS envelope where it would be impossible to get back to in-trim without maneuvers like the yo-yo. I was trying to remember if this only applied to manual trim, or also to thumb switch trim, and what the speeds/altitudes where, where this became an issue.