Originally Posted by
StuntPilot
Still gobbledygook to me.
How is safety added by not being able to trim with thumb switches? In particular, until a mechanical stop is hit? I would think it is the other way around, that hitting a mechanical stop disables trimming? Furthermore, I would think that a mechanical stop should never be hit, that there should be some microswitches on the actuator to prevent hitting a mechanical stop.
How does not being able to trim when a mechanical stop is NOT hit compensate not being able to do that when you actually need it?
Obviously hitting the mechanical limit disables trimming. It's a hard limit. But there are also electric switches preventing you from reaching the full nose down (and up) position using electric trim even before you hit the mechanical limit. Boeing regards this additional limitation as a safety feature. And indeed, in case of electric trim runaway, it can limit the amount of trim the runaway could apply, so it can improve safety in some situations.
Later edit:
You can read in more detail about that in the latest Satcom Guru article:
https://www.satcom.guru/2019/04/stab...and-range.html
In that article there is a table describing the available ranges for the various types of electric trim, compared to manual trim with the trim wheels.
For example, for the 737-800, the full trim range available with the trim wheels is -0.20 to 16.9 units. For manual electric trim with the flaps retracted the range available is 3.95 units to 14.9 units, so far away from the mechanical stops.
That's what the EASA document was talking about. You can't access the full range of mechanical trim positions using manual electric trim. With the flaps retracted on the 737-800 you would only have access to a range of 10.95 units of trim out of the total range of 17.1 trim units available with the trim wheels.
Unfortunately MCAS doesn't use the same limits as manual electric trim, so it can bring the trim closer to the full nose down limit. In the Ethiopian flight it manages to bring it to 0.4 units of trim at some point.