grasp and hold
I have read the whole thread and I'm still a bit puzzled about the relative ability to trim electrically or manually. It's been asserted several times that the electric trim is more powerful than manual trimming, but we also have the checklist item "grasp and hold" if isolating electric trim doesn't control a runaway. This has been touched on earlier, but I can't envision a simple way that the manual trim wheels could win against the electric motor (stall it) whilst then not being able to actually trim over a wider range of conditions than the electric trim. Can anyone confirm there is some form of mechanism to achieve this and provide any information as to how it works?
I'm also wondering about the revisions to the stab trim motor unit over the years and what qualification tests are likely to have been required before revisions could enter into service. It appears the current motor is a brushless DC, but it seems extremely unlikely to me that it would have been this type of motor 50 years ago. It seems more likely to me that it would have originally been an a.c. motor running directly off the 3-phase supply. These can fairly easily be made multi-speed under relay control avoiding any control electronics and there are various design parameters that can be adjusted to control the speed/torque relationship, but perhaps it was some other type of motor. I wonder how accurately a modern brushless DC motor would reproduce the original characteristics, whatever type of motor it was, and whether this would have been flight tested over the full flight envelope before being signed off i.e. is it possible that not only the manual trim has changed in its authority due to reduced wheel size, but also the electrical trim capabilities may have changed in some subtle ways.